ORDER NUMBER EA-356

AIRCRAFT

RADIO SYSTEMS

By J. Powell

BA, C Eng, MIERE, GradIMA

Reprinted in India by

HIMALAYAN BOOKS ° New Delhi-110001.

Distributed by: The English Book Store

The Aviation People

17-L, Connaught Circus New Delhi-110001 (India)

Contents

Preface

1 Historical, technical and legal context 1

Introduction 1 Historical background 1 Basic principles of radio 2 Digital systems 8 Categorization of airborne radio equipments Navigation nomenclature 13 Interference 13 Maintenance 17 Regulating and advisory bodies 18

2 Communication systems 20 Introduction 20 V.h.f. communications 20 H.f. communications 29 Selcal 35 Audio integrating systems - (Intercom) 37 Testing and trouble shooting the audio systems 43

3 Automatic direction finding 45 Introduction 45 Basic principles 45 Simplified block diagram operation 4.7 Block diagram detail 47 Sources of system error 49 Installation 52 Controls and operation 54 Characteristics 55 Calibration and testing on the ramp 55

4 V.h.f. omnidirectional range (VOR) 58

Introduction 58 Basic principles 58 Doppler VOR (DVOR) 61

Aircraft installation 63

Controls and operation 65 Simplified block diagram operation 65 Characteristics 65 Ramp testing 67

5 Instrument landing system 69 Introduction 69 Basic principles 69

Simplified block diagram operation 72 Installation 74 Controls and operation 76 Characteristics 77 Ramp testing 78

6 Hyperbolic navigation systems 79

General principles 79

Omega navigation system 83

7 Distance measuring equipment 105 Introduction 105 Basic principles 105

X and Y channel arrangements 110

The link with v.h.f. navigation 110 Installation 111 Controls and operation 112

Simplified block diagram operation 112 Range measuring and mode control 114 8 ATC transponder 121 Introduction 121 Basic principles 121 Installation 126 Controls and operation 127 Simplified block diagram operation Block diagram details 128 Characteristics 135 Ramp testing 136

128

9 Weather avoidance 139 Introduction 139 Weather radar 140 Choice of characteristics and features 140 Installation 146 Controls 147 Operation 148

Block diagram operation 150

Scanner stabilization 157

Aircraft installation delay 197 Interface 198

Other applications for weather radar 160 Weather radar characteristics 162 Maintenance and testing 164 Ryan storniscope 168 Appendix Factors affecting weather radar performance 169

Multiple installations 199 Characteristics '00 Ramp testing and maintenance 200 Appendix Sinusoidal frequency modulation 201

12 Area navigation 202 10 Doppler navigation 172 Development of airspace organization 202 Introduction 172 Generalized area navigation system 202 Doppler effect 173 VOR/DME based RNAV. principles 204 Antenna mechanization 174 Bendix nav. computer programmer NP-2041A 206 Doppler spectrum 175 ` King KDE 566 210 Beam geometry 175 Standardization 213 Transmitter frequency 176 Testing RNAV 215 Modulation 177 Over-water errors 178 13 Current and future developments 216 Navigation calculations 179 Introduction 216 Block diagram operation 179 The state of the art 216 Installation 182 The flight deck 217 Controls and operation 184 Multi-systern packages 219 Characteristics 185 Data link 219 Testing 185 ADSEL/DABS 2 21 Appendix Satcom and satnav 223 Relationships between aircraft and earth Microwave landing system 2 24 co-ordinates 186 Microwave aircraft digital guidance equipment 225 The Doppler shifts for a four-beam Janus Collision avoidance 2 29 configuration 187 The current generation of ARINC The aircraft velocity in earth co-ordinates characteristics 230 expressed in terms of Doppler shifts 188 Concluding remarks 231

11 Radio altimeter 189 Recommended reading 232 Introduction 189

Basic principles 189 Factors affecting performance 191

Block diagram operation 192 Monitoring and self-test 195 Indicat or 196 Installation 196

Glossary 234

Exercises 246

Index 252

Preface

The cockpit and equipment racks of modern aircraft, large and small, are becoming filled with ever more sophisticated systems . This book attempts to describe a certain class of such systems, namely those which rely for their operation on electromagnetic radiation. The subject matter is complex and wide-ranging, hence not all aspects can be covered in one volume In deciding where the treatment should be light or perhaps non-existent, I have asked myself two questions: (1) which aspects can most usefully be covered in a book; and ( 2 ) at which group of people involved in aviation should a book covering such aspects be aimed?

The answer to (1) must be `describe the theory'. One can, and indeed must, read or be told about how to operate the systems: how to navigate using the systems; how to solder, crimp and change items; how to use test equipment, etc. but proficiency is impossible without practice. On the other hand gaining an understanding of how a particular system works is more of a mental exercise which can be guided in a book such as this. This is not to say that more practical matters are neglected, since it would not help one's understanding of the theory of operation not to see, at least in words and pictures, how a particular system is controlled, presents its information, reacts to the environment, etc.

Having decided the main line of attack the more difficult question of depth of treatment must be answered: in other words which group should be satisfied'.' Pilots need a superficial knowledge of how all the systems work; maintenance engineers on the ramp and in the hangar a more detailed knowledge; workshop engineers must have an understanding of the circuitry for perhaps a limited range of equipments; while designers should have the greatest depth of knowledge of a11. It is virtually impossible to draw dividing lines, but it is hoped that if enough theory is given to satisfy the aircraft radio maintenance engineer then the book might be useful to all groups mentioned.

The depth of treatment varies, it being impossible to cover everything, or indeed anything, to the depth I would have liked. In particular few details of

circuitry are given since I feel most readers will be more interested in the operation of the system as a whole. Nevertheless, some circuits are given purely as examples. Should the reader need circuit knowledge, the equipment maintenance manual is the best place to find it, assuming he knows the system and he has a basic knowledge of electronics.

The state of the art of the equipment described is also varied. 1 did not see the point of describing only equipment containing microprocessors, since the vast majority of systems in service do not use them as yet. On the other hand if the life of this book is not to be too severely restricted, the latest techniques must be described. Within the pages that follow, analogue, analogue/digital, hardwired digital and programmable digital equipments all find a place.

As stated previously, the book is aimed primarily at the maintenance engineer. However, I hope several groups might be interested. This poses problems concerning the background knowledge required. For what I hope is a fairly substantial part of the book, any reasonably intelligent technically minded person with a basic knowledge of mathematics and a familiarity with aircraft will have no difficulty that two or perhaps three readings will not overcome. There are parts, however, where some knowledge of electronics, radio theory or more sophisticated mathematics is needed. In three chapters where the going gets a bit tough, I have relegated the offending material to an appendix. Some background material is covered in Chapter 1, in particular, basic radio theory and a discussion of digital systems in so far as coding and computers are concerned.

If you are one of the few people who plough all the way through the Preface to a book, you may have decided by now that this book is concerned with theory and little else. That this is not so may be clear if I outline briefly the contents of each chapter. An introduction saying a few words about the history and function of the system is followed by a fairly thorough coverage of the basic principles. In some chapters the next item is a discussion of the installation, i.e. the units, how they are interconnected, which other systems they interface

with and any special considerations such as cooling, positioning, type of antennas and feeders, etc. This, together with a description of controls and operation, puts some practical meat on to the bare bones of the theory which continues with a consideration of the block diagram operation. In certain chapters the order: installation - controls and operation -- block diagram, is reversed where 1 thought it was perhaps to the reader's disadvantage to break up the flow of the more theoretical aspects. A brief look at characteristics, in practically all cases based on ARINC publications, and -testing / maintenance concludes each chapter.

Most chapters deal with one system; none of them is exclusively military. The exceptions are, in reverse order, Chapter 13 where I look at the current scene and review some systems we should see in the next few years; Chapter 12 which is a bringing-together of some of the previously covered systems: Chapter 6 covering Omega, Decca Navigator and Loran C'; Chapter 2 which covers both radio and non-radio communications; and Chapter 1 where some chosen background material is given.

I should point out that this is not a textbook in the sense that everything is examinable in accordance with some syllabus. The reader will take from the book however big a chunk he desires, depend ing on his background knowledge, his profession, the examinations he hopes to take and, of course, his inclination. Some will have, or end up with, an understanding of all that is included herein, in which case I hope the book may be seen as a source of reference.

Acknowledgements

A number of manufacturers have given valuable assistance including the supplying of material and granting permission to reproduce data and illustrations. Without the generosity of the following, this book would have been of very limited use.

Bendix Avionics Division

Boeing Commercial Aero plane Company British Aerospace

Communications Components Corporation The Decca Navigator Company Limited Field Tech Limited Hazeltine Corporation IFR Electronics Inc King Radio Corporation Litton Systems International Inc., Aero Products Division Marconi avionics Limited MEL Equipment Company Limited RCA Limited Rockwell-Collins (UK) Limited Ryan Storm scope

Tel-Instrument Electronics Corporation (TIC)

Although I am grateful to all the above, I must reserve a special word of thanks to Mr Wayne Brown of Bendix, Mr. A. E. Crawford of King and Mr. T. C. Wood of RCA, who arranged for the dispatch of several expensive and heavy maintenance manuals in reply to my request for information. These manuals, and indeed all other information received, were used in the preparation of this book and continue to be used in the training of students at Brunel Technical College, Bristol, England.

I also wish to thank all my colleagues at Brunel who have helped, often unwittingly, in conversation. In particular my thanks go to John Stokes, Clive Stratton and Peter Kemp for proof-reading some of the chapters and also Leighton Fletcher for helping with the illustrations. May I add that, although I received technical assistance from the above, any mistakes which remain are obviously mine. I would be grateful to any reader who might take the trouble to point out any errors.

Finally, my thanks to Pauline Rickards, whose fingers must be sore from typing; to the publishers who displayed great patience as the deadline for the submission of the typescript came and went; and, most of all, to my wife Pat and son Adam who showed even more patience and understanding than Pitman’s. Bristol,

England J. P.

I Historical, technical and legal

context

Introduction

This book deals with airborne systems that depend for their operation on the generation and detection of that intangible discovery the radio wave. Such systems split naturally into two parts: communications and navigation. The former provide two-way radio contact between air and ground, while the latter enables an aircraft to be flown safely from A to B along a prescribed route with a landing safely executed at B.

An understanding of such systems requires a working knowledge of basic electronics, radio, computer systems and other topics. A book of this length cannot provide all that is necessary but it was thought that some readers might appreciate a review of selected background material. This is the objective of Chapter 1. It may be that on consulting the list of contents, the reader will decide to omit all or part of this chapter. On the other hand, some readers may decide that more basic information is needed, in which case, the list of recommended books will help point the way to sources of such material.

Historical Background

In 1864 James Clerk Maxwell, Professor of Experimental Physics at Cambridge, proved mathematically that any electrical disturbance co uld produce an effect at a considerable distance from the point at which it occurred. He predicted that electromagnetic (e.m.) energy would travel outward from a source as waves moving at the speed of light. In 1888 Hertz, a German physicist, demonstrated ,hat Maxwell's theory was correct, at least over distances within the confines of a laboratory. It was left to the Italian physicist Marconi to generate e.m. waves and detect them at a remote receiver, as he did by bridging the Atlantic in 1901. Other notable landmarks in the development of radio include: 1897 First commercial company incorporated for

the manufacture of radio apparatus: the

Wireless Telegraph and Signal Company Limited (England), later the Marconi Wireless Telegraph Company Limited.

1904 Fleming's (British) discovery of the thermionic valve - the diode.

1904 First patent for a radar-like system to a German engineer, Hulsmeyer. Workable but not accepted.

1906 De Forest's (American) invention of an amplifying thermionic valve (triode).

1911 Direction-finding properties of radio waves investigated.

1912 Discovery of the oscillating properties of De Forest's valve.

1936 The first workable pulse radar.

1939 Invention of the magnetron in Britain.

1948 Invention of the transistor by Bardeen, Brattain and Shockley (Bell Telephone Laboratories, USA).

To bring us up to date, in the early 1970s the first microprocessor appeared from Intel (USA) leading directly to present-day microcomputers.

Paralleling the progress of radio was the second of the three great developments of the twentieth century, i.e. powered flight in heavier-than-air machines. (The other two developments referred to are electronics and applications of nuclear physics; the reader is concerned with two out of three.) There can be few people who have not heard of Wilbur and Orville Wright; who designed and built the first successful powered aircraft which Orville flew for the first time at 10.35 on 17 December 1903, making a landing without damage after 12 seconds airborne. Since then landmarks in aviation, with particular reference to civil aviation, include: 1907 First fatality: Lieut. T. E. Selfridge, a

passenger in a Wright Flyer. 1909 Bleriot (French) flies the English Channel. 1912 Sikorsky (Russian) builds first multi-engined (four), passenger (sixteen) aircraft. 1914 World War I. The years 1914-18 saw

advances in performance and a vast increase in

number of aircraft, engines and pilots. 1919 Sustained daily scheduled flights begin in Europe. 1928 Whittle (British) publishes thesis on

jet engine.

1929 First blind landing by Doolittle (American) using only aircraft instruments.

1937 Flying-boat service inaugurated from Britain to the Far East. Britain to Australia took 8 days in 1938, either by KLM or Imperial Airways.

1939 Inaugural air-mail service between Britain 1939 First jet -powered flight by He 178 (German). 1939 World War II. The years 1939 -45 saw the growth of world-wide military air transport services, and the USA established as the postwar leader in civil aviation.

1944 International Civil Aviation Organisation formed at Chicago conference.

1945 American Overseas Airlines operate scheduled flights over North Atlantic with landplane (DC 4).

1952 First civil jet aircraft, the Comet 1, goes into service with BOAC.

1953 First civil turboprop aircraft, the Viscount, goes into service with BEA.

1954 Previously unknown problem of metal fatigue discovered in Comet 1. Withdrawn. 1956 Tu 104 first jet aircraft to commence sustained commercial service.

1958 First transatlantic jet service by BOAC with the Comet 4. (PAA's Boeing 707 -120 follows three weeks later.)

1965 First short -haul jet to enter service, the BAC 1 -11.

1970 Boeing 747 introduced; the first of the Jumbo Jets.

1970 First civil aircraft supersonic flights, Concorde and the Tu 144.

From the time of the Wright brothers to the present day, the non -commercial side of civil aviation, known as general aviation (business and private) has grown with less spectacular firsts than its big brother, so that now by far the largest number of civil aircraft are in this category.

It was inevitable that the new toys of radio and aircraft should be married early on in their history. Later the vast increase in air traffic made it essential that radio aids, in both communication and navigation, should be made full use of, to cope safely

with the crowded skies. In 1910 the first transmission of e.m. waves from air to ground occurred. Speech was conveyed to an aircraft flying near Brook lands Airfield (England) by means of an e.m. wave in 1916. By the 1920s, radio was being used for aircraft navigation by employing rudimentary direction -finding techniques (Chapter 3). The introduction of four -course low-frequency range equipment in 1929 provided the pilot with directional guidance without the need for a direction -finder on the aircraft.

Steady progress was made up to 1939, but it was world War I I which gave the impetus to airborne Radio innovations . Apart from very high frequency (v.h.f.) communications, introduced during the war, a number of radio navigation aids saw the first light of day in the period since 1939. These systems are described in the following chapters.

Basic Principles of Radio

Radiation of Electromagnetic (e.m.) Waves and Antennas If a wire is fed with an alternating current, some of the power will be radiated into space. A similar wire parallel to and remote from the first will intercept some of the radiated power and as a consequence an alternating current will be induced, so that using an appropriate detector, the characteristics of the original current may be measured. This is the basis of all radio systems.

The above involves a transfer of energy from one point to another by means of an e.m. wave. The wave consists of two oscillating fields mutually perpendicular t o each other and to the direction of propagation. The electric field (E) will be parallel to the wire from which the wave was transmitted, while the magnetic field (H) will be at right angles. A 'snapshot' of such a wave is shown in Fig. 1.1 where the distance shown between successive peaks is known as the wavelength.

The velocity and wavelength of an e.m. wave are

Fig. 1.1 An electromagnetic wave

directly related through the frequency of the alternating current generating the wave. The law is:

c=Y`f where: c is the speed of light (3 X 108 m/s). Y` is the wavelength in metres. f is the frequency in Hertz (cycles/s). A radiating wire is most efficient when its length

is equal to half a wavelength. Thus for a frequency of 100 MHz the wire should be (3 X 10 8)/(2 X 100 X 106) = 1.5m long, in which case it is known as a dipole. In practice many airborne radio systems do not make use of dipole antennas since their size is prohibitively large, except at very high frequencies, and the radiation patt ern is not suited to applications where energy needs to be transmitted in or received from a certain direction.

A close relative of the dipole is the unipole antenna which is a X/4 length conductor mounted vertically on the metal fuselage which acts as a ground plane in which a reflection of the unipole is `seen' to form a dipole. Thus a v.h.f. communication (comm.) unipole would be less than 60 cm long (centre frequency of the band is 127 MHz). Two unipoles are sometimes mounted back to back on the vertical stabilizer to function as a dipole antenna for use with VOR (Chapter 4) or ILS (Chapter 5).

At frequencies in the region of 2-30 MHz (h.f.) a dipole would be between 5 and 75 m. Since the dimensions of aircraft fall, roughly speaking, within this range of lengths it is possible to use the aircraft as the radiating or receiving element. A notch or slot cut in a suitable part of the airframe (e.g. base of vertical stabilizer) has a large oscillating voltage applied across it, so driving current through the fuselage which in turn radiates. The notch/airframe load must be `tuned' to the correct frequency for efficient transmission. Without tuning, little energy would be radiated and a large standing wave would be set up on the connector feeding the notch. This is due to the interaction of incident and reflected energy to and from the antenna. An alternative type of antenna for this band of frequencies is a long length of wire similarly tuned, i.e. with variable reactive components.

For frequencies within the range 10-100 kHz the maximum dimension of even large aircraft is only a small fraction of a wavelength. At these frequencies capacitive type antennas maybe used. One plate of the capacitor is the airframe; the other a horizont al tube, vertical blade or a mesh (sometimes a solid plate). The aircraft causes the field to become

intensified over a limited region near its surface.

The resulting comparatively strong oscillating E field between the capacitor's plates causes a current to flow in twin feeder or coaxial cable connected across the antenna. The airborne systems operating in the relevant frequency band are the receive-only systems covered in Chapter 6 (Omega, Decca and Loran C). Although ADF (Chapter 3) receives sign als in the band of frequencies immediately above those considered in this paragraph, one of its two antennas (sense) utilizes the principles discussed.

An alternative to the capacitance antenna is the loop antenna which is basically a loop of wire which cuts the H field component of the e.m. wave. The field is intensified by use of a ferrite core on which several turns are wound. Use of two loops mounted at right angles provides a means of ascertaining the direction of arrival (ambiguous) of an e.m. wave. Such antennas are used for ADF (loop) and may also be used for Omega.

At frequencies above, say, 3000 MHz the properties of waveguides may be used. A waveguide is a hollow metal tube, usually of rectangular cross -section, along which an e.m. w ave can propagate. If the end of a waveguide is left open some energy will be radiated. To improve the efficiency, the walls of the waveguide are flared out, so providing matching to free space and hence little or no reflected energy back down the guide. Such an antenna is called a horn and may be used for radio altimeters (Chapter 11).

Associated with the wave propagated along a waveguide are wall currents which flow in specific directions. A slot, about 1 em in length, cut in the waveguide so as to interrupt the current flow will act as a radiator. If several slots are cut the energy from them will combine several wavelengths from the antenna to form a directional beam. The direction depends on the spacing of the slots. Such antennas may be used for Doppler radar (Chapter 10) and weather radar (Chapter 9).

The theory of some of the more esoteric antennas used on aircraft is a little sketchy and design is finalized, if not based, on empirical data. However the antenna is designed, it will only s ee service if it performs its function of transmitting and/or receiving e.m. waves in and/or from required directions. The directivity of an antenna, or the lack of directivity, is most clearly defined by means of a polar diagram. If we take a transmitting antenna and plot points of equal field strength (one value only) we have such a diagram. The same antenna used for receiving would, of course, have the same polar diagram. If the diagram is a circle centred on the antenna, as would be the case if the plot were in the plane perpendicular

to a dipole, then the antenna is said to be omnidirectional in the plane in which the measurements were made. A practical antenna cannot be omnidirectional in all planes, i.e. in three dimensions.

Table 1.3 Approximate bands for microwave frequencies

Letter designation

Frequency range (GHz)

The e.m. Spectrum and Propagation

As can be seen from the previous paragraph, the frequency of the radio wave is an important consideration when considering antenna design. In addition the behavior of the wave as it propagates through the earth's atmosphere is also very much dependent on the frequency.

However, before considering propagation, we will place radio waves in the spectrum of all e.m. waves (Table 1.1). In doing so we see that the range of frequencies we are concerned with is small when

L S C X K Q

1-3 2-5-4 3.5-7.5 6-12.5 12.5 -40 33-50

Table 1.4 Airborne radio frequency utilization (exact frequencies given in relevant chapters)

System Frequency band

Table 1.1 The electro magnetic spectrum

Hz Region

1025 Cosmic rays

1021 Gamma rays

10' 9 X rays 1017 Ultraviolet 1015 Visible 1014 Infra-red 1011 Radio waves

compared with the complete spectrum. By general agreement radio frequencies are categorized as in Table 1.2. There is less agreement about the letter designations used for the higher radio frequencies which are tabulated with approximate frequency ranges in Table 1.3. Finally, Table 1.4 lists the frequencies used for airborne radio systems by international agreement.

Table 1.2

Name

Very low frequency Low frequency Medium frequency High frequency Very high frequency Ultrahigh frequency Superhigh frequency Extremely high frequency

Radio frequency categorization

Abbreviation Frequency

VIE l.f. m.f. h.f. v.h.f. u.h.f. s.h.f. e.h.f.

3-30 kHz 30-300 kHz 300-3000 kHz 3-30 MHz 30-300 MHz 300-3000 MHz 3-30 GHz 30-300 GHz

Omega Decca Loran C ADF h.f. comm. Marker ILS (Localizer) VOR v.h.f. comm. ILS (Glideslope) DME SSR Radio altimeter Weather radar (C) Doppler (X) Weather radar (X) Doppler (K)

10-14 kHz 70-130 kHz 100 kHz 2-25 MHz 75 MHz 108-112 MHz 108-118 MHz 118-136 MHz 320-340 MHz 960-1215 MHz 1030 and 1090 MHz 4.2-4.4 GHz

In free space, all radio waves travel in straight lines at the speed of light. Such a mode of propagation is known as the space wave. In addition, two other modes of propagation are used with airborne radio equipment: the ground wave and the sky wave. A fourth mode known as tropospheric scatter is used only for fixed ground stations since elaborate and expensive. equipment must be used at both ends of the link due to the poor transmission efficiency.

The ground wave follows the surface of the earth partly because of diffraction, a phenomenon associated with all wave motion which causes the wave to bend around any obstacle it passes. In addition, the wave H field cuts the earth's surface, so causing currents to flow. The required power for these currents must come from the wave, thus a flow

of energy from wave to earth takes place causing bending and attenuation. The attenuation is a limiting factor on the range of frequencies which can be used. The higher the frequency the greater the rate of change of field strength, so more attenuation is experienced in maintaining the higher currents. Ground waves are used for v.l.f. and l.f. systems.

Radio waves striking the ionosphere (a s et of ionized layers lying between 50 and 500 km above the earth's surface) are refracted by an amount depending on the frequency of the incident wave. Under favourable circumstances the wave will return to the earth. The distance between the transmit ter and point of return (one hop) is known as the skip distance. Multiple hops may occur giving a very long range. Above about 30 MHz there is no sky wave since insufficient refraction occurs. Sky wave propagation is useful for h.f. comm. but can cause problems with l.f. and m.f. navigation aids since the sky wave and ground wave may combine at the receiver in such a way as to cause fading, false direction of arrival or false propagation time measurements. At v.l.f. the ionosphere reflects, rather than refracts, with little loss; thus v.l.f. navigation aids of extremely long range may be used.

Above 30 MHz, space waves, sometimes called line of sight waves, are utilized. From about 100 MHz to 3 GHz the transmission path is highly predictable and reliable, and little atmospheric attenuation occurs. Above 3 GHz attenuation and scattering occur, which become limiting factors above about 10 GHz. The fact that space waves travel in a straight line at a known speed and, furthermore, are reflected from certain objects (including thunderstorms and aircraft) makes the detection and determination of range and bearing of such objects possible.

Modulation

Being able to receive a remotely transmitted e.m. wave and measure its characteristics is not in itself of much use. To form a useful link, information must

be superimposed on the e.m. wave carrier. There are several ways in which the wave can carry information and all of them involve varying some characteristic of the carrier (amplitude or frequency modulation) or interrupting the carrier (pulse modulation).

The simplest, and earliest, way in which a radio wave is made to carry information is by use of Morse Code. Switching the transmitter on for a short time-interval, corresponding to a dot, o r a longer time-interval, corresponding to a dash, enables a message to be transmitted. Figure 1.2 illustrates the transmission of SOS, the time-intervals shown being typical.

In radar the information which must be superimposed is simply the time of t ransmission. This can easily be achieved by switching on the transmitter for a very short time to produce a pulse of e.m. energy.

When transmitting complex information, such as speech, we effectively have the problem of transmitting an extremely large number of sine waves. Since the effect of each modulating sine wave on the radio frequency (r.f.) carrier is similar, we need only consider a single sine wave modulating frequency. The characteristics of the modulating signal which must be transmitted are the frequency and amplitude. Figure 1.3 shows three ways in which a pulsed carrier may be modulated by a sine wave while Figs 1.4 and 1.5 show amplitude and frequency modulation of a continuous wave (c.w.) carrier.

Both amplitude modulated (a.m.) and frequency modulated (f.m.) carriers are commonly used for airborne systems. With a.m. the amplitude of the carrier represents the amplitude of the modulating signal, while the rate of change of amplitude represents the frequency. With f.m. the amplitude and frequency of the modulating signal is represented by the frequency deviation and rate of change of frequency of the carrier respectively.

Both a.m. and f.m. waves have informative parameters associated with them. With a.m. if the

y

Radio frequency transmitted

, Time

x=0 .1s, y=0 -3s

Fig. 1.2 Morse code: SOS

--, Time

Fig. 1.4 Amplitude modulation

Fig. 1.3 Pulse modulation - from top to bottom: unmodulated carrier, modulating waveform, pulse amplitude modulation, pulse width modulation and pulse position modulation

carrier amplitude is Y"e and the modulating signal amplitude is V„, then the modulation factor is Vt,-,, , 'C. This fraction can be expressed as a percentage, in which case it is known as the percentage modulation or depth of modulation (note sometimes depth of modulation is quoted as a decimal fraction). Figure 1.4 shows 100 per cent modulation. With f.m. the parameter is the deviation ratio which is given by the ratio of maximum frequency deviation (fd max) to maximum modulating frequency (fm max). The ratio fd/fr„ is called the modulation index and will only be constant and equal to the deviation ratio if the modulating signal is fixed in frequency and amplitude.

In Figs 1.3, 1.4 and 1.5 the modulated signal is illustrated in the time domain, i.e. with time along the horizontal axis. It is instructive to look at the frequency domain representations as shown in

Fig. 1.5 Frequency modulation

Fig. 1.6 where a single sine wave of frequency fT, is the modulating signal. It can be seen that several frequencies are present, so giving rise to the idea of bandwidth of a radio information channel. The most significant difference between a.m. and f.m. is that the a.m. bandwidth is finite whereas, in theory, the f.m. bandwidth is infinite. In practice the f.m. bandwidth is regarded as finite, being limited by those extreme sidebands which are regarded as significant, say 10 per cent of amplitude of the largest frequency

Carrier (fc)

Fig. 1.6 Amplitude modulation and frequency modulation spectrums for a pure sine wave modulating signal of frequency fm

component. The relative amplitudes of the carrier and sidebands depend on modulation factor and index for a.m. and f.m. respectively.

In any information link there is a relationship between the bandwidth and the amount of information which can be carried, hence high-fidelity stereo broadcasts occupy a wide bandwidth. It is not, however, desirable to have as wide a bandwidth as possible since (a) the number of available channels is reduced; (b) electrical noise, generated at all frequencies by electrical equipment and components, and by atmospheric effects, will be present in the receiver channel at a greater power level the wider the bandwidth. The signal power to noise power ratio is a limiting factor in the performance of receiving equipment.

The information in an a.m. wave is repeated in each of the sidebands; the carrier frequency component has no information content. As a consequence, at the expense of more complicated transmitting and receiving equipment, we need transmit only one sideband. Single sideband (s.s.b.) transmission conserves bandwidth, with attendant advantages, and is found in airborne h.f. comm. systems.

Multiplexing

In most airborne systems the required number of channels is obtained by allocating non-overlapping bands of frequencies centred on specified discrete carriers. This is known as frequency multiplexing. Shannon's sampling theory shows that a sine wave of frequency fm can be completely specified by a series of samples spaced at no more than 1/2 fm second (s). To transmit speech where the highest

frequency component is 3000 Hz we need only transmit a sample of the instantaneous amplitude every 1/6000 = 0-000 166 7 s (= 166.7 ps). Thus we have time-intervals during which we can transmit samples of other signals. The number of signals we can time multiplex on one carrier link depends on the duration and frequency of each sample. The shorter the sample duration the greater the bandwidth required, confirming the statement made earlier that more information requires wider bandwidths.

Basic Receivers and Transmitters

A much simplified transmitter block diagram is shown in Fig. 1.7. This could be called the all-purpose block diagram since it could easily be converted to a

Fig. 1.7 Basic radio transmitter block diagram

low-level a.m. transmitter, (little if any amplification of the carrier before modulation), a high-level a.m. transmitter (little if any amplification of the carrier after modulation), an s.s.b. transmitter (introduce a band pass filter after the modulator) or an f.m. transmitter (introduce a frequency multiplifer after the modulator). Obviously in the above examples the circuit details would vary greatly, particularly in the modulators, and if detailed block diagrams were drawn the underlying similarities in structure would be less obvious.

The most basic type of receiver is a tuned radio frequency (t.r.f.), however this is rarely used. The standard receiver configuration is the superheterodyne (superhet) shown in Fig. 1.8. The desired r.f. is converted to a constant intermediate frequency by taking the difference frequency after mixing the received signal with the output from a local oscillator (Lo.). Since most of the amplification and selectivity is provided by constant frequency and bandwidth stages the design problem is eased.

In both the transmitter and the receiver, r.f. oscillators have to b e tuned to different frequencies. In the transmitter it is the m.o. (master oscillator),

while in the receiver it is the Lo. Modern practice is

Fig. 1.8 Basic superhetrodyne receiver block diagram

to use a frequency synthesizer with a single crystal toprovide stability dnd accuracy.

Digital Systems

CodingMost of the airborne systems in use are basicallyanalogue, i.e. they deal with signals which representvarious quantities continuously and smoothly. Forexample in DME a very small increase in range resultsin a corresponding increase in time;we say time is ananalogue of distance. With a digital system,information is represented by a number encoded insome suitable way.

Since it is difficult to detect many different voltageor current levels only two are used, and this leadsnaturally to expressing numbers to the base 2 (binarycode) where the only digits are 0 and l. It remains todefine electronic representations of 0 and I in an runambiguous way. Various methods are used with(a) being by far the most common, in thenon-exhaustive list which follows.

(a) Voltage level

(b) Pulse polarity

(c) Pulse position

(d) Phase change

In all of the above the logic may be reversed (positiveand negative logic). Thus we can represent a binarydigit (bit) by an electrical signal, but if the number tobe represented is larger than l, we must combine bitsinto some intelligible code.

Binary code has been mentioned; this is simplycounting to the base 2 rather than the base l0(decimal code) as we do normally. Unfortunately,binary numbers soon become very large, for example9l ro = I 0 I I 0 I 12 ( thesubscr ipts indicat ingthebase), so octal (base 8 = 23) and hexadecimal(base l6 = 24) may be used. The machine may stilldeal with a 1/0 situation but the numbers are moremanageable when written down, for example91 ro = l33s = 5816. Note, in the examples given, ifwe split the binary number into groups of three fromthe right (least significant bit, l.s.b.) we havel , 0 l l , 01 12 = 1, 3, 3s, i .e . each group is the b inarycode for an octal digit. Similarly1 0 1 , l 0 l l 2 = 5 , 8 1 6 .

Binary and hexadecimal codes are used in digitalcomputers, octal code is used for the ATCtransponder (Chapter 8). The task of frequencyselection is one which lends itself to coding, andamong several which have been used, the two mostcommon are binary coded decimal (b.c.d.) and twofrom five (2/5). Both of these codes retain thedecimal digit 'flavour' of the number to be encodedat the expense of using extra bits. To represent 9l 16we consider the decimal digits 9 and I separately togive:

9 l r o = 1 0 0 1 0 0 0 1 6 . 9 . 6 . ,

9 l r o = 1 0 0 0 1 1 1 0 0 0 2 7 5

Equivalents for all the codes mentioned are given fordecimal numbers 0 to l5 in Table 1.5.

It can be seen from the above that more bits thanare absolutely necessary are used for b.c.d. and2l5.

no voltagehigh voltagepositivenegativea time interval is split intwo halves:pulse in first half = |pulse in second half = 0at specified read time asine wave:changes phase

. (180"C) = |does not changephase = Q

- 0- t= l= Q

Tabh 1.5 Various code equivalents 4.5 .6 .7 .

conversion from binarY to b.c.d.;b.c.d. fed to frequency synthesizer;conversion from b.c.d. to special code;

special code fed to readout device.Base Code

1 0 2 8 1 6 BCD 2ls

0 0000 0I 000r I2 0010 23 00 l l 34 0100 45 0 l0 l 56 0 1 1 0 6? 0 l l l 78 1000 l09 1001 I Il 0 l 0 l 0 l 2l l l 0 l l l 312 I 100 1413 l l 01 1514 l l l 0 l 61 5 l l l l l 7

000000010010001 I01000l0 l0 l 100 l l l1000l00l

0001 00000001 00010001 00100001 00l t0001 01000001 0101

0l 001l 1000101000l 1000 l 0 l q001 l0001010001 I1001010001

11000 0100111000 1100011000 1010011000 0110011000 01010u000 00110

If the bits are transmitted serially, one after the other

in time down a line, then more time is needed for the

transmission. of a number than would be needed if

binary code were used. If the bits are transmitted in

parallel, one bit per line, then more lines are needed.

This has a certain advantage in that the redundancymay be used to detect transmission errors, for

example I 0 I I Ocouldnotbe a2/5 codeandI 0 I 0 could not be b.c.d.

Error checking can also be used with binary codes.We will always be restricted to a certain maximumnumber of bits, one of which can be designated aparity bit used solely for error detecting. Suppose we

had eight bits available, each group bf eight-bits wouldbe call-ed a word of length 8 (commonly called a

byte). Ttre first seven bits of the word would be used

to encode the decimal digit (0 to 127) while the

eighth would be the parity bit. For odd parity we set

the parity bit to 0 or I so as to make the total

number of ones in the word odd; similarly for evenparity. Thus 61e = 00001l0l odd parity or

3,o j oooot 100 even parity. Error correcting (as

opposed to detecting) codes exist but do not find use

in airborne equiPment as Yet.To consider a practical application qf the above -

Srppose a particular frequency is selected on a

control unit, we may have the following sequence of

events:

l. information from controller: 215 code',2. conversion from2l5 to binary;3. microcomputer processes binary data;

So far we have only discussed the coding of

numerical data. The ISO (lnternational Standards

Organisation) alphabet No. 5 is a seven-bit word code

which can be used to encode upper and lower case

letters, punctuation marks, decimal digits and various

other characters and control symbols' The full code

may be found in most of the latest ARINC

characteristics and will not be repeated here, however'

examples are A = I 0 0 0 0 0 I'

n = i O I 0 0 1 0 , e t c . A p a r i t y b i t m a y b e a d d e dto give a byte.

Wh"r. . limited number of actual words need to be

encoded, e.g. 'distance', 'speed', 'heading', etc' special

codes may be designated' Such codes are described in

AR INC spe cification 429'2 digital in fo rm ation

transfer system (DITS) which is discussed in

Chapter 13.

MicrocomputersThe microprocessor has brought powerful computers

on to aircraft to perform a number of functions,

including the solution of navigation equations, in a

more sophisticated way than before' A

microcomputer consists of a microprocessor and

several peripheral integrated circuits (chips), to help

the microprocessor perform its function'

There are four basic parts to computers, micro or

otherwise: memory, arithmetic logic unit (ALU)'

control unit and the input/output unit (l/O)' In a

microcomputer the ALU and control unit are usually

combined on a single chip, the microprocessor or

central processing unit (CPU)' Figure l '9 illustrates a

basic system.The memory contains both instructions and data

in the form of binary words' Memory is of two basic

types, ,ead only (ROM) and random access (RAM)'

The ROM does not remember any previous state

which may have existed; it merely defines a functional

relationshrp between its input lines and its output

lines. The RAM could be termed read and write

memory; since data can be both read from memory

urd written into memory, i.e. its state may change'

trnformation in RAM is usually lost when power is

switched off.The ALU contains the necessary circuitry to allow

it to carry out arithmetic operations, such as addition

and subtiaction, and logical functions such as Boolean

algebra operations (combinations of NANDs and

NORs etc.).

0I23456I

89AEcDEF

f':FA. t.9 Basic microcomputer organization

The control unit provides timing instructions andsynchronization for all other units. The controlsignals cause the other units to move data, manip_ulatenumbers, input and output information. All thisactivity depends on a set of step-by-step instructions(known as the program) which reside in memory.

The l/O unit is the computer's interface with theoutside world.

From Fig. 1.9 it can be seen that the units areinterconnected by three main buses. A bus is severalelectrical connections dedicated to a particular task.A unidirectional bus allows data flow in one directiononly, unlike a bidirectional bus where flow is two-way.ln a microcomputer we usually have:

l. address bus: sixteen unidirectional l irfes;2. data bus: eight or sixteen bidirectional l ines;3. control bus: the number of l ines varies with the

system and may have both unidirectional andbidirectional l ines.

. :To operate, each step-by-step instruction must be

fetched, in order, from memory and executed by theCPU. To keep track of the next step in the program,a program counter is used which incremelts each timean hstruction is fetched. Before an instruction canbe executed. it must be decoded in the CPU todetermine how it is to be accomplished.

On switch-on, the program counter is set to thefirst stored instruction. The address (location) of thisfirst instruction is placed on the address bus by thepro$am counter causing the instruction to be fetched

1 0

from memory, on the data bus, to the control unitwhere it is decoded. The program counterautomatically increments by one count, and after thecurrent instruction has been executed the nextinstruction is fetched. This basic cvcle of:

fetchdecodeincrementexecute

is repeated continuously. During the execution of aninstruction data may have to be fetched frommemory, for example to add two numbers theinstruction will need to tell the CPU not only that anaddition operation is necessary, but the location. inthe memory, of the numbers to be added.

The rate at which instructions are executeddepends on the complexity of the instrtrction and thefrequency of the systern clock. Each pulse from theclock init iates the next action of the system; severalactions per instruction are needed. Often the clockcircuit is on the CPU chip. the only externalcomponent being a crystal.

The I/O data flows via logical circuits called porrs.These ports may be opened in a similar way to that inwhich memory is addressed. In some systems the I/Oports are treated as if they were RAM - an addressopens a particular port and data flows in or out ofthat port depending on whether a read or write signalis present. A variety of chips are used for l/O, some

J

of whi_ch are very basic; others (programmable ports)more flexible.

The program which is resident in ROM issrbdivided into routines. Some routines will berunningcontinuously unless stopped; others may onlybe called for when the need ariseJ. For example, a

-

navigation computer will continuously compute theaircraft position by running the mainioufini (orbop) which instructs the ALU as to whichcalculations must be carried out using data availablein memory. This data must be updated periodicallyby accepting information from, siy, u ,"dionavigation sensor. When data is available from theextemal equipment, an interrupt sigral is generatedand fed to the microcomputer on an interiupt line.Such a signal causes the computer to abandon themain routine and commence a service routine whichwill supervise the transfer of the new data intorrrmory. After transfer the main routine willrccommence at the next step, remembered by a CpUregister.

The topics discussed in the paragraphs above canall be classified as hardware or softwaie. Thehardware is the sum total of actual components

T"king up the computer: chips, active and passivediscrete components, and interwiring. Softwarecomprises_programs, procedures and the languages orcodes used for internal and external commu-nication.Software determines the state of the hardware ar anyparticular time. In an airborne computer both thesoftware and hardware are fixed Uy itre designer.The operator does not have to program the computerin the sense that he must write a routine; however,he plays his part in how the computer will functionb1,, for example, selecting a switCh position whichwill cause certain data to be presenied to him by thecomputer, inserting atard (hardware), on whichcoded instructions or data (software) have been*Titten, into a card reader, etc.

Examples of the use of microcomputers arecoruidered in some of the chapters to follow. Theseapplications, and the above brief discussion. shouldgive the reader a basic idea on how computers work;for details of circuitry and programminj consult thereadily available specialist literiture.

CaGgorization of Airborne RadioEqripments

Frcqucy and Modulation

T* 9" techniques involved vary greatly with ther.f. and type of modulation used, itls ofien useful tocrtegonze equipment as to the band of frequencies in

which it operates (see Tables 1.2 and 1.3) and asbeing pulsed, a.m. or f.m. From both the desigr andmaintenance point of view, the frequency at whichequipment operates is perhaps more important thanthe modulation used, at least in so far ai the choice ofcomponents and test equipment is concerned.

The higher the frequency the greater the effect ofstray capacitance and inductance, sigral transit timeand skin effect in conductors. In thi microwaveregion (s.h.f. and the high end of u.h.f.) wavezuidereplaces co-axial cable, certainly above 5 GHz] andspecial components whose dimensions play a criticalpart in their operation are introduced (klystrons,magnetrons, etc.).

Analogue-DigitalThese terms have already been mentioned and certainaspects of digital systems have been discussed. Inmodern airborne systems the information in the radioand intermediate frequency stages, including the'wireless'

r.f. link, is usually in analogue form (theexception being secondary surveillan ce rcdar (seeChapter 8), to be joined in future by microwavegtail8 systems, data link and the replacement forSSR (see Chapter l3)). In addition Commonly usedtransducers such as synchros, potentiometers,microphones, telephones and speakers are all analoguedevices. Not all transducers are in the analogue

Tlegory, a shaft angle encoder used in encodingaltimeters is basically an analogue to digital converter.

. With the exception of the above almost everything

else in current equipment is digital, whereaspreviously systems were all analogue. There is afurther subdivision within digital iquipment intothose using a combination of hardware and software(computer-controlled) and those using only hardware(hardwired logic). The trend is towards thl former.

FunctionThe two basic categories with regard to function arecommunications and navigation. If navigation isdefined in its widest sense as safe, economical passagefrom A to B via selected points (waypoints) thencommunications systems eould be considered asbelonging to the navigation category. If, however,communications systems are regarded as thosesystems capable of transmitting speech over radio orwire links, and all other systems as navigation, we iueobeying a sensible convention. The introduction ofdata links will require some amendment to thedefinition of communications systems, since

, non-navigational data will be transmitted but not as aspeech pattern.

Navigation systems may be subdivided into radio

t 1

and non'radio, but only the radio systems concemls

;;;":'-A;;ih.i possible sub d iv i si on is p osi t i on- t txmg

ffi; ;;rfieiiht-nndrng, r'ndlnc, ui9: 111 "^.,.environment-monitoring' For the latter we nave

weather avoidance systems' while radio a]tim,etlrs

fi "* ; ;it -l.i-ghlri"olng

cate go rv and inst rument

landing systems belong to the landing-aids category;

itfr*t?i',vpes of these subdivisions of the category

lf nuuigution systems will be considered in Chapters

S. f f uiO 5 respectively' Position-fixing systems

I ^ "' u.'i" t,ft.i subtlivide d into self'co ntaine d and

ilffi;;,i"t-tutta' The former uses dead

( @ e r 4 ' ' l . j $ *

re{S 80 IIITEGRATED NAVIGATIONSYSTEM (VOR/DME/RNAV/ILS)

<-NAV & GS RECEIVER

Fh. l.l0 KNS 80 integrated navigation system

(JurtesY King Radio CorP')

12

\COI,IPUTER BOARD

reckoning to compute the aircraft's position whilethe latter uses a variety of methods: rho'theta,rho-rho, rho-rho-rho, theta-theta and hyperbolic.

The Greek letters p (rho) and 0 (theta) are used to

represent distance (range) and angle (bearing) to afixed point of known location. The pilot candetermine (fix) his position if he knows:

(a) p and 0 to one fixed Point:(b) p to three distinct fixed Points;(c) 0 to two distinct fixed Points.

A rho-rho system gives an ambiguous fix unless the

aircraft is at the midpoint of the line joining the two

stations to which the range is known. With

hyperbolic systems position-fixing is achieved by

measuring differences in range; ambiguity may be

avoided by various techniques (Chapter 6).One item of navigation equipment overlaps the

boundhries between the different methods of position-

fixing, namely Omega; this uses dead reckoning in

conjunction with rho-rho, rho-rho'rho or hyperbolicmethods. Two systems, VOR (Chapter 4) and DME(Chapter 7) are used together to gfue a rho-theta fix.

With miniaturization of circuitry, it is now possibleto house several systems, which were previouslyphysically separate, into one box' It is still possibleto categorize by function, but we must bear in mindthat the circuit implementation may be intimatelyconnected. Such an example is given by theKing KNS 80 integrated navigation system (Fig. I ' l0)which incorporates VOR, DME and ILS (Chapters 4,

7 and 5) as well as area navigation facilities (Chapter

l2). Other equipment may group together systemsoperating within the same band of frequencies suchas v.h.f. comm. and v.h.f. nav. (VOR and ILS).

Figure l.l I illustrates the navigation systems in use

on a Boeing 747 with a typical fit; different operators

may takeop different options. This diagram includesnon-radio (mainly in the top half) as well as radiosystems, and illustrates the interrelationshipsbetween them especially with regard to display(right-hand side). A similar diagram for thecommunications systems is included in Chapter 2(Fig. 2.15). The large number of radio navigationsystems, some duplicated or even triplicated forsafety, present the problem of where to position theantennas. The solution for the Boeing 747 is shownin Fig. 1.12.

Navigation Nomenclature

Figure I .13 and Table I .6 define the most commonly

used terms in aircraft navigation. All of the quantities

defined, with the exception of heading, can be found

using radio systems, or are input by the pilot at some

stage of the flight, usually prior to take off.

Interference

The e.m. environment of an aircraft radio system is

such that it may suffer from interfering signals-and/ornoise, man-made or natural, ind cause interferenceitself to other systems. Interference may be either

radiated or conducted.As the aircraft fliei through the atmosphere, it

picks up electrical charge due to frictional contact

with atmospheric particles (precipitation static) and

also while flying through cloud formations, within

which very strong electric fields exist (electrostatic

induction). An uneven distribution of charge will

cause currents to flow in the aircraft skin; possibly in

the form of a spark, between parts of unequalpotential. Any spark results in a wide band ofradiated r.f. which will be picked up by radio systemsas noise and possibly mask wanted sigrals. To avoid

this type ofinterference, a bonding system is used

comprising numerous metal strips which present very

low iesistance links between all parts of the aircraft.In addition to discharges within the aitcraft, a

discharge will occur to atmosphere if a sufficientlylarge difference in potential exists. The discharge

cannot be avoided, but in an attempt to keep the

activity as far from antennas as possible, static

dischaigers are fitted to the trailing edge of the

mainplane, tailplane and vertical stabilizer in order to

provide an easy path for it. By providing a number of

discharge points at each discharger the voltage is kept

low. The bonding system carries the large currents

involved to those parts of the airframe where the

static dischargers are fitted. Lightning conductors,such as on the inside surface of the non-conductingnose radome, and lightning dischargers connected to

the lead-in of wire antennas and some notchantennas, help conduct any strike to the bulk of the

airframe, so preventing damage to equipment. A wire

antenna will also have a high resistance path between

it and the airframe to allow leakage of any static

build-up on the antenna.Sparks occur in d.c. motors and generators, enginc

igrition systems, etc. Capacitors are used to provide

a low resistance r.f. path across brushes, commutatorsand contacts, a form of protection known as

nrppression.Another form ofinterference is capacitive and

inductive pick-up and cross-talk between adjacent

13

Wcathcr radarsystcm

Loranindicetorsinstrurnant

Radio rnagindic6tors

dircctor in.

lffic'_llbcrqr 3yst. I lbcacon ird.l

Fig. l.l I Boeing 747: typical navigation systems fit(courtesy Boeing Comntrcial Aeroplane Co.)

cables. Pick-up is the term used when the interferingsource is a.c. power (400 Hz in aircraft), whilecross-talk is interference from a nearbysignal-carrying cable. The problem arises out of thecapacitance and mutual inductance which edstsbetween the cables. A pair of wires may be twistedtogether to reduce both types ofinterference - thepick-up or cross-talk on adjacent loops, formed bythe twist, tending to cancel out. An earthed metallicscreen or shield will provide an effective reduction incapacitive interference but low-frequency inductive

14

pick-up is not appreciably affected by thenon-magnetic screen. At high frequencies skin effectconfines the magnetic fields of co-axial cables to theirinterior. Most signal-carrying cables are both screenedand twisted; some, where integrity is especiallyimportant, e.g. radio altimeter output, may havedouble screening.

The screen around a wire must be earthed in orderto be effective. However if both ends of the screenare earthed, an earth loop may be formed since thecomplete circuit through the screen, remote earth

Wirthor ndu

Locrlizrr No.

LocrlizcrNo. 1 end tlo. 2

Loft tnd right AFTnos. 9e8r door glidcslopo tracl end capturrantcnna sy3tams Ccntor .furolegcJ

aqulpment cantcrLow rmgc rrdio altimotrr

ADf loop No. 1

Fig. l.l2 Boeing747.. typical navigation systems aeriallocations (courtesy Boeing Commercial Aeroplane Co.)

points and the airframe is of non-zero resistance. As aconsequence, interfering sources may cause apotential difference to edst between the ends of thescreen. The resulting current flow and its associatedH field would cause interference in the innerconductor. Earth loops are a particular problem inaudio systems and must be avoided.

The earth points for screened cables and a.c. powermust be remote from one another. If a screen were tobe connected directly to an a.c. power earth,conducted mains interference may result. Anotherform of conducted interference is cross-talk where a

l{rrtrrbatcon

ADF srnse

ADF rns.antomat{o. 1

entcnna No. 2

number of signal carryiqg wires are brought together,e.g. audio signals being fed to an interphone amplifier.Suitably designed potential divider networks keepthis conducted cross-talk to a minimum (Chapter 2).

Adequate separation of antennas operating withinthe same frequency band is necessary to preventmutual interference by radiation. Frequency andtime domain filtering may be used in helping to avoidsuch interference, the former in c.w. systems, thelatter in pulsed systems. Different polarization (Efield direction) will assist in preventing cross-couplingbetween antennas.

ADF loopllo. 2

V O R N o . 1VOR No. 2

1 5

,/WPT O

(

\Mnd dircctiondnd velocatY

On tnck

\Mnd dircctionand velocity

Fit; l.l3 Navigation nomenclature (courtesy Litton Systems

lnternational lnc., Aero hoducts Division)

16

HDG

TK

Table 1.6 Navigation nomenclature - abbreviations

Abbreviation Meaning

electrical equipment will interfere with the magneticcompass. Units are marked with their 'compass safedistance' as appropriate but care should also be takenwith cables, particularly for d.c. power.

Maintenance

This is not the place to go into great detail on thisimportant practical topic but some notes of a generalnature are in order to supplement the notes includedin most of the following chapters. In practice themaintenance engineer relies on his training andexperience and also regulations, schedules andprocedures laid down or approved by national bodiesresponsible for aviation in general and safety inparticular.

The aircraft maintenance enginber, of whateverspecialism, is responsible for regular inspections ofequipment as laid down in the aircraft schedule. Forthe radio engineer an inspection will consist of athorough examination of all equipment comprisingthe radio installation for cracks, dents, chafing, dirt,oil, grease, moisture, buming, arcing, brittleness,breakage, corrosion, mechanical bonding, freedom ofmovement, spring tension, etc" as applicable. Incarrying out specific tasks the engineer should lookfor damage to parts of the airframe or its equipmentwhich are not directly his or her responsibility.Vigilance is the key to flight safety.

Carrying out functional tests when called for in th0schedule, or when a fault has been reported, shouldbe done in accordance with the procedure laid downin the aircraft maintenance manual. A word ofwaming should be given here, since procedures are notalways what they should be: often testing of certainaspects of a system's function are omitted and,rarely, there m.ay be errors in the procedure.A thorough knbwledge of the system is the bestguard against mistakes or omissions which, if noticed,strould be amended through the proper channels.

Modern equipment usually has sufficient built-intest equipment (BITE) and.monitoring circuits tocarry out a comprehensive check of the system. Withradio systems, however, special portable testequipment must be used in addition to BITE in orderto be in a position to certify the system as serviceable.Test sets should be capable of testing by radiation andof simulating the appropriate sigrals to test allfunctions not covered by BITE.

The r.f. circuits, including antennas and feeders,are often neglected when functional tests are carriedout. In particular test set antennas should becorrectly positioned if a false impression of the

DTK

Heading - angle, measured clockwisebetween North and the direction in whichthe aircraft is pointing,Track - direction in which the aircraft ismoving.Desired'Track - direction in which the pilotwishes the aircraft to move.Drift Angle - angle between hcading andtrack measured to port (left) or starboard(right).Track Angle Error - angle bctween trackand desired track, usually quoted as left orrght.Ground Speed - spded of the aircraft in thedirection of the track.in the'plane' parallelto the earth's surface (map speed). .Compare with air speed which is the speedof the aircraft relative to the air massthrough which it is moving.Position.Waypoint - a significant point on the routewhich may t c used for reporting to AirTraffic Control, turning or landing.Distance to go from position to waypoint,Cross Track - the perpendicular distancefrom the aircraft to the line joining the twowaypoints between which the aircraft isflying.Estimated time of arrivalETA

Adjacent channel interference occurs when areceiver's bandwidth is not sufficiently narrow toattenuate unwanted sigrals close to the requiredsigtal. Second or image channel interference mayoccur in superhet receivers when an unwanted sigral

{ separated froin the required sigrral by twice theintermediate frequency and lies on the opposite sideof the local oscillator frequency. A high intermediatefrequency will help reduce second channelinterference since the image will be outside the r.f.bandwidth, the separation being greater.Unfortunately for a given Q factor, the bandwidth ofthe intermediate frequency amplifiers will be wide forthis solution to the second channel problem.Increasing channel separation is not really acceptablesince demand for more channels is forever rising.Some receivers employ two intermediate frequenciesproduced by two mixer stages and two localoscillators; this can give good adjacent and seconddrannel rejection.

Magrretic fields associated with electronic and

DA

TAE

GS

POSWPT

DISXTK

receiver sensitivity or power output is to be avoided.When fault-firtdhg a complete functional test

should be carried out as far as posible in order toobtain a full list of symptoms. Naturally symptomssuch as a smell of burning or no supply must call ahalt to the procedure. Fault-finding charts inmaintenance manuals are useful but there is nosubstitute for knowledge of the system.

One should not forget the possible effects ofnon-radio systems and equipment when investigatingreported defects. Poor bonding, broken staticdischargers, open circuit suppression capacitors, lowor inadequately filtered d.c. supplies, low voltage or

lincorrect frequency a.c. supplies, etc. will all give rise\o symptoms which will be reported by the pilot asradio defects.

Sometimes symptoms are only present when theaircraft is airborne and the system is subject tovibration, pressure and temperature changes, etc.A functional test during engine runs will gc part wayto reproducing the conditions of flight.

One should mention the obvious hazard of loosearticles; so obvious that many aircraft accidents havebeen caused in the past by carelessness. Tools andtest equipment, including leads, must all be accountedfor when a job is finished. A well-run store withsigning-in and signing-out of equipment is an addedsafeguard to personal responsibility.

Installation of equipment should be in accordancewith the manufacturer's instructions which will coverthe following,

-

l. Weight of units: centre of gravity may beaffected.

2. Current drawn: loading of supplies should becarefully considered and the correct choice ofcircuit-breaker made.

3. Cooling: more than adequate clearance shouldbe left and forced air-cooling employed ifappropriate. Overheating is a major cause offailure.

4. Mounting: anti-vibration mounts may benecessary which, if non-metallic, give rise to aneed for bonding straps.

5. Cables: length and type specified. Usuallymaximum length must be observed but in some

. cases particular lengths are necessairy. Types of' cable used must provide protection againstinterference and be ablerohandle currentdrawn or supplied. Current capabilties arereduced for cables in bunches.

6. Antenna: approved positions for particulartypes of antenna or particular types of aircraftare laid down by aviation authorities.

t 8

Strengthening of the structure around theantenna in the form of a doubler plate wil lprobably be necessary. A ground plane isessential and must not be forgotten if theantenna is to be mounted on a non-conductingsurface. If the antenna is movable, adequateclearance should be left. Any alignmentrequirements must be met.

7. Interface: qompatibility with other systems/units must be ensured. Both impedancematching (including allowing for capacitive andinductive effects) and signal characteristicsshould be considered. Loading of outputsshould be within limits. Particular care shouldbe taken in deciding where synchro devicesobtain their reference supplies. Programmingpins for choice of outputs and/or inputs mustbe correctly connected.

8. Compass: safe distance.9. Radiation hazards.

The last item in the above non-exhaustive list raisesthe topic of safety. Electric shock is an obvioushazard w'ren working on aircraft and it should beremembered that one is liable to receive a shock fromradiating antennas, particularly h.f. antennas.A radiation hazard exists with all transmittingantennas, thus the operator should ensure no-one isworking, particularly doping or painting, near anantenna when the associated transmitter is on. Theparticular hazards of microwave radiation areconsidered in Chapter 9. It is up to all personnelworking on aircraft to become aware of the dangersof harmful substances, the use (and position) of fireextinguishers, the dangers of mixing oil or grease withoxygen, elementary first aid, warning symbols, etc.

Regulating and Advisory Bodaes

All countries set up bodies which are responsible formatters concerned with aviation e.g. CAA (UK),FAA (USA), Bureau Veritas (France), etc. Thesebodies draft air law and issue regulations concernedwith the hcensing of engineers and aircrew, aircraftoperations, aircraft and equipment manufacture,minimum equipment fits (including radio), air trafficcontrol, etc. They are also the bodies charged withseeing, by means of examinations and inspections,that the law is obeyed.

Aviation is an international activity andco-operation between countries is essential. Thisco-operation is achieved mainly through the ICAO,an agency affiliated to the United Nations. All

ARINCATAAEECCAACAPFAAlcAoIFRBCCIRITUTSOWARC

nations which are signatories to the ChicagoConvention on Civil Aviation 1944 are member-sratesof the ICAO which was an outgrowth of thatconvention.

Table 1.7 Organizations, orders and conferenceconcerned with aircraft radio systems

Abbreviation Orsanization

radio, the Chicago convration provides that aircraftregistered in contracting states may carry radiotransmitting apparatus only if a licence to install andoperate such apparatus has been issued by theappropriate authorities of the state in which theaircraft is registered. Furthermore, radio transmittingapparatus may only be used over the territory ofcontracting states, other than the one in which theaircraft is registered, by suitably licensed flight crew.

Various non-regulatory bodies exist with a view toextending co-operation across national boundaries inrespect to aircraft equipment and maintenance.ARINC is one such organization. It is a corporationthe stockholders of which are drawn from airlines andmanufacturers, mostly from the USA. As well asoperating a system of aeronautical land radio stationsARINC sponsors the AEEC, which formulatesstandards for electronic equipment and systemsdesigned for use in airliners as opposed to generalaviation. Characteristics and specifications publishedby ARINC do not have the force of law butnevertheless are, in the main, adhered to bymanufacturers who wish to sell their equipment tothe airlines.

A specification relating to the presentation ofmaintenance information is the ATA 100. A standardIayout for technical publications relating to aircrafthas been promulgated and widely adopted. Ofparticular interest to readers are Chapters 23 and34of the maintenance manual which covercommunications and navigation respectively. Inaddition to prescribing layout, a set ofstandardsymbols for electrical wiring diagrams has been issued.

So far, bodies concerned with aircraft and theirequipment have been considered; in additionorganizations concerned with telecommunicationsstrould be mentioned. The ITU is an agency of theUnited Nations which exists to encourageinternational co-operation in the use and developmentof telecommunications. The CCIR is a committee setup by the ITU to deal with radio communications.Among topics of interest to the CCIR are spectrumutilization and aeronautical mobile services. TheIFRB has also been set up.by the ITU for theassignment and registration of radio frequencies ina master frequency list. In November 1979 aninternational conference (WARC '79), withrepresentatives from 154 countries, met in Geneva toconsider radio regulations and re-allocatefrequencies. The results of WARC '79 will not bepublished while this book is being written but it isunlikely that the frequencies allocated to aeronauticalmobile services will suffer significant amendment -the cost would be too great.

Aeronautical Radio lnc.Air Transport AssociationAirlines Electronic Engineering CommitteeCivil Aviation AuthorityCivil Aviation PublicationFederal Aviation AgencyInternational Civil Aviation OrganizationInternational Frequency Registration BoardInternational Radio Consultative CommitteeInternational Telecommunications UnionTechnical Standard OrderWorld Administrative Radio Conference

The ICAO issues annexes to the convention.Annex l0 being of particular significance to aircraftradio engineers since it is concerned with aeronauticaltelecommunications and, among other things, laysdown a minimum specification for airborne radiosystems. The material published by the ICAO doesnot automatically become the law or regulations in allmember-states; ratification is necessary and may nottake place without considerable amendment, if at all.In particular, system specifications emergd in formsconsiderably different from Annex 10, although similarin content. In the USA specifications are issued in theform of TSOs while in the UK there is CAP 208.Volume I with its companion Volume 2 listingapproved equipment under various classifications.

Licensing of engineers is one area in which, as yet,there is little international standardization inaccordance with Annex l. The licensed aircraft radiomaintenance engipeer is unknown in the USA but,of course, organizations operating with the approvalof the FAA do so only if they employ suitablyqualified personnel. In the UK the [censed engineerreigns supreme, except in the certification ofwide-bodied jets and supersonic transports where asystem of company approval of personnel exists, thecompany itself being approved by the CAA for theoperation and maintenance of such aircraft. Francehas no system of state licensing, it being left to theoperators to assess the compettncy of its maintenancepersonnel under the watchful eye of officials.

Of further interest to those concerned with aircraft

h

19

2 Com munication sYstems

lntrodoction

There is a fundamental need for communicationbeiween aircrew and ground controllers, among the

aircrew and between aircrew and passengers. External

communication is achieved by means ofradio-telephone (R/T) link while internalcommunication'(intercom or audio integrating system)

is by wire as opposed to wireless. Although intercom'

is not a radio system, it is included in this chapterbecause of its intimate relationship with the aircraftradio systems. Voice recorders and in-flightentertainment systems are also considered since theyare usually the responsibility ofthe aircraft radiotechnician/engineer-

The first items of radio equipment to appear on

aircraft were low-frequency (l'f.) communicationssets in the World War I days of spark gap transmitters.lntercom was by means of a Gosport (speaking) tube'

By the 1930s the early keyed continuous wave (c'w')(radio-telegraphy) was beginning to be replaced by

R/T although 'key-bashing'had its place as long as

aircraft carried radio operators. Early R/T was within

the l.f. and h.f. bands, the sets operating on only one

or very few frequencies. With airfields widely spaced

and low-powered transmission, there was littleinterference and so the need for many channels did

not arise.The situation has drastically changed since World

War II; air traffic and facilities have increased with the

consequent demand for extra channels which cannot

be provided in the Lf., m.f. or h.f. bands.Fortunately v.h.f. equipment has been successfully

- developed from early beginnings in World Wu II

figtrter control.The current situation is the v.h.f. is used for

*tort-range communication while h'f. is used for

long-range. A large airliner, such as aBoeing74T ,carries three v.h.f.s and dual h'f. In addition, in such

aircraft, selective calling (Selcal) facilities areprovided by a dual installation such that a ground

station can call aircraft either singly or in groups

without the need for constant monitoring by the

crew. Provision for satellite communication (Satcom)

m

on v.h.f. lrequencies is often found; unfortunately

aeronauticaliommunications satellites are not to be

found (1979).The audio integrating system (AIS) complexity

depends on the type of aircraft. A light aircraft

,yrt.* may provide two transmit/receive channels for

dual u.h.f. iomms and receive only for dual v'h'f'

nav., ADF, DME and marker. Each receive channel

has a speakeroff'phone switch while the microphone

can be switched bitween v'h.f. comms I and v'h'f'

comms 2. A multi-crew large airliner has very many

more facilities, as described later.

V.H.F. Gommunications

Basic PrinciplesAn aircraft u.h.f. comrns transceiver is comprised of

either a single or double conversion superhet receiver

and an a.m. transmitter' A modern set provides 720

channels at 25 kHrz spacing between I l8 MHz and

135'975 MHz; until recently the spacing was 50 kHz

giving only 360 channels. The mode of operation is

iinglJ.itutntl simplex (s,c.s.), i.e. one frequency and

onJ antenna for both receiver and transmitter' If

provision for satellite communication is included in

iccordance with ARINC 566 then in addition to

a.m. s.c.s. we will have f.m' double channel simplex(d.c.s.), i.e. different frequencies for transmit and

re ceive..Communication by v.h.f. is essentially

'line of

sight'by direct (space) wave. The range available can

be approximated by I '23 (\/.\ + y'ft1)nm where ftt is

*re trilght, in feet, above sea level of the receiver

while ftl.is the same for the transmitter' Thus, with

the ground station at sea level, the approximate

,nr*I*urn range for aircraft at l0 000 and 1000 ft

(30 000 and 3000 m) would be 123 and 40 nm

respectivelY.

' trstallationA single v.h.f. installation consists of three parts'

namJy control unit, transceiver and antenna' In

addition crew phones are connected to the v'h'f' via

Frg. 2.1 KY 196 v.h.f. comm. transceiver (courtesy KingRadio Corp.)

Fi& a2 CN-201I v.h.f. comm./nav. equipment(courtesy Bendix Avionics Division)

selection switches in the AIS. Light aircraft v.h.f.susually have a panel-mounted combined transceiverand control unit, an example being the King KY 196illustrated in Fig. 2.1. The current trend is forcombined COM/NAV/RNAV; Fig. 2.2 illustrates theBendix CN-2011, a general aviation panel-mountedunit comprising two comms transceivers, two nav.receivers, glidepath receiver, marker receiver,

frequency control for internal circuits and d.m.e. andlast but not least, audio selection switches. Suchequipment will be considered in Chapter 12.

Figure 2.3 shows one of a triple v.h.f. commsinstallation as might be fitted to a large passengertransport aircraft: VHF2 and VHF3 are similar toVHFI but are supplied from a different 28 V d.c. busbar and feed different selection switches in the AIS.

21

ATE

liitool

ov.h.f. COMM

cO rcool@

Freq.

FWDMTR PWR REFoFF-\ t fPwa

\ t . /r\l \ I\_./

SOUELCH DISABLE(o)

PHONE O O MIC.

Ars

M i c

P.t.t.

Rcv Audio

Sidetone

foAerial

v.h.f. No. 2 v.h.f No.

Fig. 2.3 Typical v.h.f.l installation

The transceiver. which is rack-mounted, containsall the electronic circuitry and has provision for themaintenance technician to connect mic. and telsdirect, disable the squelch, and measure VSWR.These provisions for testing are by no means universalbut if the system conforms to ARINC 566 a plug isprovided to which automatic test equipment (ATE)can be connected. A protective cover for the ATEplug is fitted when the unit is not in the workshop.

The antenna can take various forms: whip, bladeor suppressed. In a triple vl.f. comms installationthese may be two top-mounted blade antennas andone bottom-mounted: an altemative would be twoblade and one suppressed within the fincap dielectric.The whip antenna is to be found on smaller aircraft.All antennas are mounted so as to receive andtransmit vertically polarized waves.

The blade antenna may be quite cornplex. It willbe self-resonant near the centre ofthe band withbandwidth improvement provided by a short'circuitedstub across the feed terminal or a more complicatedreactive network built in which will permit height andhence drag reduction.

Controls and OperationIt is common to have in-use and standby frequenciesavailable, the former controlling the transceiverfrequency. This is the situation in Fig.2.3 where wehave two sets of frequency controls and two displays,the in-use one being selected by the transfer switchand annunciated by a lamp above the display.

z2

RcvAudioTo Selcal

Frequency control is achieved by concentric knobs,the outer one of which varies the tens and units whilethe inner pne varies the tenths and hundredths. Analternative is shown in Fig. 2.1 where there is onefrequency control and two displays' On rotating thefrequency knobs clockwise or anticlockwise, thestandby frequency only will increment or decrementrespectively. Standby may then become in-use byoperation of the transfer switch. There are manycontrollers in service with only in-use selection.

Some or all of the following switches/controls maybe provided by manufacturers on request.

Volume Control A potentiometer, which allowsvariable attenuation of audio, prior to feeding the AISmay be fitted as a separate control or as a concentricknob on the frequency selector(s). Such a volumecontrol may have sidetone coupled through it ontransmit.

$uelch Control A squelch circuit disables thereceiver output when no sigrals are being received sopreventing noise being fed to the crew headsetsbetween ground transmissions. The squelch control is

a potentiometer which allows the pilot to set the levelat which the squelch opens, so allowing audio outputfrom the receiver. When the control is set tominimum squelch (fully clockwise) the Hi and l,osquelch-disable leads, brought to the control unitfrom the transceiver, should be shorted, so giving adefinite squelchdisable.

28 V d.c.StbyBus

Mode Selector Control Provides selection of normala.m., extended range a.m. or Satcom. If the Satcomantenna has switchable lobes such switching may beincluded in the mode switch, or could be separate.

On-Off Switch Energizes master power relay intransceiver. The switch may be separate, incorporatedin mode selector switch as an extra switch position, organged with the volume or squelch control.

Receiver Selectivity Switch Normal or sharpselectivity. When Satcom is selected sharp selectivityautomatically applies.

Block Diagram Operation (KY 196)Ftgure 2.4 is a simplified block diagram of the KingKY 196 panel-mounted v.h.f. comm. transceiver.This equipment, intended for the general aviationmarket, is not typical of in-service transceivers sincefrequency and display control is achieved with the aidof a microprocessor;however within the lifetime ofthis book such implementation will becomecommonplace.

Receiver The riceiver is a single conversion superhet.The r.f. stage employs varactor diode tuning, utilizingthe tuning voltage from the stabilized masteroscillator (s.m.o.). Both the r.f. amplifier and mixerare dual gate field-effect transistors (f.e.t.). The r.f.amplifier f.e.t. has the input signal applied to gate Iwhile the a.g.c. voltage is applied to gate 2. Themixer connections are: gate l, signal;gate 2, s.m.o.The difference frequency from the mixer, I l'4 MHz,is passed by a crystal filter, providing the desirednarrow bandpass, to the i.f. amplifiers. Two stages ofa.g.c.-controlled i.f. amplification are used; the first ofwhich is a linear integrated circuit.

The detector and squelch gate utilize transistors onan integrated circuit transistor array. A further arrayis used for the squelch-control circuitry. Noise at8 kHz from the detector output is sampled and usedto close the squelch gate if its amplitude is asexpected from the receiver operating at full gain.When a sigral is received, the noise output from thedetector decreases due to the a.g.c. action; as aconsequence the squelch gate opens allowing theaudio signal to pass. The squelch cap be disabled by

t

t;:I

F

FF

F

Frequency display

P.t.t.

' _ _ - - _ _ - - . 1

MHz Code kHz Codc

lncremenVDecrement

tFA(FF ,

118.70 121.90Use 1 Standby

Fig. 2.4 King KY 196 simplified block diagram

a

R.F.Input

TuningVolts{s.m.o.}

Fig. 2.5 King KY 196 simplified receiver block diagram

means of a switch incorporated in the volumecontrol. When the received sigral has excessive noiseon the carrier, the noise-operated squelch would keepthe squelch gate closed were it not forcarrier-operated or backup squelch. As the carrierlevel increases, a point is reached where the squelchgate is opened regardless of the noise level.

The mean detector output voltage is used todetermine the i.f. a.g.c. voltage. As the i.f. a.g.c.voltage exceeds a set reference the r.f. a.g.c. voltagedecreases.

The detected audio is fed via the squelch gate,low-pass lilter, volume control and audio amplifier tothe rear panel connector. A minimum of 100 mW

-audio power into a 500 O load is provided.

Tiansmitter The transmitter (Fig. 2.6) feeds l6 W ofa.m. r.f. to the antenna. Modulation is achieved bvsuperimposing the amplified mic. audio on thetransmitter chain supply. The carrier frequencycorresponds to the in-use display.

Radio frequency is fed from the s.m;o. to an r.f.amplifier. This input drive is switched by thetransmit receive switching circuits, the drive beingeffectively shorted to earth when the press totransmit (p.t.t.) button is not depressed. The

24

transmitter chain comprises a pre-driver, driver andfinal stage all broad band tuned, operated in Class Cand with modulated collectors. The a.m. r.f. is fedviaa low-pass filter, which attenuates harmonics, to theantenna. On receive the t.r. diode is forward biasedto feed the received signal from the antenna throughthe low-pass filter to the receive r.f. amplifier.

The modulator chain comprises microphonepre-amplifier, diode limiting, an f.e.t. switching stage,integrated circuit modulator driver and two modulatortransistors connected in parallel. The pre-amp outputis sufficient to subsequently give at least 85 per centmodulation, the limiter preventing the depth ofmodulation exceeding 100 per cent. The mic. audioline is broken by Jhe f.e.t. switch during receive.

Stabilized Master Oscillator The s.m.o. is aconventional phase locked loop with the codes for theprogrammable divider being generated by amicroprocessor. Discrete components are used forthe voltage controlled oscillator (v.c.o.) and bufferswhile integrated circuits (i.c.) are used elsewhere.

The reference sigrral of 25 kHz is provided by anoscillator divider i.c. which utilizes a 3'2 MHz crystalto give the necessary stability. Only seven stages of afourteen-stage ripple-carry binary counter are used to

R.F.{s.m.o.}

P.t.t.

Fig. 2.6 King KY 196 simplified transmitter block diagram

Phasedetector v.c.o.

MHz cont. MHz cont.lrom pp lrom gp

Fig. 2.7 King Ky 196 simplified programmabte dividerbkrck diagram

gtve the necessary division of 21 = l2g. This reference,

l9e9lner with the outpur of the programmabledivider, is fed to the phase detecior-which is part ofan i.c., the rest of which is unused. The pulsating d.c.on the output of the phase detector tras a a.O.component which after filtering is used to control thefrequency of the v.c.o. by varactor tuning. If there isa synthesizer malfunction, an out-ofloclisignal fromthe phase detector is used to switch off the

"s.m.o.

feed to the transmitter.

. The programmable divider consists basically ofthrce sets of counters as shown in Fig. 2.7 . Tie!r1f!rea- v.c.o. output is first dividedUy "ittrei+O o,41, the former being so when a discreti MHz selection

is made; i.e. zeros after the displayed decimal point.The prescaler which perlbrms this division is a u.h.f.programmable divider (+ l0/l l) followed by adivide-by-four i.c. The whole MHz divicler uses a74LSl62 b.c.d. decade counter and a 74LSl63 b inarycounter which together can be programmed to divideby an integer between I l8 and 145, hence theprescaler and whole MHz divider give a total divisiono,t^llSO (40 X I l8) to 5800 (40 X 145) in steps of40. Thus a required v.c.o. output of, say, t IO.OO UHzwould be achieved with a division of 5200 (40 X 130)since 130 MHz + 5200 = 25 kHz = referencefrequency.

The 25 kHz steps are obtained by forcing the

25

prescaler to divide by 41, the required number oftimes in the count sequence. Each time the divisionratio is 41, one extra cycle of the v.c.o. frequency isneeded to achieve an output of 25 kHz from theprogrammable divider. To see that this is so, considerthe previous example where we had a division ratio of5200 to give 130'00 MHz, i.e. 5200 cycles at130'00 MHz occupies 40 ps = period of 25 kHz.Now a prescaler division ratio of 4l once during 40 psmeans 5201 cycles of the v.c.o. output occupy 40 psso the frequency is 5201/(40 X 10-6) = 130.025 MHzas required. The prescaler ratio is controlled by thefractional MHz divider, again employ ing a 7 4LS | 62and 74LS163. The number of divide-by-41 events in40 ps is determined by the kHz control code from themicroprocessor and can be anywhere from 0 to 39times. Therefore each whole megacycle can haveN X 25 kHz added where N ranges from 0 to 39. Thisproduces 25 kHz steps from 0 kHz to 975 kHz.

Microprocessor and Display The microprocessorused, an 8048, contains sufficient memory for theprogram and data required in this application to bestored on the chip. In addition to this memory and,of course, an eight-bit c.p.u., we have an eight-bittimer/counter and a clock on board. Throughtwenty-seven I/O lines the 8048 interfaces with theprogrammable divider, display drive circuits andnon-volatile memory.

Fig. 2.8 8048 eieht-bit microcornputcr (courtery KingRadio Corp.)

216

The 8048 has been programmed to generate abinary code for the 'use' and 'standby' frequencies.The code, as well as being stored in the 8048, is alsostored in a l40o-bit electrically alterable read onlymemory (EAROM). This external memory iseffectively a non-volatile RAM, the data and addressbeing communicated in serial form via a one-pinbidirectional bus, the read/write/erase mode beingcontrolled by a three-bit code. When power isapplied the microprocessor reads the last frequenciesstored in the EAROM which are then utilized as theinitial 'use'and 'standby' frequencies. In the event offailure of the EAROM the microprocessor will display120'00 MHz as its initial frequencies. The EAROMwill store data for an indefinite period without power.

The 'standby'frequency is changed by clockwiseor counterclockwise detent rotation of the frequencyselect knobs. I MHz, 50 kHz and 25 kHz changes canbe made with two knobs, one of which incorporates apush-pull switch for 50125 kHz step changes. Themicroprocessor is programmed to increment ordecrement the 'standby'frequency by the appropriatestep whenever it senses the operation of one of thefrequency-select knobs.

The code for the frequency in use is fed to theprogrammable dividers from the microprocessor.'Use' and 'standby' frequencies are exchanged onoperation of the momentary transfer switch. Whenthe transceiver is in the receive mode themicroprocessor adds I l'4 MHz to the 'us€' frcquencycode since the local oscillator signal fed to thereceiver mixer should be this amount higher than thedesired received carrier in order to give a differencefrequency equal to the i.f.

Both luse'and 'standby'codes are fed to thedisplay drivers. The'use'code represents thetransmit frequency and is not increased by I l'4 MHzin the receive mode. Each digit is fed in tum to thecathode decoder/driver, an i.c. containing a seven-segment decoder, decimal point and comma drivesand programmable current sinks. The decimal pointand comma outputs (i and h) are used to drive thesegments displaying'l ' , ' . 'and 'T' (see Fig. 2.10).The 'T'is illumfrrated when in the transmit mode.

The display is a gas discharge type with itsintensity controlled by a photocell located in thedisplay window. As the light reaching the photocelldecreases the current being supplied to theprogramming pin of the cathode decoder/driver fromthe display dimmer circuit decreases, so dimming thedisplay.

Time multiplexing of the display dri'res is achievedby a clock sipal being fed from the microprocessor to

Clock 1024 wordsprogrammemory

64 wordsdatamemory

__JI___J'l(

8-bi rCPU

8-bitTimer/evont counter

27I /g l ines

qC2

C3

Fig. 2.9 Electrically alterable read only memory, e.a.r.o.m.(courtesy King Radio Corp.)

f lh l

t l ' l o.Ll.

d

o hT h

llrito?yarrayIOO x 14

TCnstddf

ess

LSB

1O-bits

Display

A1 A2 A3 A4 A5 A6 A7 A8

&'-AA lf" ",,Anode'driver

Cathodedecoder/driver

A B C D1f I l T I TMultiplexer B.C.D.

codeDimmingcurrent

Clocft

A7A1- Anode

drive

t-lt:l

llU

t_-tU .

l - l l - l -l - t l l

l-ll-1.

I lr_- l

B.C.D.code

l/1lo sec.

Fig.2.l0 King KY 195 simplified display drive blockdiagram

n

a r u l o f J u r r s r / r r r u u P

(Al to A8) are switched scquentially. As the anodedrives are switched the appropriate b.c.d. informationfrom the microprocessor is being decoded by thecathode decodery'driver, the result being that thenecessary segments of each digit are lighted one digitat a time at approximately I l0 times per second.A synchronization pulse is sent to the multiplexerfrom the microprocessor every 8 cycles to maintaindisplay synchronization.

Characteristics

The selected characteristics which follow are drawnfrom ARINC Characteristic 566 covering airbomev.h.f. communications and Satcom Mark l. Detailsof Satcom and extended range a.m. are not included.

System Unitsl. V.h.f. transceiver;2. modulation adaptor/modem - f.m. provision

for Satcom;3. power'amplifier - Satcom and extended range;4. pre-amplifier - Satcom and extended range;5. control panel;6. remote frequency readout indicator - optional;7. antennas - separate Satcom antenna.

Note: I and 2 may be incorporated in one linereplaceable unit (l.r.u.).

Frequenry Selection720 channels from I l8 through 135.975 MHz,25 kHz spacing.Receiver muting and p.t.t. de-energization duringchannelling.2i 5 channel selection.Channelling time: ( 60ms.

Recciver

Sensitivity3 pV, 30 per cent modulation at 1000 Hz to giveS + N / N > 6 d 8 .

SelectivityMinimum 6 dB points at I l5 kHz (t 8 kHz sharp).Maximum 60 dB points at I 31.5 kHz (t l5 kHzsharp).Maximum 100 dB points at i 40 kHz (t l8'5 kHz

-sharp).

Qoss ModulationWith simultaneous receiver input of 30 per cent

2A

dcsired signal, ttrc resultant audio output shall notexceed -10 dB with reference to the output prcducedby a desired sigral only when modulated 30 per cent(under specified sigtal level/off resonance conditions).

Undesired ResponsesAll spurious responses in band 108-135 MHz shall bedown at least 100 dB otherwise, including image,at least 80 dB down.

Audio Output

GainA 3 pV a.m. sigral with 30 per cent modulation at1000 Hz will produce 100 mW in a 200-500 Q load.

Frequmcy ResponseAudio power output level shall not vary more than6 dB over frequency range 300-2500 Hz.Frequencies > 5750 Hz must be attenuated by atleast 20 dB.

Harmonic Distortion[.ess than 7'5 per cent with 30 per cent modulation.Irss than 2O per cent with 90 per cent modulation.

AGCNo more than 3 dB variation with input signals from5 gV to 100 mV.

Transmitter

StabilityCarrier frequency within t 0'005 per cent underprescribed conditions.

Power Output25-40 W into a 52 O load at the end of a 5 fttransmission line.

SidetoneWith 90 per cent a.m. at 1000 Hz the sidetone outputstrall be at least 100 mW into either a 200 or 500 Oload.

Mic. InputMic. audio input circuit to have an impedance of150 O for use with a carbon mic. or a transistor mic.operating from the (approx.) 20 V d.c. carbon mic.supply.

AntennaVertically polarized and omnidirectional.

,J'i6,$

$

To match 52 O with VSWR ( l'5 : l.

Ramp TestingAfter checking for condition and assembly andmaking available the appropriate power supplies thefollowing (typical)'checks should be made at eachstation using each v.h.f.

l. Disable squelch, check background noise andoperation of volume control.

2. On an unused channel rotate squelch controluntil squelch just closes (no noise). Press p.t.t.button, speak into mic. and check sidetone.

- 3. Establish two-way communication with aremote station using both sets of frequencycontrol knobs, in conjunction with transferswitch, if appropriate. Check strength andquality of signal.

NB . Do not transmit on I 2l '5 MHz (Emergency).Do not transmit if refuelling in progress.Do not interrupt ATC-aircraft communications.

H.F. Communicataons

Basic PrinciplesThe use of h.f. (2-30 MHz) carriers for communicationpurposes greatly extends the range at which aircrewcan establish contact with Aeronautical MobileService stations. This being so, we find that h.f.comm. systems are fitted to aircraft flying routeswhich are, for some part of the flight, out of range ofv}t.f. service. Such aircraft obviously include publictransport aircraft flying intercontinental routes, butthere is also a market for general aviation aircraft.

The long range is achieved by use of sky waveswhich are refracted by the ionosphere to such anextent that they are bent sufficiently to return toearth. The h.f. ground wave suffers quite rapidattenuation with distance from the transmitter.Ionospheric attenuation also takes place, beinggreatest at the lower h.f. frequencies. A significantfeature of long-range h.f. transmission is that it iszubject to selective fading over narrow bandwidths(tens of cycles).

The type of modulation used, and associateddetails such as channel spacing and frequencychannelling increments, have been the subject ofmany papers and orders from users, both civil andmilitary, and regulating bodies. ARINC CharacteristicNo. 559A makes interesting reading, in that it revealshow conflicting proposals from various authorities(in both the legal and expert opinion sense) can existat the same time.

The current and future norm is to use singlesideband (s.s.b.) mode of operation for h.f.communications, although sets in service may haveprovision for compatible or normal a.m., i.e. carrierand one or two sidebands being transmittedrespectively. This s.s.b. transmission and receptionhas been described briefly in Chapter I andextensively in many textbooks. A feature ofaircrafth.f. systems is that coverage of a wide band of r.f. anduse of a resonant antenna requires efficient antennatuning arrangements which must operateautomatically on changing channel in order to reducethe VSWR to an acceptable level.

InstallationA typical large aircraft h.f. installation consists oftwo systems, each of which comprises a transceiver,controller, antenna tuning unit and antenna. Each ofthe transceivers are connected to the AIS for mic.. tel.and p.t.t. provision. In addition outputs to Selcal.decoders are provided. Such an installation is shownin Fig. 2.1 l .

The transceivers contain the receiver, transmitter,power amplifier and power supply circuitry. They aremounted on the radio rack and provided with a flowof cooling air, possibly augmented by a fan. Atransceiver rated at 200 W p.e.p. needs to dissipate300 W when operated on s.s.b. while on a.m. thisfigure rises to 500 W. Telephone and microphonejacks may be provided on the front panel, as might ameter and associated switch which will provide ameans of monitoring various voltages and currents.

Coupling to the antenna is achieved via theantenna tuning unit (ATU). Some systems mayemploy an antenna coupler and a separate antennacoupler control unit. The ATU provides,automatically, a match from the antenna to the 50 Qtransmission line. Closed-loop control of matchingelements reduces the standing wave ratio to l'3 : Ior less (ARINC 559A).

Since the match must be achieved between line andantenna the ATU is invariably mounted adjacent tothe antenna lead-in, in an unpressurized part of theairframe. For high-flying a'ircraft (most jets) the ATUis pressurized, possibly with nitrogen. Some unitsmay contain a pressure switch which will be closedwhenever the pressurization within the tuner isadequate. The pressure switch may be used forohmmeter checks or, providing switch reliability isadequate, may be connected in series with the keyline thus preventing transmission in the event of aleak. Altematively an attenuator may be swit;hed into reduce power.

Light aircraft h.f. systems in service are likely, for

A

Mic.

Tcl.

No. Ip.t.t.

No. 2interlock

No. Iinterlock

28V2av

No. 2p.t.t.

Fig 2.1I Typical dual h.f. installatbn

financial reasons, to have a fixed antenna coupler.Such a system operates on a restricted number ofchannels (say twenty). As a particular channel isselected, appropriate switching takes place in thecoupler to ensure the r.f. feed to the antenna is viapreviously adjusted, reactive components, whichmake the effective antenna length equal to a quarterof a wavelength, thus presenting an impedance ofapproximately 50 O. The required final manualadjustment must be carried out by maintenancepersonnel on the aircraft.

The antenna used varies greatly, depending on thetype of aircraft. For low-speed aircraft a long wireantenna is popular although whip antennas may befound on some light aircraft employing low-poweredh.f. systems. The aerodynamic problems of wire

c,

antennas on aircraft which fly faster than, say, 400knots, have led to the use ofnotch and probeantennas which effectively excite the airframe so thatit becomes a radiating element.

Modern wire antennas are constructed ofcopper-clad steel or phosphor bronze, giving a reducedr.f. resistance compared with earlier stainless.steelwires. A covering ofpolythene reduces the effects ofprecipitation static. Positioning is normally a singlespan between forward fuselage and vertical stabilizer.I:rger aircraft will have twin antennas while a singleinstallation, possibly in a 'V' configuration, is morecommon for smaller aircraft. The r.f. feed is usuallyat the forward attachment via an antenna mast. Therear tethering is by means of a tensioning unit.

The anienna maqt is subject to pitting and erosion

Mic.

Tel.

No. 1Xmit No. I

28V---2-� t.r.

- l

28vl ruo z +

of the leading edge; a neoprpne covering will providesome protection, nevertheless regular inspections arecalled for. Protection against condensation within themast may be provided by containers of silica gelwhich should be periodically inspected for a change incolour from blue to pink, indicating saturation.Hollow masts are usually provided with a water-drainpath which should be kept free from obstruction.

The two most important features of the reartethering point are that the wire is kept under tensionand that a weak link is provided so as to ensure thatany break occurs at the rear, so preventing the wirewrapping itself around the vertical stabilizer andrudder. On light aircraft a very simple arrangement ofa spring, or rubber bungee, and hook may be used.The spring maintains the tension but if this becomesexcessive the hook will open and the wire will be freeat the rear end. On larger aircraft a spring-tensioningunit will be used to cope with the more severeconditions encountered due to higher speeds andfuselage flexing. The unit loads the wire by means ofa metal spring, usually enclosed in a barrel housing.A serrated tail rod is attached to the tethering pointon the aircraft and inserted into the barrel where it issecured by a spring collet, the grip of which increaseswith tension. The wire is attached to a chuck unitwhich incorporates a copper pin serving as a weak linkdesigred to shear when the tension exceeds about180 lbf. Some units incorporate two-stage protectionagainst overload. Two pins of different strengths areused; should the first shear, a small extension (3/ 1 6 in.)of overall length results, thus reducing tension andexposing a yellow warning band on the unit.

Notch antennas consist of a slot cut into theaircraft structure. often at the base of the verticalstabilizer. The inductance of the notch is oseries-resonated by a high-voltage variable capacitordriven by a phase-sensing servo. Signal injection is viamatching circuitry driven by a SWR sensing servo.Since the notch is high 'Q' the input is transformed toa voltage across the notch which is ofthe order ofthousands ofvolts. This large voltage provides thedriving force for current flow in the airframe whichserves as the radiator.

A probe antenna, which is aerodynamicallyacceptable, may be fitted at either of the wing-tips oron top of the vertical stabilizer. Again series tuningprovides the necessary driving force for radiation.The probe antenna, as well as the wire antenna, isliable to suffer lightning strikes, so protection in theform of a lightning arrester (spark gap) is fitted.Any voltage in excess of approximately l6 kV on theantenna will cause an arc across the electrodes of thehydrogen-iilled spark gap, thus preventing discharge

through the h.f. equipment. Build-up of precipitationstatic on antennas, particularly probes, is dealt withby providing a high resistance static drain (about6 MSl) path to earth connected between the antennafeed point and the ATU.

It is important in dual installations that only oneh.f. system can transmit at any one time;this isachieved by means of an interlock circuit. This basicrequirement is illustrated in Fig. 2.11 where it can beseen that the No. I p.t.t. line is routed via a contactof the No. 2 interlock relay, similarly with No. 2p.t.t. The interlock relays will be external to thetransceivers often fitted in an h.f. accessory box.While one of the h.f. systems is transmitting the othersystem must be protected against induced voltagesfrom the keyed system. In addition, with someinstallations, we may have a probe used as atransmitting antenna for both systems and as areceiving antenna for, say, No. I system. The No. 2receiving antenna might be a notch. It follows that onkeying either system we will have a sequence ofevents which might proceed as follows.

HF I keyed:l. HF 2 keyline broken by a contact of HF I

interlock relay;2. HF 2 antenna grounded;3. HF 2ATU input and output feeds gounded and

feed to receiver broken.

HF 2 keyed:l. HF I keyline broken by a contact of HF 2

interlock relay;2. HF I probe antenna transferred from HF l;

ATU to HF 2 ATU;3. HF 2 notch antenna feed grounded;4. HF I ATU input and output feeds grounded

and feed to receiver broken.

C,ontrols and OperationSeparate controllers are employed in dual installations,each having'in-use' frequency selection only. Oldersystems and some light aircraft systems have limitedchannel selection where dialling a particular channelnumber tunes the system, including ATU, to apre-assigned frequency, a channel/frequency chart isrequired in such cases. With modern sets, indicationofthe frequency selected is given directly on thecontroller.

The controls shown in Fig. 2.1 I are those referredto in ARINC 559A; variations are common and willbe listed below.

Mode Selector Switch. OFF-AM-SSB Thc'turn off'function may be a separate switch or indeed may not

F

31

be crnployed at all; snritching on and offbeingachieved with the master radio switch. The 'AM'

position may be designated 'AME'(AM equivalent orcompatible) and is selected whenever transmissionand reception is required using a.m. or s.s.b. plus fullcarrier (a.m.e.). The 'SSB' position provides fortransmission and reception of upper sideband only.

Although use of the upper sideband is the normfor aeronautical h.f. communications some controllershave 'USB' and 'LSB'positions. In addition 'DATA'

and 'CW'modes may be available. The former is forpossible future use of data links by h.f. using theupper sideband - the receiver is operated atmaximum gain. The latter is for c.w. transmission andreception, morse code, by 'key bashing', being theinform ation-carrying medium.

Flequency Selectors Frequency selecton consist of,typically, four controls which allow selection offrequencies between 2.8 and 24MHz in I kHz steps(ARINC 559A). Military requirements are for afrequency coverage of 2 to 30 MHz in 0.1 kHz steps,consequently one will find systems offering 280 000'channels'meeting these requirements in full or28 000 channels meeting the extended range but notthe 0'l kHz step requirement.

When a new frequency is selected the ATU mustadjust itselfsince the antenna characteristics willchange. For this purpose the transmitter is keyedmomentarily in order that SWR and phase can bemeasured and used to drive the ATU servos.

Squelch Control Normel control of squelchthreshold may be provided. As an alternative an r.f.sensitivity control may be used, but where Selcal isutilized it is important that the receiver operates atfull sensitivity at all times with a squelch circuit beingemployed only for aural monitoring and not affectingthe output to the Selcal decoder.

Audio Volume Control Provides for adjustment ofaudio level. Such a control may be located elsewhere,such as on an audio selector panel, part of the AIS.

C:lanfter This control is to be found on some h.f.controllers. With s.s.b. signals while the phase of thcre-inserted carrier is of little consequence itsfrequency should be accurate. Should the frequencybe inconect by, say, in excess of t 20 Hzdeterioration of the quality of speech will result.A clarifier allows for manual adjustment of there-inserted carrier frequency. Use of highly accuratemd stable frequency synthesizers make the provisionof such a control unnecessary.

32

Indicator A meter mounted on the front panel of thecontroller may be provided in order to give anindication of radiated power.

Block Dhgram Operation

Tlansceiver Figure 2.12 is a simplified block diagramof an a.m./s.s.b. transceiver. The operation will bedescribed by function.

Amplitude Mo dulated Transm issio n The frequencyselected on the controller determines the output fromthe frequency synthesizer to the r.f. translator whichshifts the frequency up and provides sufficient drivefor the power amplifier (p.a.). The mic. input, afteramptfication, feeds the modulator which produceshigh-level amplitude modulation of the r.f. amplifiedby the p.a. The r.f. signal is fed to the ATU via theantenna transfer relay contact.

The PA output signal is sampled by the sidetonedetector which feeds sidetone audio via the contactof the deenergized sidetone relay and the sidetoneadjust potentiometer to the audio output amplifier.

Single Sideband Transmission Low-level modulationis necessary since there is no carrier to modulate atthe p.a. stage, hence the mic. input, /n., is fed to abalanced modulator together with a fixed carrierfrequency, /., from the frequency synthesizer. Thebalanced modulator output consists of both sidebandsf" + f^ and f" - f^, the carrier being suppressed.The required sideband is passed by a filter to the r.f.translator after further amplifi cation.

Ifwe consider an audio response from 300 to3000 Hz we see that the separation between thelowest Es.b. frequency and the highest l.s.b.frequency is only 600 Hz. It follows that the filterused must have very steep skirts and a flat bandpass.A mechanical filter can be used in which an inputtransducer converts the electrical signal intomechanical vibrations, these are transmitted bymechanically resonant metal discs and coupling rodsand finally converted back to an electrical signal byan output transducer.

Frequency translation is by a mixing processrather than a multiplicative process since if theu.s.b. /, + /n' were multiplied by try' we wouldradiate a frequency of//(/c + /n,') rather thanft + f " + /.. The amount by which the u.s.b. istranslated, fi, is determined by the frequency selectedon the controller. Final amplification takes place inthe p.a. prior to feeding the r.f. to the ATU.

To obtain sidetone from the p.a. stage a carrierwould need to be re-inserted. A simpler method,

Sidetoncrelay

To r.f./i.f. stages

f " + f -

f " - f .

t

Fq.2.l2 Typical h.f. a.m./s.s.b. trmsceiver block diagram

which nevertheless confirms that a sigral has reachedthe p.a., is to use the rectified r.f. to operate asidetone relay. When energized the contact of thisrelay connects the amplified mic. audio to the outputaudio amplifier.

Amplitude Modulated Reception The received signalpasses from the ATU via the de+nergized antennatransfer relay contact to an r.f. amplifier and thenceto the r.f. translator. After the translator normal.a.m.detection takes place, the audio so obtained being fedto the output stage. A variety ofa.g.c. and squelchcircuits may be employed.

Single Sideband Reception The circrit action ons.s.b. is similar to that on a.m. until after thetranslator when the translated r.f. is fed t6 the productdetector along with the re-inserted 'carrier'

/". Theoutput ofthe product detector is the required audio

signal, which is dealt with in the same way as before.

Antenna Tuning Unit Figure 2.13 illustrates anautomatic ATU simplified block diagram. Onselecting a new frequency a retune sigrral is sent tothe ATU control circuits which then:

l. keys the transmitter;2. inserts an attenuator in transceiver output line

(Fig.2.t2);3. switches on the tuning tone sigral generator

(Fig.2.l2) and drives a tune warning lamp(optional);

4. switches on reference phases for servo motors.

The r.f. signal on the input feed is monitored by aloading servo system and a phasing servo system. Ifthe load impedance is high then the line current, /L,is low and the line voltage ZL is high. This isdctected by the loading s€rvo discriminator which

El

I

TuneRctunc tone

TxkeYAru

Transceiver

p;g.2.13 Typical h.f. a.t.u. block diagrarn

appL:: the appropriate amplitude and polarity d.c.sigral to a chopper/amplifier which in turn providesthe control phase for the loading servo motor. Theauto transformer tap is driven until the loadimpedrnce is 50 O.

Should Iy and Vynot be in phase this is detectedby the phasing servo discriminator which applies theapprop:iate amplitude and polarity d.c. signal to achopper/amplifier which in turn provides the controlphase for the phasing servo motor. The reactiveelemenis, inductance and capacitance, are adjusteduntri.Il and Vy are in phase.

As a result of the action of the two servo systems aresistive load of 50 O is presented to the co-axial feedfrom the transceiver. When both servos reach theirnull positions the control circuits remove the signalslisted previously.

CtaracteristicsThe following brief list of characteristics are those ofa system which conforms with ARINC 559A.

frequency SelectionAn r.f. nnge of 2'8-24 MHz covered in I kHzincrements.Method: reentrant frequency selection system.Orannelling time less than I s.

Mode of OperationSingle channel simplex, upper single sideband.

g

TlansmitterPower output: 400 W p.e.p. (200 W p.e.p.operatiohal).Absolute maximum power output: 650 W p.e.p.Mic. input circuit frequency response: not more thanI 6 dB variation from 1000 Hz level through therange350 Hz to 2500 Hz.Spectrum control: components at or below/" -100 Hz and at or above f" +29O0 Hz should beattenuated by at least 30 dB.Frequency stability: ! 2O Hz. Shop adjustment nomore often than vearly. Pilot control (e.g. clarifier)not acceptable.lnterlock: only one transmitter in a dual systemstrould operate at a time on a 'first-come, first-served'basis, this includes transmitting for tuning purposes.

ReceiverSensitivity: 4 pV max.; 30 per cent modulation a.m.(l pV s.s.b.) for l0 dB signal and noise to noise ratio.A.g.c.: audio output increase not more than 6 dB forinput signal increase from 5 to I 000 000 pV and nomore than an additional 2 dB up to I V input signallevel.Selectivity:s.s.b.,6 d-B points atf"+ 300 Hz and /. + 3100 Hz,t 35 dB points at f"andf" + 3500 Hz.A.m.: toensure proper receiver operation (noadjacent ch4nnel interference) assuming operations on6 kHz spaced a.m. channels.

Overall response: compatible with selectivity but inaddition no more than 3 dB variation between anvtwo frequencies in the range 300-1500 Hz (forsatisfactory Selcal operation).Audio output: two-wire circuit isolated from ground,300 O (or less) output impedance supplying 100 mW(0'5 Selcal) into a 600 O load.

Ramp Testing ard MaintenanceWhilst regular inspection of all aircraft antennas iscalled for, it is particularly important in the case ofh.f. antennas and associated components. Anymaintenance schedule should require frequentinspection ofantenna tensioning units and tetheringpoints in the case of wire antennas, while for bothprobe and wire antennas the spark gap should bein3pected for signs of lightning strikes (cracking

. and/or discolouring).A functional test is similar to that for vh.f. in that

two-way communication should be established with aremote station: all controls should be checked forsatisfactory operation and meter indications, if any,strould be within limits. Safety precautions areparticularly important since very high voltages arepresent on the antenna system with the resultingdanger ofelectric shock or arcing. No personnelshould be in the vicinity of the antenna whentransmitting, nor should fuelling operations be inprogress. Remember with many h.f. systems a changeof frequency could result in transmission to allowautomatic antenna tuning.

Selcal

The selective calling (Selcal.) system allows a groundstation to call an aircraft or group of aircraft usingh.f. or vir.f. comms without the flight crew havingcontinuously to monitor the station frequency.A coded sigral is transmitted from the ground andrcceived by the v.h.f. or h.f. receiver tuned to theappropriate frequency. The output code is fed to aSelcal decoder which activates aural and visual alertsif and only if the received code'corresponds to thecode selected in the aircraft.

Each transmitted code is made up of two r.f.bursts (pulses) each of-l t 0'25 s separated by aperiod of 0.2 t 0'l s. During each pulse thetransmitted carrier is 90 per cent modulated with twotones, thus there are a total of four tones per call; .the frequencies of the tones determine the code.

The tones available are given by the formula

[y = antilog (0'054(/V- t) + 2O),

w h e r e N = 1 2 , 1 3 . . . 2 7 ,

giving a total of sixteen tones betwecn 312'6 and1479.1Hz. The tones are desigrated by lettes A to Somitting I, N and O so a typical code might b,:AK-DM. The re ue 297O codes available forassigrment using the first twelve tones, the additionof tones P, Q, R and S (1976) bring the total to10920. Codes or blocks ofcodes are assigned onrequest to air carrier organizations who in turn assigtcodes to their aircraft'either on a flight number oraircraft registration-related basis.

Figure 2.14 illustrates a single Selcal systemlarge passenger transport aircraft would norma,ly'carry two identical systems. The decoder willrecognize a received combination of tones on rny offive channels which corresponds to that combrnationselected on the code select and annunciator pa:;el.When the correct code is recogrized the chime switchand appropriate lamp switch is made. The lamp .witchsupply is by way of an interrupter circuit so that thelamp will flash. A constant supply to the chimesvitch causes the chimes to sound once. Each lanrpholder, designated HF I , HF I I etc. incorporates a resetswitch which when depressed will release the latchedlamp switch and chime switch. The tone filters in thedecoder will typically be mechanically resonantdevices.

Variations in the arrangement shown anddescribed are possible. Mechanically the control andannunciator panel may be separate units. Should theoperator require aircraft registration-related codesthere will be no need for code select switches. theappropriate code being selected by jumper leads onthe rear connector ofthe decoder.

Although five reset leads will be provided theymay be connected individually, all in parallel to asingle reset switch or to the p.t.t. circuit of theassociated transmitter. In this latter case isolationdiodes (within the decoder) prevent'sneak' circuits,i.e. keying one transmitter causing one or moreothers to be keyed.

The lamp and chime srrpplies shown can bechanged at the operator's option. Possibilities are toreverse the situation and have steady lights andmulti-stroke chimes, or have steady lights andsingle-stroke chime, in which case the interrgptcircuit is not used.

The Selcal systems which do not comply withARINC 596 may not provide facilities for decodingoffive channels simultaneously. A switch is provided onthe control panel with which the single desiredchannel can be selected; in this case only Selcal codesreceived on the corresponding receiver will be fed to

b L*-ntgs

35

v.H.F.1

v.H.F. 2

V.H.F. 3

H . F . 2

the decodcr. Only one annunciator lamp is required.Code selection in an ARINC 596 system is achieved

by means of a 'b.c.d.' format. Each of the four toneselectors has four wires associated with it; for anyparticular tone an appropriate combination of thewires will be open circuit, the rest grounded. If the

3G

Sslf test [ampdrive(5 wiresl

tones A to S arc numbered I to 16 (0) the open wireswill be as given by the corresponding binary number;e.g. tone M-12-l l(X), so with the wires designated8,4,2 and I we see 8 and 4 will be open. Note this isnot really b.c.d. but is nevertheless termed so.

Testing of Selcal is quite straightforward. If

Fig. 2.f4 Typical Selcal block diagram

G

possible a test rig, consisting ofa tone generator inconjunction with a v.h.f. and h.f. transmitter shouldhe used, otherwise permission to utilize aSelcalequipped ground station should be sought.

Audio Integrating Systems (AlS) - lntercom

IntroductionAll the systems in this book exhibit a variety ofcharacteristics but none more so than AIS. In a light.aircraft the function of the audio system is to providean interface between the pilot's mic. and tel. and theselected receiver and transmitter; such a 'system'

might be little more than a locally manufacturedpanel-mounted junction box with a built-in audioamplifier and appropriate switching. ln contraSt alarge multi-crew passenger aircraft has several

sub-systems making up the total audio system. Theremainder of this chapter will be concerned with theAIS on aBoeing747.

It is unusual to consider all the systems andsub-systems which follow as part of AIS, a termwhich should perhaps be restricted to the systemwhich provides for the selection of radio system audiooutputs and inpuis and crew intercommunications.However a brief description of all systems whichgenerate, process or record audio signals will be given.The following services comprise the complete audiosystem:

l. flight interphone: allows flight deck crew tocommunicate with each other or with groundstations;

2. cabin interphone: allows flight deck and cabincrew to communicate:

Attendant's chimecall

PA

VOR/ILS NAVsystomMarker bsaconsystomLow rangeradio alt imetersystom

ATC system

DME systom

ADF system

bverride

Visualaudio

lSatcom II sysrem Itjgg'"q"rlj

Fig. 2.15 Boeing 747: typical communications fit(courtesy Boeing Commdrcial Aeroplane Co.)

Pass. ent.(motion pic.)system

HF comnrun ication

system

Headsetsandmicroph,

37

3. service interphone: allows ground staff tocommunicate with each other and also with theflight crew;

4. passenger address (PA): allows announcementsto be made by the crew to the passengers;

5. passenger entertainment system: allows theshowing of movies and the piping of music;

6. gound crew call system: allows flight andground crew to attract each other's attention;

7. cockpit voice recorder: meets regulatoryrequirements for the recording of flight crewaudio for subsequent accident investigation ifnecessary.

It *rould be noted that the above are not completelyseparate sysrems as illustrated in Fig. 2.15 anddescribed below. The dividing lines betweenzub-systems of the total audio system are somewhatarbitrary, and terminologl is varied; however thefacilities described are commonplace.

Flight InterphoneThis is really the basic and most esential part of theaudio system. All radio equipments having mic.inputs or tel. outputs, as well as virtually all otheraudio systems, interface with the flight interphonewhich may, in itself, be termed the AIS.

A large number of units and components make upthe totd system as in Table 2.1 with abbreviatedterms as listed in Table 2.2. Figuie 2.16 shows theflight interphone block diagram, simplified to theextent that only one audio selection panel (ASP),jack panel etc. is shown. An ASP is shown inFig. 2.17.

A crew member selects the tel. and mic. signalsrequired by use of the appropriate controls/switcheson an ASP. The various audio signals entering an ASpa.ie'selected by twelve combined push select andvolume controls. Each ASP has an audio bus feedinga built-in isolation amplifier. The v.h.f. and h.f.comm. ADF, interphone and marker audio signals arefed to the bus via the appropriate select buttons and

-volume controls. The vh.f. nav. and DME audio isfed to the bus when voice and range are selected withthe Voice pushbutton; with voice only selected theDME audio is disconnected while the vJr.f. nav. audiois pased through a sharp 1020 Hz bandstop filter(FLl) before feeding the bus. With the flii-normalswitch in the fail position only one audio channel canbe selected (bypassing the amplificr) and the pAaudio is fed direct to the audioout lines. Radioaltimeter audio is fed direct tb the audio-out lincs.The above audio switching arrangements are illustratedh Fig. 2.18. Note the serics resistors in the input

38

Table 2.1 Flight interphone facilities

UPT FlO FIE OBSI OBS2 M.E.

ASP

Jack panel

Int - R-Tp.Lt.

Handheldmic.

llcrdrct

Boom mic.headset

Oxygenmask mic.

lnterphonespeaker

X Jack

Jack Jrct

trck lack Jrck

x x x x x

x x i x x

Jack

Iack

x

x

Jack

x

Jack

X

x

feck

hck

x

J.ct

x

Jack

Jac*

A'X'indicates the particular unit or component is fittedat that station (column),

'Jack' indicates a jack plug b fitted to enable use of theappropriate mic. and/or tel.

Table2.2 Abbreviations

CAPT - CaptainF/O - First OfficerOBS - Observerm.e. - Main Equipment

Centremic. - Microphone

a.s.p. - Audio Selector Panelint. - Interphonerlt - Radiotelephonep.t.t. - hes to Transmit

tel. - Telephone

a\dio lines which, together with loading resistors inthe interphone accessory box, form an anti-cross talknetwork; if one crew member has, say, h.f.l selectedon his ASP then the resistive network will greatlyattenuate say h.f.2 whidr would otherwise be audibleshould another crew member have selected h.f.l andh.f.2.

Six mic. select buttons are provided on an ASP;three vJr.f. @mm., two h.f. cornm. and PA. Additiondsritches asociated with mic. select and transmissionare the boom-mask and r.t.-int. p.t.t. on each ASP andalso p.t.t. buttons on the hand-held microphones.jack panels and the captain's control wheel(R/T-int.).

To speak orrer interphone a crew member shouldselect interphone using the r.t..int. switch on thea.s.p. which will connect mic. high (boom or mask)

c . E @ { r - 8 . . | &

r ^ . & r E r . @ * c c r d

' E g - B

rrcw rs

r.Mro s* *rcx

E]

l r .slucuO'El

Fi& 2.lt Ardb signel selectbn (courtcsy BoeiruCommercial Aeroplane Co.)

i_p-

Fig. 2.16 Boeirg ?4?: night intqphonc (courtcry BoeirgCommercial Aeroplane Co.)

Fig. l.l7 Audio selection panel (c€urtesy BoeingCommercial Aeroplane Co.)

to the interphone mic. high output feeding the flightinterphone amplifier in the interphone accessory box.Alternatively the captain can seiect interphone on hiscontrol wheel p.t.t. switch which will energize relayK2 thus making the mic. higft connection as before.Note that the ASP r.t.-int. p.t.t. switch does not relyon power reaching the ASP for relay operation (see

aaaaa

rE ,r'i El

xrt .3r 'l l -F .

f ' IC'Off hIGN

r|rrrftsa {rcaet{d rgro rL rc i r r r ta t ,da

E E

Fi& 2.19 Microphone signal selection (courtesy BocingCommercial Aeroplane Co.)

or ' ,a.. , .r I i ID + - i l i rl r t : l l l - - - - l

l ! l r i 4t t u gr r r , r l : J Jn i ! | i + l s # - . "

t r ! I I I+= | 1, . i , . .* .:l---+i i :i L-re'g!'e$!sret-l

F.--d i i---r '"* T------1#--i ij-#S-.i i r

i l - - r - - - l t I

i l ' F . ' L i " r i - i i i i

EOOM

tr trtr trtrtr

I t

l l t l lo o o o o .l-"i"_.]o o C I

**" --fio,,

a t r @l-oo'ls o

-G-ql# rrrrNr

o

39

Fig. 2.19). Interphone mic. signals from all ASh arefed to the flight interphone amplifier which combinesthem and feeds the amplified interphone audio to allASPs for selection as required.

Pressing a mic. select button on the ASP willconnect the corresponding system mic. input lines torelay K2 and to contacts on the ASP r.t.-int. p.t.t.switch. Thus when a p.t.t. switch is pressed, the mic.lines will be made by either the contacts of K2 or bythe ASP p.t.t. switch in the r.t. position. In Fig. 2.19the h.f.2 select switch is shown as typical of all comni.select switches. When the PA select switch is pressedthe flight interphone mic. circuit is interrupted andPA audio is applied to the fail-normal switch; inaddition the mic. lines to the PA system are made.Operation of any p.t.t. switch mutes both interphonespeakers to prevent acoustic feedback.

Cabin InterphoncThe cabin interphone is a miniature automatictelephone exchange servicing several subscribers:the cabin attendants and the captain. In additionthe system interfaces with the PA to allowannouncements to be made.

Numbers are dialled by pushbuttons on thetelephone type handsets or on the pilot's controlunit. Eleven two-figure numbers are allocated to thesubscribers, plus additional numbers for PA invarious or all compartments, an 'all-attendants' calland an 'all-call'. Two dialling codes consist of letters:P-P is used by an attendant to alert the pilot (calllight flashes on control unit and chime sounds once)while PA-PA is used by the pilot to gain absolutepriority over all other users of the PA system. Thedirectory is listed on the push-to-talk switchincorporated in each handset to minimize ambientnoise.

All dialling code decoding and the necessary trunkswitching is carried out in the central switching unit,CSU (automatic exchange). The CSU also containsthree amplifiers, one of which is permanentlyallocated to the pilot on what is effectively a privatetrunk. Of the five other available trunks, two areallocated to the attendants, two to the PA system and-one for dialling. (Note a trunk is simply a circuitwhich can connect two subscribers.)

The cabin interphone and service interphoneqystems may be combined into a common networkby appropriate selection on the flight engineer'sinterphone switch panel, captain's ASP and cabininterphone control unit. Any handset may then belifted and connected into the network (dial'all-call').In a similar way the flight interphone circuits may beused to make specific calls over.the cabin interphonesystem.

n

Crll light

Attondrnt's .t tion3(typrcrl)

Ft. 2.20 Boeing 7rt7: cabin interphonc (courtcsy Boeitg

Commercial Aeroplane Co.)

.** - -o;*

The system is more complex than has beensuggested above but a basic description has been given,zupported by Fi1.2.20.

Service InterphoneA total of twenty-two handset jacks are located invarious parts of the airframe in order that groundcrew can communicate with one another using theservice interphone system. The system is rathersimpler than those considered above. Mic. audio fromall handsets, with 'press to talk' depressed, arecombined in and amplified by the service interphoneamplifier in the interphone audio accessory box.The amplified signal is fed to all handset tels.Volume control adjustment is provided by a presetpotentiometer.

With the flight engineer's interphone switchselected to ON the input summing networks for bothservice and flight interphone systems are combined.All mic. inputs from either system are amplified andfed to both systems.

PassengQr Address'The system comprises three PA amplifiers, tape deck,annunciator panel, attendant's panel, PA accessorybox, control assemblies, speaker switch panel andfifty-three loudspeakers. The various PA messageshave an order of priority assigred to them: pilot'sannouncemen ts, attendant's announcements,pre recorded ann ouncements and finally boardingmusic. All PA audio is broadcast over the speakersystem and also, except for boarding music, overrides

c.:to!:S"hin!Itntcrphonc Ilrudio acccrll box I

L - - - - - - - -

r - - - - - - -

I

,:.*Q

*.*Ai

L-

L _ _ - - - - - - . Jf r a L a c l i o i r 6 c r i c u l l

r ! r6xr , i [ma

f euo'o

? i l 6

Fig,2.2l Boeing ?47: service interphone (courtesy Boeing

Commercial AeroPlane Co.)

L____r

F1g.2.22 Boeing ?4?: passenger address (courtesy Boeing

Commercial AeroPlane Co')

amplifiers. When the aircraft is on the ground with

;;;i;g gear locked down and ground powgl applied

the lev-efof speaker audio is reduced by 6 dB'

The tape deck contains up to five tape cartridges

apart from the necessary tape'{rive mechanism'

piayback head and a pre-amplifier' Boarding music is

Ltectea at an attendint's panel while prerecorded

imnouncements are selected by means of twelve

pushbuttons on the annunciator panel'

Passenger Entertainment SYstem

The pa-ssenger entertainment system of the Boeing

747 and an! other modern large airliner is perhaps

the mmt complex of 3ll airbome systems' lt is also

the system liklly to caus€ most trouble and,

fortunatelv, teait litcely to affect the safety of the

aircraft unless bad servicing leads to a fire or ^loose-article hazard. Even on the same type ol

aircraft a variety of services will be available since

different op.r"iott will offer different entertainment

in a bid to capture more customers' In view of the

above comments, the following description is

particularly brief and does not do justice to the

complexitY involved.

T----1

l-*.,^f-J, .*,,"."I ,".",* ,.1-liii',T:f..i ' ' ' '6--'"'"" 'l;n: *llF^*.; ; *,,*,* ^*', " * l f , b _ _ ^ - r c ,

entertainment audio fed to the passenger stethoscope

headsets. A prerecorded emergency announcement

may be initiated by the pilot or an attendant, or

automatically in the event of cabin decompression'

A chime is generated when the pilot tums on 'fasten

seat-belt' or 'no smoking' siglts.The passenger address amplifiers are fed via the

flight or cabin interphone systems for pilot or-

atiendant announcements respectively. Distribution

of audio from the amplifiers to the speakers in various

zones depends on the class configuration, since some

*noun.itntnts may be intended for only a certain

class of passengers.The necessaiy distribution is achieved by means of

switches on the speaker switching panel. Audio is also

fed to the flight interphone system for sidetonepurposes.-

Number 2 and number 3 ampliliers ere slaved to

number I for all'class announcements. Should

separate class announcements be required the parallel

control relay is energized, so separating the number I

audio from that of number 2 and 3. The control

assemblies in the PA accessory box containpotentiometers used to set the gain of the PA

41

Movieaudio

P.A.override

Othersubmultiplexers

Seats1 2 3

Other seatcolurnns

1 2 3Seats

simplified passenger entertainment

Channel select

Other seatdemult iolexers

Fig.2.23 Boeing 747:system

Both movies and music are provided, the movieaudio being fed to individual seats via the musicportion of the system. Ten tape-deck channels, fourmovie audio channels and one p.a. channel (totalfifteen) are provided using time multiplexing. A timeinterval, ternred a liame, is divided into fifteenchannel t imes during which the signal amplitude ofeach channel is sampled. The audio sigrral amplitudesare binary coded (twelve bits) and transmitted,together with channel identification, clock and sync.pulses, over a co-axial cable running throughout theaircraft.

The music channels (five stereo, ten monaural or amixture) are multiplexed in the main multiplexer, theresulting digital signal being fed to six submultiplexersin series, the final one being terminated with a suitableload resistor. Movie and PA audio are multiplexedwith the music channels in the zone submuliiplexers,each of which feeds three or four columns of seatdemultiplexers. Channel selection is made by thepassenger who hears the appropriate audio over his

- stethoscope headset after digital to analogueconversion in the demultiplexer. Alternate zonesubmultiplexers are used as back-up in the event ofprime submultiplexer failure (class priorities exist iffailures mean some passengers must have theen tertainment service discontinued).

The controls necessary for activation of theentertainments system are located on attendants'control panels.

Ground Crew Call SyrtemGround crew call is hardly worthy of the title

42

'system', as can be seen from the schematic diagramn Fig.2.?4. The horn and fl ight-deck call button arelocated in the nose wheel bay while tl're ground-crewcall (with i l lumination) and aural warning box are onthe flight deck. Operation is self-explanatory fromthe diagram. Should horn or chinte sound, the groundcrew, or flight crew respectively, will contact eachother using one of the interphone systems.

Fig.2.24 Boeing 747: ground crew call (courtesy BoeingConrmercial Aeroplane Co.)

Cockpit Voice RecorderAn endless tape provides 30 niin recording time foraudio signals input on four separate channels. Thechannel inputs are captain's, first officer's and flightengineer's transmitted and received audio and cockpitarea conversation. Passenger address audio may besubstituted for the flight engineer's audio in anaircraft certified to fly with two crew members.

The microphone inputs should be from so-called'hot mics', i.e. microphones which are permanentlylive regardless of the setting of ASP or controlcolumn switches. The area microphone (which may

C F O C i C W C A L L+.

Flt. eng.hot mic.

tel.

lst. off.hot mic.

tel.

Recordhead

loQ PlaybackI headArea Mic. Pre-amp-

Erase TestJack

4,Landing parking

P"::; ffifFt1.2.25 Typical cockpit voice rccorder block diagram

be s.eparate from the control panel) is strategicallysituated so that it can pick up night crew speech andgeneral cockpit sounds.

., While the control panel is situated in the cockpit,

the recorder unit (CVR) is located at .:he other end ofthe aircraft where it is least likely to suffer damage inthe event of an accident. The CVR is constructed soas to withstand shock and fire damage, and additionallyis painted in a fire-resistant orange paint to assist inrecovery from a wreck.

The recorded audio may be erased providing thelanding gear and parking brake interloik relav'contacts are closed. As a further safeguard aiainstaccidental erasure a delay is incorporited in the bulkerase circuit which requires thp operator to depressthe 'erase'

switch for two secondi before "r"a*"commences.

Test facilities are provided for all four channels,

Essontirlf l t. inst.bus bar

separately or all together. A playback head andmonitor amplifier allows a satisfactory test to beobserved on meters or heard over a headset viajackplug sockets. Pressing the test button on the controlpanel or the all-test button on the CVR causes thechannels to be monitored sequentially.

The power supply for the system should be from asource which provideq maximum reliabilitv. Since thetape is subject to wear and thus has a limiied life, theCVR should be switched off when nqt in use. Asuitable method would be to remove power to theCVR whenever external ground power is connected.

Testing and Trouble Shooting the AudioSystems

Various self-test facilities may be provided by which

tl:l

tones may be generated and heard over headsets. finding short circuits or howls due to coffee-inducedHowever, to test properly all switches should be tel.-mic. feedback (i.e. spilt l iquid providing aoperated and all mic. and tel. jacks, as well as conducting path between tel. and mic. circuits).speakers, should be checked for the required audio. Where one has a number of units in series, e.g.This should be sufficiently loud, clear and noise-free. demultiplexers in an entertainment system,Amplifier gain presets in accessory boxes may need to disconnecting can be a particularly rapid method ofbe adjusted. A full functional test is best done by fault-finding; it is usually. best to split the run in half,two men, although it is not impossible for one man then in half again, and so on until the faulty unit orwith two headsets and an extension lead to establish connection is found. Continuity checks on very longtwo-way contact between various stations. cables can be achieved by shorting to earth at one end

Faults can be quite difficult to find owing to the and then measuring the resistance to earth at thecomplicated switching arrangements. However the other. The resistance to earth should also bewide range of switching can be used to advantage in measured with the short removed in case a naturalorder to isolate suspect units or interconnections. short exists.Disconnecting units provides a good method of

4

3 Automatic direction finding

Introduction

Most readers will have come across the principle onwhich ADF is based when listening to a transistorradio. As the radio is rotated the signal becomesweaker or stronger, depending on its orientation withrespect to the distant transmitter. Of course it is theantenna which is directional and this fact has beenknown since the early days of radio.

In the 1920s a simple loop antenna was used whichcould be rotated by hand. The pilot would positionthe loop so that there was a null in the signal from thestation to which he was tuned. The bearing of thestation could then be read off a scale on the loop.Tuning into another station gave rise to anotherbearing and consequently a fix. Apart fromposition-fixing the direction-finding loop could beused for homing on to a particular station. Thisprimitive equipment represented the first use of radiofor navigation purposes and came to be known as theradio compass.

The system has been much developed since thoseearly days and in particular its operation has beensimplified. Within the band 100-2000 kHz (I.f./m.f.)there are many broadcast stations and non-directionalbeacons (NDB). An aircraft today would have twin

receivers which, when tuned to two distinct stationsor beacons, would automatically drive two pointers onan instrument called a radio magnetic indicator (RMI)so that each pointer gave the bearing of thecorresponding station. The aircraft position is wherethe two directions intersect. Since such a systemrequires the minimum of pilot involvement the nameradio compass has come to be replaced by automaticdirection finder (ADF).

Basic Principles

The Loop Antenna'l

he first requirement of any ADF is a directionalantenna. Early loop antennas were able to be rotatedfirst by hand and subsequently by motor,automatically. The obvious advantage of having nomoving parts in the aircraft skin-mounted antenna hasled to the universal use of a fixed loop and goniometerin modern equipments, although some older types arestill in service.

The loop antenna consists of an orthogonal pair ofcoils wound on a single flat ferrite core whichconcentrates the magnetic (H) field component of thee.m. wave radiated from a distant station. The plane

Athwartshipsloop

Forc and aft loop

Fb.3.l Loop entcnna and goniometcr

Rotot(sctrch coil)

45

ol one coil is aligned with the aircraft longitudinalaxis while the other is aligned with the lateral axis.

The current induced in each coil wil l depend onthe directidn of the nragnetic f ield. When the planeof the loop is perpendicular to the direction ofpropagation, no voltage is induced in the loop sincethe l ines of f lux do not l ink with it. l t can be seenthat if one loop does not l ink with the magnetic f ieldthe other wil l have maximum linkage. Figure 3.1shows that the loop currents flow through the statorwinding of a gonionreter (resolver) where. providingthe characteristics o1'each circuit are identical, themagnetic f ield detected by the loop wil l be recreatedin so far as direction is concerned. We noweffectively have a rotating loop antenna in the formof the eoniometer rotor or search coil. As the rotorturns tf,rough 360o there wil l be two peaks and twonulls of the voltage induced in it. The output of therotor is the input to the ADF receiver which thus seesthe rotor as the antenna. Such an arrangement isknown as a Bell ini-Tosi system.

Since we are effectively back with a rotating loopsituation we should consider the polar diagram ofsuch an antenna as we are interested in its directionalproperties.

In Fig. 3.2 we have a vertically polarized t.e.m.wave from the direction shown. Tl.rat component ofthe H field linking with the loop will be H sin 0, so aplot of the loop current against I produces a sinecurve as shown. The polar diagram of such anantenna wil l be as in Fig. 3.3. It can be seen thatbecause of the sinusoidal nature of the plot the nullsare far more sharply defined than the peaks.

The above has assumed a vertically polarized wavewhich is in fact the case with NDBs and most

Fig, 3.3 Loop aerial polar diagram

broadcast stations. However a vertically polarizedsignal travell ing over non-homogeneous earth andstriking reflecting objects, including the ionosphere,can arrive at the loop with an appreciablehorizontally polarized component. The current inthe loop wil l then be due to two sources, the verticaland horizontal cornponents, which will in general givea non-zero resultarrt null, not necessarily in thedirection of the plane of the antenna. Thispolarization error dictates that ADF ihould only beused with ground wave signals which in the l.f./m.f.bands are useful for several hundred miles. However.they are contaminated by non-vertically polarized skywaves beyond, say,200 m at 200 kHz and 50 m at1600 kHz, the effect being much worse at night(night effect)

$*o read ,

The Sense AntennaThe polar diagram of the loop (Fig. 3.3) shows thatthe bearing of the NDB will be given as one of twtr

Plane of looP

l /tl

Direction of l,/propagatron -------------+

i

( l\

| . - 0 , ,

H f i e l d

E f i e l dO

tt6

F8. 3.2 To illustrate degrcc of coupling of loop acrbl

figures, l80o apart, since there are two nulls. Inorder to determine the correct bearing furtherinformation is needed and this is provided by anomnidirectional sense antenna. In a verticalivpolarized field an antenna which is omnidireitional inthe horizontal plarre should be of a type which isexcited by the electric (E) field of the t.e.m. wavei.e. a capacitance antenna. The output of such anantenna will vary with the instantaneous fieldstrength while the output of a loop antenna varies asthe instantaneous rate of change of field strength(Faraday's law of induced e.m.f.). As aconsequence, regardless of the direction of the t.e.m.wave, the sense antenna r.f. output wil l be in phasequadrature with respect to the search coil r.f.butput.In order to sense the direction of the NDB the twoantenna outputs must be combined in such a way aseither to cancel or reinforce, and so either the senseor the loop signal must be phase shifted by 90..

A composite signal made up of the search coiloutput phase shifted by 90' and the sense antennaoutput would appear as if it came from an antennathe polar diagram of which was the sum of those forthe individual antennas. Now the figure-of-eight polardiagram for the loop can be thought of as beinggenerated as we consider the output of a fixed searchcoil for various n.d.b. bearings or the output of arotating search coil for a fixed n.d.b. beaiing, either

1v%the separate halves of the figure-of+ight will berdu out ot phase. As a consequence the senseantenna polar diagram will add to the loop polardiagram for some bearings, and subtract foiothers.The resultant diagram is a cardiod with only one null,

although not as clearly defined as the nulls for thefigure-of-eight (Fig. 3.4).

Simplif ied Block Diagram Operation

Automatic direction finding (ADF) is achieved bymeans of a servo loop. The search coil is driven ro astable null position, a second null being unstable.- The search coil o-utput, after amplification, is

phase-shifted by 90" so as to be either in phase or outof phase with the sense antenna output, dipending onthe direction of the NDB. prior to adding to thesense signal the phase-shifted loop signal is switchedin phase in a balanced modulator at a rate determinedby a switching oscillator, usually somewhere betweena 50 Hz and 250 Hz rate. When the composite signalis formed in a summing amplifier it will beamplitude-modulated at the switching frequency sincefor one half period the two input signals will be inphase while for the next half period they will be inantiphase (see Figure 3.6).

The amplitude modulation is,detected in the laststage of a superhet receiver. The detected output willbe either in phase, or in antiphase, with the switchingoscillator output and so a further 90" phase-shift isrequired in order to provide a suitable control phasefor the servo motor. The motor will drive eitherclockwise or anticlockwise towards the stable null.When the null is reached there will be no search coiloutput hence no amplitude modulation of thecomposite signal so the reference phase drive will bezero and the motor will stop.

Should the servo motor Le in such a position thatthe search coil is at the unstable null the sliehtestdisturbance will cause the motor to drive aiay fiomthis position towards the stable null. The senie of theconnections throughout the system must be correctfor the stable null to give the bearing.

. A synchro torque transmitter (STTx), mounted onthe search coil shaft, transmits the bearine to a remoteindicator.

Block Diagram Detail

TuningModern ADFs employ so-called digital tuningwhereby spot frequencies are selected, as opposed toolder sets where continuous tuning *.s usurl.A conventional frequency synthesizer is used togenerate the local oscillator (first l.o. if doublesuperhet) frequency. The tuning voltage fed to thev.c.o. in the phase lock loop is also used for varicap

- - a - - \ -

. t \ \/ / \ \, / l \ \

/ t l \/ t l \

/ \ t

r l' . \

' l /' . \ /\ - \ . / z '-\-__-i--

Fig. 3.4 Composite polar diagram

47

Loop antenne

Summing amP.

Fig. 3.5 An ADF simplified block diagram

tuning in the r.f. stages. Remote selection is by b.c'd.(ARINC 570) or some other code such as 215.

Balanced ModulatorFigure 3.7 shows the balanced modulator used in theKing KR 85. Diodes CR 1 l3 and CR I l4 are turnedon and off by the switching oscillator (Q 3l I and

Q 312) so alternately switching the loop signal to oneof two sides of the balanced transformer T I 16. Theoutput of Tl l6 is thus the loop sigral with its phaseswiiched between 0o and 180" at the oscil lator rate.

ReceiverA conventional superhet receiver is used with an i.f.frequency of 14l kHz in the case of the KR 85 ; i.f.and r.f. gain may be manually controlled but in anycase a.g.c. is used. An audio amp, with normal gaincontrol, amplifies the detectsd signal and feeds theAIS for identification purposes. A beat frequencyoscillator (b.f.o.) can be switched in to facilitate theidentification of NDBs transmitting keyed c.w- The

/t8

b.f.o. output is mixed with the i.f. so as to produce

an audio difference frequency. Good sensitivity is

required since the effective height of modern

low-drag antennas gives a low level of signal pick-up'

Good selectivity is required to avoid adjacent channel

interference in the crowded I'f./m.f. band.

Indication of BearingIn all indicators the pointer is aligned in the direction

of the NDB. The angle of rotation clockwise from a

lubber line at the top of the indicator gives the relative

bearing of the NDB. If the instrument has a fixed

scale ii is known as a relative bearing indicator (RBD'

More common is a radio magnetic indicator (RMI)

which has a rotating scale slaved to the compass

heading. An RMI will give the magretic bearing of

the NDB on the scale as well as the relative bearing by

the amount of rotation of the pointer from the lubber

line. Figure 3.8 illustrates the readings on RBI and

RMI for a given NDB relative bearing and aircraft

heading. An RMI normally provides for indication of

Synchro. torque Tx o lP

NDB 2 to r ight

I\A/\A

Ira_I

M,|AAI

AAA -iAA

I

I

l-.-1r i r -

Assume search coi l al igned

NDB 1 to left

Loopr . t .

Switchingvoltage

Balancedmod. O/P

Senser.f .

Compositesignal

DetectedRx out

Referencephase

N.8. Waveshapes and relative timescales are not exactly as shown.

Fig. 3.6 Diagram showing ADF phase relationships

two magnetic headings from a combination of twoADF receivers and two VOR receivers. Figure 3.9shows a typical RMI while Fig.3.l0 shows the RMIcircuit and typical switching arrangements which maybe internal or external to the RMIs.

Sources of System Error

Automatic direction finding is subject to a number ofsources of error, as briefly outlined below.

hdg.

ANDB 1

ANDB 2

with zero bearing

N.D.B.

A

R.M. l .

Fig. 3.8 Diagram of RMI and RBI readings

T116

R . B . I

Astablemult ivibratoro311-312

Fig. 3.7 King KR 85 balanced modulator - simplified

49

NoADF

Night Effect This is the polarization error mentio,nedpreviously under the heading of the loop antenna.The effect is most noticeable at sunrise clr sunsetwhen the ionosphere is changing most rapidly.Bearing errors and instabil ity are least when tuned toan NDB at the low end of the frequency range of theADF.

Coastal Refraction The differing properties of landand water with regard to e.m. ground wave absorptionleads to refraction of the NDB transmission. Theeffect is to change the direction of travel and so giverise to an indicated bearing different from the actualbearing of the transmitter.

Mountain Effect If the wave is reflected bymountains, hil ls or large structures, the ADF maymeasure the direction of arrival of the reflected wave.The nearer the reflecting object is to the aircraft thegreater the error by the geometry of the situation.

Stotic Interference Static build-up on the airframe

V O R N o . l VOR No. 2.

Fig. 3.9 KNI 581 RMI (courtesy King Radio Corp.)

ADF No. 1

No. 1 .cto

No. 2.cto

2 6 V4OO HzRef.

No. 2. R.M.l

t ,/tr\ No' 1 R'M'l'- - - / (w E)

/ Red \9/ A_-A

50

Fig. 3.10 Radio magnetic indicator: simplified circuit

and the consequent discharge reduces the effectiverange and accuracy of an ADF. Thunderstorms arealso a source of static interference which may giverise to large bearing errors. The ability of ADF topick up thunderstorms has been used by onemanufacturer to give directional warning of stormactivity (Ryan Stormscope).

Vertical or Antenna Effect The vertical limbs of thecrossed loops have voltages induced in them by theelectric component of the e.m. wave. If the plane ofa loop is perpendicular to the direction of arrival ofthe signal there will be no H field coupling and the Efield will induce equal voltages in both vertical limbsso we will have a null as required. Should, however,the two halves of the loop be unbalanced, the currentinduced by the E field will not sum to zero and so thedirection of arrival to give a null will not beperpendicular to the plane of the loop. Animbalance may be due to unequal stray capacitance toearth either side of the loop; however in awell-designed Bellini-Tosi system, where each loop isbalanced by a centre tap to earth, this is not a severeproblem.

Station Interference When a number of NDBs andbroadcast stations are operating in a given area atclosely spaced frequencies station interference may

Direction ofarrival

result. As previously mentioned high selectivity isrequired for adequate adjacent channel rejection.

Quadrantal Enor (QE) It is obvious that the twofixed loops must be identical in electricalcharacteristics, as must the stator coils of thegoniometer. If the signal arrives at an angle 0 to theplane of loop A in Fig. 3.1 I the voltage induced inloop A will be proportional to cos 0 and in loop B tocos(90-d) = sin0. If now the search coil makes anangle d with the stator P then the voltage induced inthe search coil will be proportional to(cosO X cos@) - (sinO X sin@) provided there is nomutual coupling between the interconnecting leads.So when the search coil voltage is zero:

c o s O X c o s @ = s i n O X s i n @

or:

cot0 = tan0

and:

0 = 6 + 9 0 + i f X 1 8 0

where y'y' is 0 or any integer. This is simply amathematical model of the situation previouslyddscribed under the heading of the loop antenna.

Now consider the two loops not electricallyidentical so that the ratio of the maximum voltagesinducpd in the two loops by a given signal is r. Thecondition for zero voltage in the search coil is now:

Fig. 3. I I Diagram showing search coil signal as a function ofdirection of arrival

51

c o t O = r X t a n @ '

when

0 = O c o t p = o

therefore

t a n @ ' = e s o Q ' = 9 0 + N X l 8 0

when

0 = 9 0 c o t 0 = 0

therefore

t a n 6 ' = 0 s o 0 ' = 0 + N X l 8 0

In these two cases (also when g = 180 or 270) we

have the same s i tuat ion as bc lore l .e . p= 0 ' s t t no

error.At intermediate angles there wil l be an error' so the

bearing indicated by the search coil wil l be incorrect'

Since this type of error has a maximum value tlnee in

each quadrant it is called quadrantal error'

Now the t.e.m. wave from the NDB will cause r'f '

currents to flow in the metal structure of the aircraft '

Each of the loops will receive signals direct from the

NDB and also re-radiated signals trom the airframe'

Since the aspect ratio of the aircraft fuselage and

wings is not I : I the effect of the re-radiated energy

on th. t*o loops wil l be different: this is equivalent

to making two physically identical loops electrically

dissimilar. The resulting quadrantal error could be up

to 20" maximum.Fortunately, compensation can be nrade by using a

QE corrector loop equalizer and possibly QEcorrection built into the loop. Nt>rrnaliy the combined

r.f. f ield produces a gleater voltage in the longitudinal

loop than in the lateral loop if the loops are identical'

This being the case some loop antennas have more

turns on the lateral loop than the longitudinal locp,

typical correction Uelng t Z|' in the middle of the

quadrants.

Loop Alignment Error If the longitudinal loop plane

is not parallel to the aircraft longttudinal axis then a

constant loop alignment error wil l exist.

Field Alignment Error If the loop antenna is offset

from the aircraft centre line the maxima of the

quadrintal error will be shifted, as will the zeros'

Consequently the situation where the NDB is at a

relative bearing of 0, 90, 180 pr 2?0o will not give

zero error.

Loop Connector Stmy Coupling Reactive coupling

between the loop connections or between external

52

circuits and the loop connections will lead to errors in

the search coil Position.

lnstallation

A typical transport aircraft ADF installation is shown

in Fig.3. l2 ; No. 1 system only is shown, No' 2 being

sirnilir except that different power bus bars will be

used. Main power is 28 V d.c', the 26 V, 400 Hz

being used to supply the synchros' lt is vital that the

26 V 400 Hz fed to the ADF receiver is from the

same source as that fed to the RMI'The loop antenna and its connecting cable form

part of the input circuit of the receiver and so must

irave a fixed known capacitance (C) and inductance

(L). This being so the length and type of loop cable is

ipecified by the manufacturer of the loop. The

length specified must not be exceeded, but it can be

made shorter provided compensating C and L are

correctly placed in the circuit.The QE corrector loop equalizer contains the

necessary reactive components to compensate for a

short loop cable and to provide QE correction' A

typical ciicuit is given in Fig. 3.13. Cl,C2,Ll,L2

una C:, C4,L3, L4 provide compensation (loop

equalization) while L5, L6,L7 provide QE correction

by attenuating the current in the appropriate stator of

the goniometer. The QE corrector loop equalizer is

mounted close to the IooP.Similar considerations apply to the sense antenna

which is required to present a specified capacitance to

the receiver. Again we have a given length of cable

which must not be exceeded but can be made shorter

proviclecl an iqualizer is f itted. Often both an

equalizer and a suscepti-former are used to achieve

the statetl input capacitance to the receiver' The

susceptlformer is a passive matching device which

util izes an auto transformer to increase the effective

capacitance of the sense antenna. Typical units are

shbwn in Fig. 3.14. As an alternative the necessary

matching and equalization may be achieved in a single

sense antenna coupler. The matching/equalizing

unit(s) are mounted close to the antenna'

Tire loop antenna will consist of the crossed coils

wound on a ferrite slab and encapsulated in a

low-drag housing. On high-speed aircraft the loop will

be flush with the skin but on slower aircraft the

housing may protrude slightly, giving better signal

pick-up.- The sense antenna can take many forms' On large

Ft transport aircraft a suppressed c-apacitiveplate is

comrnon, whereas on slower aircraft a 'towel rail'

type of antenna may be used. General aviation

No. 1 28 Vd.c .4OO Hz

Panell igh tssupply

Correctorbox

Senseaerial

Fig.3. l2 Typical ADF instal lat ion

Fig. 3.13 Quadrantal error corector/loop equalizer(straight-through connections not shown)

lnsulatedsense ae.terminal

lnnerscreen

Fig; 3.t4 Sense aerial matchin!

aircraft might use a wire antenna or, as an alternative,

a whip antenna. Some manufacturers now produce a

combined loop and sense antenna for the general

aviation market.The position of both antennas is important. The

loop should be mounted on, and parallel to, the-

cenire line of the aircraft with nomore than 0'25"

alignment error. While the loop may be on top or

INo. IVOR

No.or No.

From Compass2 ADF hdg2 VOR

o i-cc

Sense ae. cableequalizer

53

bottom of the fuselage it should not be mounted nearthe nose, tail, large or movable protuberances or nearother system antennae. Similar considerations applyto the sense antenna, although being omnidirectionalalignment is not a problem. Ideally the sense antennawill be mounted at the electrical centre of the aircraftin order to give accurate over-station turn-around ofthe bearing pointer.

The interconnections in the system must take intoaccount that the phasing of voltages produced bysense and loop antennas will be different for top andbottom mounting. The method used wil l depend onthe manufacturer but if the system conforms toARINC 570 the synchro repeater connections wil l beas in Table 3.1. If, as in some light aircraft

Table 3.1 Synchro connections for alternate aeriallocations. Indicator synchro receiver corrections

Aerial position

ottO'oo'

S ISynchro 52transmitter S 3corrections Rl

R2

s ls2S 3R2R I

S3S2S IR IR 2

S 3s2slR2R I

installations, the goniometer is in the indicator andthe bearing is presented directly rather than by synchrofeed then the following corrections are necessary:

l. loop from top to bottom: longitudinal coilconnections to goniometer stator reversed:

2. sense from top to bottom: search coilconneCtionS reverSed.

Obviously one must check fcrr which positior.r. t()p (rrbottom, the connections are rnadt- in tht supplicdunit.

Protection from interference is of vital intportance.and to this end adequate screening of cables should beemployed. ARINC 570 calls for four individuallyshielded co-axial cables insulated and twisted. thenjacketed. The sense antenna connector should usedouble shielding (tri-axial) cable. Thc. cable runsshould be clear of any high-level trlnsrrritt ing cablesor a.c. power cables.

Controls and Operation

A standard ARINC 570 control prrrci rs i l lustrated irrF ig . 3 .15 .

54

Fig. 3.15 ARINC 570 contro l panel ( typical)

Functiort Switch. OFF-ANT-ADF In the antennaposition (ANT) the receiver operates fronr the senseantenna only, the beaLing pointer being parked at 90"re lat ive bear ing. Thi : posi r ion may be used lbrtuning and NDB, ls t l t ion ident i f icat ion. In the ADFposition signals frorn both loop and sense antennaprovide nolnra l ADF operat ion, the RMI indicat ingthe bear ing o l ' the stat ion.

Fretluenq, St,/ecl (rrobs Three knobs are used; oneis nrorrnted co-arially with the function switch, toselect f requency, in 0.5, l0 and 100 kHz increments.Dig i t r l tvpe t ' requene y d isp lay segments indicate theselected flr 'qrrencv. The information is passed to therecei rer us p l r l lk ' l b .c . t i .

Ilcat F-requt'rtct' Oscillatr'r Sx,itch Selects the BFOlirr use u'hen the NDB selec:ted is identif ied byt ' rn-o l - i - ker ing o l ' the e arr ier .

A nurrrber of other su,itches nray be found onvar ious corr t ro l lers. as bLie l ly dcscr ibed below.

Ftur<'tit,rr Srt,irclr. OFI-'-ANT-ADI:-l.OOP An extraposi t ion r r l ' t l re funct io t r swi tch nta l be provided to()pr'ri le the receiver fronr the loop ;rc.rial only. Thisposi t ion. LOOP. would be used in coniunct ion wi th aIoop contro l .

Ittop Corttrol Spring loaded to ot'f. When op!'ratedclockwise or anticlockwise the search coil rotates inthe selected d i re.c t ion. This contro l c ln br . used fornranual d i rect ion- l ind ing. the- seaLch coi l be ing rotateduntil an audio null is achit 'ved or. i l 'pr,rvidcd. I visualtuning indicator ind icates u nul l . A l thotrgh r tot usedin most rnodern equipments this does huve theadvantage over ADF that the nulls arr. sharper; ADFoperation would lrave to be used to scnse the correctnu l l .

Gain Cotttntl An auil io gain control is usuallyprovided and rnay he annolated volume. On at leastone svstr'nr the gain 0f'tht' R.F. amps is manually

Bottom Bottornloop, loop,bottorn topsense scnse

Toploop,

topsense

Toploop,bottomsense

s ls2s3R IR2

adjustable wherr ANT or LOOP is selected, whereasaudio gain is controlled on ADF.

Beat Frequency Oscillator Tone A rotary switch,giving b.f.o. on-off, and a potentiometer may bemounted on the same shaft turned by the b.f.o.control. When switchetl on the frequency of theb.f.o. can be adjusted, so varying the tone in theheadset.

heselect Frequency Capability Provision can bemade for in use and standby frequencies sele'cted bymeans of a transfer switch. When frequency selectionis made only the standby frequency changes.Switching the transfer switch (TFR) will now reversethe roles of in-use and standby frequencies. Bothfrequencies are displayed and clear annunciation ofwhich is in use is required.

Characteristicrs

The following characteristics are selected andsummarized from the ADF System Mark 3 ARINC570.

Frequency SelectionRange: 190-1750 kHz; spacing: 0.5 kHz; channell ingtime less than 4 s; parallel b.c.d. frequency selectionwith provision for serial b.c.d.

ADF Accuracv+ 2o excluding q.e. for any field strength from50 !V/m to 100 000 pV/m, assuming a sense aerialquality factor of 1.0. (Sense aerial quality factor =effective height X square root of capacitance,i.e.i i-root-cap).13- excluding q.e. for a field strength as low as25 pY lm.+ 3" after q.e. correction.

ADF Huntingkss than I 1" .

SensilivitySignal + noise to noise ratio 6 dB or better with35 pV/m field strength moduiated 30 per cent at1000 Hz and l.ri-root-cap = l '0.

Station InterferenceAn undesired signal from a source 90o to that ofthe.desired signal at the frequencies and relativc signallevels listed in Table 3.2 shall not cause a change inindicated bearing of more than 3".

Receiver SelectivityPassband at least l '9 kHz at -6 dB points not morethan 7 kHz at -60 dB points. Resonant frequencywithin 1 175 Hz of selected frequencv.

Calibration and Testing on the Ramp

Loop SwingThe procedure for determining the sign and size oferrors in an ADF installation is known as a loopswing. On init ial installation a swing should be;arried out at l5o heading intervals. Check swingsshould be carried out whenever called for in themaintenance schedule, after a lightning strike, whenan airframe modification close to the ADF antenna iscompleted or when a new avionic system is installed.The check swing is carried out at 45o intervals. Asving should not be carried out within + 2 h of sunsetor sunrise to avoid night effect.

The loop swing may be carried out in the air dr onthe ground. The advantage of an air swing is that theaircraft is operating in its nclrmal environment awayfrom external disturbances but, in some ways, aground swing is to be preferred, since readings may betaken more accurately. If the loop is mounted on thebottom of the t'uselage the swing may be affected bythe close proximity of the ground, in which case anair swing should be carried out. An installationshould be checked by air test after q.e.s have beencorrected.

Ground Swing A groun{ loop swing must be carriedout at a site known not to introduce bearing errors.A base suitable for compass swings will not necessarilybe suitable for loop swings. A survey using portabledirection finding (D/F) equipment must be carriedout if the site is doubtful.

The loop may be swung with reference to true ormagnetic north. Using true north has the advantagethat the loop swinging base may be permanentlymarked out. If the swing is with reference tomagretic north the loop should be calibrated using a

Table 3.2 Station interference conditions. withreference to desired frequency

Undesired frequency Unde sir e d sig nal s tre ngth

t2kHzt 3 k H zt 6 k H zt7 kHz

-4 dB-10 dB-55 dB-70 dB

55

datum compass, such as the medium landing compass'whic,h should be aligned with the longitudinal axis of

the aircraft and positioned about 100 ft from the

aircraft. To sight the longitudinal ais, sighting rods

or clearly visible plumb lines may be fixed to theaircraft centre line. Use of an upright nose'mountedpropeller and the vertical stabilizer may sufficeproviding sighting is carried out carefully from asuitable distance. For a check swing the aircraft gyro

magnetic compass may be used provided this has been

recently swung, corrected and a calibration chartmade out.

The aircraft must contain its full complement ofequipment. Doors and panels in the vicirfity of the

ADF antennas must be closed. Internal power

supplies should be used whenever possible since the

external generator and lead may cause errors in thereadings.

The ADF is tuned to a station or NDB withinrange and of a known magnetic bearing from the site.With the aircraft on the required number of headingsthe ADF reading and the aircraft magnetic heading

Table 3.3 Loop swing record chart

A/C Tail No ............. A/C Type............-..Date....................,.... Time10.00 Base'.. ' ...........-...... ' .Station - Droitwich Freq.200kHz Mag'Brg (A)029'5

Magnetic heading Autornattc direction Correctiondatum compass linding relative bearing (D)(B) (c)

are recorded on a loop swing record chart, a specimen

of which is shown as Table 3.3.The correction (D) is the sigred angle which must

be added to the indicated magnetic bearing of the

station (B + C) in order to give the true ma^gneticbearing ie). So, for example, adding -5'5o to 41" +

354' gives 389'5' = 29'5o as required.When completed the values obtained in the final

column should be plotted as a q.e- correction curve,

as shown in Fig. 3.16. The average of the absolutevalues of the peaks gives the amount of correctionrequired. the polarity being given by the sign of the

correction in the first quadrant. So in the examplegiven we have:

12.5 + 12.5 + 16 + l7'5 = +14'625"4

as the required correction. The correction is made by

suitable choice of components in the QE corrector

loop equalizer as instructed by the manufacturer.The correction should be more or less the same for

identical installations in a particular aircraft type.

Once the prototype has had a calibration swing and

the component values are chosen, subsequent swings

on series aircraft should show the error bounded byI 3" as required.

Loop alignment error is given by the average of the

peaks. So in the examPle we have:

l2 '5 - 16+ l2 '5 - l7 '5 = -2 '125

4

Since this is in excess of t 0'25o this error should be

taken out by re-alignment of the loop.The line correction = -2'125 has been drawn on

Fiq. 3.16. The correction curve should cross this l ine

atb, gO, 180 urd 270' if there is no field alignment

error. Within the limits of the accuracy of the plot

and the scale we can see that this is the case in our

example. If there were a field alignment error it

would be measured along the horizontal axis.

Air Swing An *ir swing should be carried out in

smooth air conditions in order to eliminate drift

errors. There are various methods which may be

employed but all involve flying a particular pattern

over a clearly defined point or points sgme distance

from the transmitter which is to be used for the swing'

Magnetic heading and ADF relative bearing are noted

forl number of headings, every 10" or I 5o, depending

on the pattern flown. The aircraft should be inland

of the transmitter when readings are taken to avoid

coastal refraction problems' Recording and plotting

is as for the ground swing.

028041054.5073089105'5r22135153t72184198213230242257271.528630231733234s358

001.5354346333318298272247224206197187179169160r481341 1 5088063u503202l

0-5.5- l 1 . 0- l6 '5- 17 .5-14.0-4.5

7 -512.5I1 .58.54.5

-2.5-9.5-12.5- 15 .5- 16.0- 1 1 . 5-0.5

9'512.5t2'5 '

10.5

s6

Fig. 3.16 Quadrantal error correction curve for Table 3.3

Two possible methods are position-line swingingand single-point swinging. With the first method aseries of landmarks which give a ground reference linealigned with a distant transmitter are chosen. Azig-zag pattern is flown, both toward and away fromthe transmitter, the readings being taken'as theaircraft crosses the line at various headings. With thesecond method a clearly defined point, some distancefrom the transmitter, is chosen. A cloverleaf patternis flown centred on this point, readings being taken on

various headinp when overhead of the referenoepoint.

Sense Antenna Capacitance CheckThe total capacitance of the sense antenna and feedershould be checked when called for in the maintenanceschedule. whenever the sense antenna or feeder ischanged, upon initial installation or if a possible faultcondition is suspected. A capacitance bridge or Qmeter operating at 650 kHz should be used for themeasurement.

Functional TestIdeally there will be at least one station or NDBwithin range in each quadrant; in busy regions this willccrtainly be so, for example, at London Heathrowmore than twenty beacons can be received under goodconditions. The true magnetic bearing of thosebeacons to be used must be known. An accuracycheck is performed by tuning into a beacon in eachquadrant and ensuring that for each the pointerindicates the bearing to within the limits laid down inthe procedure, say t 5o. The figure given will only beachievable if the aircraft is well away from large metalobjects, and if the test is not carried out within 2 h ofsunrise or sunset. Use of external power may alsogive erroneous readings.

All controls should be operated to ensure correctfunctioning. In particular if a loop control is^provided the pointer should be displaced 170" firstclockwise and then anticlockwise from the correctreading, to which the pointer should return withoutexcessive hunting.

6'

4 V.h.f . omnidirect ional range(voR)

lntroduction

Prior to World War II it was realized that thepropagation anomalies experienced with low- andmedium-frequency navigation aids l imited theirusefulnes3 as standard systems for a sky which wasbecoming ever more crowded. A system calledfour-course low-frequency range was widelyimplenrented in the United States during the 1930s;this gave four courses to or from each ground stationand fltted in quite nicely with a system of f ixedairways. A problenr with the four-course system isthat each station only provides for two intersectingairways; a more complex junction requires rnorecourses. The above, coupled with increased altitudeof f lying rnaking l ineof-sight frequencies useful atlonger ranges, and the developtnent of v.h.f. comms,led to the adoption of v.h.f. omnidirectional range(VOR) as standard in the Uni ted States in 1946 andinternationally in 1949. The competit ion ftrr aninternational standard system was fierce, the leadingcontender after VOR being Decca Navigator. lt isdebatable whether the technically superior svstemwas chosen, but certainly VOR was cheaper. had theadvantage of a large home market, and has done thejob adequately ever since.

The VOR system operates in the 108-l !8 MHzband with channels spiced at sO kHiffiT[shared with ILS localizer tt. ff i ing allocated to160 of the 200 available channels. Of these 160cfrannels 12O are allocated to VOR stations intendedfor en route navigation while the other forty are fortClrn!ryLYQR statim!_(TVORL The output porverof an en route stat ion wi l l be about 200 W provid insa service up tL49-laulieal n,iles.itTfleouency wilfbe wi th in t l re band I l2- l l8 MHz. A TVOR wi l lhavean output power of Ebgnl5)-t{-providing a service ofup to about ]5_!sg.l[sal miles, its frequency wil l bewithin the bund I9&I2_I4I&, this belng the part ofthe total band shared with ILS localizer.

The crew of an approp.iately equipped aircraft cantune into a VOR station within range and read thebearing to the station and the relative bearing of thestation. Should the flight plan call for an approach

58

to, or departure from, a station on a particularbearing, steering information can be derived from thereceived VOR signals. It is this latter facil i ty whichmakes VOR so useful in airways flying, stations canbe placed strategically along so-called Victor airwaysand the pilot can then, by selection of the appropriateradials, f ly from station to station either by obeyingsteering commands or by feeding the same to theautopilot.

To obtain a position fix liom VOR one needsbearings to two separate stationst when used in thisway VOR can be considered a theta-tl-reta system. If aVOR station is colocated with a DME station an \aircraft can obtain a fix using the pair as a rho-theta isystem. The VOR/DME system ircurrentl iTilinternational short-range navigation standard. lnrecent years this systenr has become even moreversati le u'ith the advent of airborne equipment whichcan effectively reposition an existing VOR/DMEstat ion to g ive a 'phantom beacon'complete wi thradials which can be flown using VOR-derivedsteering inforrnation. This development is consideredin Chapter 12.

Basic Pr inc ip les

A sinrple analogy to VOR is given by imagining alig;hthouse which enrits an omnidirectional pulse ofl ight every time the beam is pointing due north. Ifthe speed of rotation of the beam is known, a distantobserver could record the time interval betweerrseeing the onrnidirectional f lash and seeing the beam,and l 'rence calculate the bearing of the l ighthouse.In reality a VOR station radiates v.h.f. energymodulated with a retbrence phase signal - theomnidirectional l ight - and a variable phase signal -

the rotating beam. The bearing of the aircraftdepends on the phase difference between referenceand variable phases -- t ime difference between lightand beam.

The radiation frorn a qgnventional VOR (CVOR)station is a horizontally polarized v.h.f. wavemodulated as follows:

$el{*,!

Sin pt coc

Cos qrt

Variable d3O Hz a .m.

\a/

Cos nt rcosII

III

2r.3O Hz

2n.1" MHz

t .2 .

Fig.4.l Ground station block diagram, v.o.r,

30 Hz a.m.: the variable phase signal. at Xo magnetic bearing/roz the station the variable9960H2a.m.: th is isazubcarr ier f requency, phasewi l l /agtherefer"encepnut"uy i ; . r igures+. t ,modulated at 30 Hz with a deviation of 4.2 and4.3 illustrate th. b.ii" priniiples.1480 Hz. The 30 Hz signal is the reference The airborne equipment receives the compositephase' signal radiated by ihe station to which the receiver isl02Q Fz a'm': identification signal keyed to tuned. After deiection the various modulating signalsprovide morse code identification at least three- are separated by filters. The 30 Hz reference iigrul isgjnes each 39-s. Where a voR and5ffitre-co-located the identif ication transmissions are

3 .

synchronized (associated identity, seeChapter 7).

4. Voice a.m.: the VOR system can be used as aground-to-air communication channel as long asthis does not interfere with its basic navisationalfunction. The frequency range of the vo]cemodulation is limited to SQOXZ-

The 30 Hz variable phase is space modulated inthat the necessary amplitude variation in the receivedsignal at the aircraft is achieved by radiating a cardioidpattern rotating at 1800 r.p.m. The frequencymodulated 9960 Hz sub-carrier amplitude modulatesthe r.f. at source before radiation. It is arransed thatan aircraft due north of the beacon rryill receiievariable and reference signals in phase, for an aircraft Flg 4.2 Frequency spectrum: CVOR space signals

59

:l

1,sI

REFER€'{CE PHASE

S I G N A L ( F M )

A L L R A O I A L S

FESU LI ilI

R O T A T T N G P A T T E R i l

VARIAEL€ PHASE

S I G N A L ( A M )

I O o R A D i A LI 996otsr SU8CAnRIER

F R € O M O O A T 3 o x t

ib sEcoNo

I stcono

V O L T A G E S A T A I R C R A F T

O N 2 4 O ' R A D I A L

.o r R € c T l O N O F P O S r r V E L O S E

O F R O T A T I N G A T T E N N A

UNIO0ULATE0R O f A T I N G

o I P O L E P A T T € R I

+sEcoro -

-"2to'

R E f E R E T C E P H A S € V O L T A G E

I A F T E R f X O E T E C T I O N )

240"

vaRrAEL€ IHASE VOLTIGE oT A F T E R A T O E T E C T I O t r )

Fig 4.3 Phase relationships, CVOR (courtesy King Radio

Corp.)

= ' .c <

Z l

a

phase compared with the variable signal, the difference'ln

ohase eivine the bearing from the station' The

actual reiding presented to the pilot is the bearing to

the station ralher than from, so if the difference in

ohase bet*een variable and reference signal is 135'

ift" ' to' bearing would be 135 + 180 = 315", as shown

in Fig.4.4.-- liio.p*s information (heading) is combined with

the VORderived bearing the relative bearing of the

60

station ian be presented to the pilot' Figure 4'4

itfurt*t.t thatihe relative bearing to the station is the

Aiii.r.n.. between the magnetic bearing-to the station

.na tn. aircraft heading. An RMI is used to.display

the information. Such instruments are considered in

air;; i.r 3. In this application the card is driven by

th. .n*putt, as normal, so that the card reading at

ttre lubber line is the aircraft heading' At the same

titnt u pointtr is driven to a position determined by

r-f-F-1- -r-o r t t t o n o

a

I

. l

-&a:i*,--

135"(Froml

Fig.4.4 To/from magnetic bearings and relative bearing

the difference b€tween the bearing to the station andthe heading. A differential synchro or resolver is usedto give the required angular difference. Figure 4.5strows the RMI presentation corresponding to thesituation diagram shown in Fig.4.4. Only one pointeris shown, for clarity.

The previous two paragraphs refer to 'automatic'

VOR, so called since the pilot need do no more thansvitch on and tune in to an in-range station in orderto obtain bearing information. 'Manual'VOR

requires the pilot to select a particular radial on whichhe wants to position his aircraft. The actual radial onwhich the aircraft is flying is compared with thedesired radial. If the two are different the appropriate

fly-left or fly-right signals are derived and presented tothe pilot.

A complication is that radial information dependsonly on the phase difference between modulatingsignals and is independent ofheading; hence thefly-right or fly-left information may send the aircraftthe 'long way round'. Further, when an aircraft is oncourse, i.e. the steering command is nulled, theaircraft may be heading either toward or away fromthe station on the selected radial. A TO/FROMindication removes the ambiguity. With the aircraftheading, roughly, towards (away from) the stationand the TO/FROM indicator indicating TO (FROM),the steering information gives the most direct path inorder to intercept the selected radial.

If the reference phase (R) is phase shifted by theselected course (C) and then compared with thevariable phase, a fly-right indication will be given ifR + C lags V, while if R + C leads V, the commandwill be fly-left. If we now add 180 to thephase-shifted relbrence phase we have R + C + 180which will, on addition, either cancel V, partially orcompletely, in whict case a TO indication will begiven, or reinforce V, partially or completely, inwhich case a FROM indication will be given.

Figure 4.6 shows two possible situations. ln bothcases the selected course is 042,i.e. the pilot wishesto fly towards the station on the 222 radial or awayfrom the station on the 042 radial. With aircraft Awe have a fly-left and a TO indication; with aircraft Bwe have a fly-right and a FROM indication. Note thatif the headings of the aircraft were reversed, theindications would be the satne, so sending them the'long way round'. Figure 4.7 shows an electronicdeviation indicator corresponding to aircraft B. Theindication at top right shows the aircraft to be on the022 radial from a second VOR station.

Doppler VOR (DVOR)

The use of CVOR leads to considerable site errorswhere the station is installed in the vicinity ofobstructions or where aircraft are required to fly overmountainous terrain while using the station. Theerror is caused by multi-path reception due toreflections from the obstructions, and gives rise tocourse scalloping, roughness and/or bends when theaircraft is flown to follow steering commands. Theterms used describing the course under theseconditions refer to the nature of the departure from a

straight line course. DVOR is relatively insensitive to

siting effects which would render CVOR unusable.Although the method of modulation is completely

different DVOR is compatible with CVOR in thatFig. 4.5 RMI presentation

61

rightFlv

O42 (Froml

Fly left

From

O42 (To)

R + 1 8 0 0 + 4 2 "

Fig' 4.6 Fly'left/flv-right and'to/from' situation diagram

, i * - :airbome equipment will give the correct indicationswhen used with stations of either type. In the DVORthe reference sigrral is 30 Hz a.m. while the variablesigrral is 30 Hz f.m. on a 9960 Hz sub-carrier. Sincethe roles of the a.m. and f.m. are reversed with respectto CVOR the variable phase is arranged to lead thereference phase by X" for an aircraft at X" magteticbearingfrom the station (cf. CVOR).

In a double sideband DVOR (DSB'DVOR) thecarrier, /", with 30 Hz (and identification) a.m. isradiated from an omnidirectional antenna. Twounmodulated r.f. sideband sigrals, one 9960 Hz above

/c, the other 9960 Hz below /", are radiated fromantennas diametrically opposite in a ring of aboutfifty antennas. These latter sigtals are commutated

62

R + 4 2 0

Fromfly right(R + 42' lags V)

R + 1 8 0 0 t 4 2 '

R + 4 2 '

i l , : ; i i

, ' i '

Tofly left(R + 42' leads V)

at 30 Hz anticlockwise around the ring of antennas.To a receiver, remote from the site, it appears as if thesignal sources are approaching and receding, andhence the received sigral suffers a Doppler shift(see Chapter l0). -With a diameter of l3'5 m androtation speed of 30 r.p.s. the tangential speed at theperiphery is n X l3'5 X 30 = 1272 m.p.s. At thecentre frequency of the v.h.f. band, I l3 MHz, one

rycle occupies approximately 2'65 m, thus themaximum Doppler shift is 127212'65 = 480 Hz.ln the airborne receiver the sidebands mix with thecarrier at /. to produce 9960 t 480 Hz. Singlesideband and altemate sideband DVOR are possible,but since they compromise the performance of thesystem they will not be discussed.

v.-

Fig.4.7 IN-2014 electronic course deviation indicator(courtesy Bendix Avionics Division)

Reference ry'30 Hz a.m.

9960t +8o Hz

Fig.4.8 Frequency spectrum - DVOR space signals

Aircraft lnstallation

Since VOR and ILS localizers occupy the same bandof frequencies they invariably share the same receiverwhich will also contain the necessary circuits toextract the required information. It is notuncommon for v.h.f. comm. and v.h.f. nav. to sharethe same receiver, particularly with general aviationequipment. It is expected that the 'all v.h.f. in onebox' trend wil l continue-

A large airliner, and indeed most aircraft fromtwins up, has a dual v.h.f. nav. installation. The

instruments on which VOR information is displayedare multi-function hence quite complex switchingarrangements are involved. Figure 4.9 shows oneVOR/ILS system of a typical dual instatlation; onlythose outputs from VOR are shown.

The antenna may serve ILS as well as VOR butsome aircraft have separate antennas, particularly ifall-weather landing is a requirement when the.optimum position for the localizer antenna may notsuit VOR. If separate antennas are used with acommon r.f. feed to the receiver the switching logicwill be derived from the channel selection made atthe control unit. The VOR antenna employshorizontal polarization with an omnidirectionalradiation pattern. A horizontal dipole is often usedwith the dipole elements forming a 'V' shape to give amore nearly omnidirectional pattern. Since thedipole is a balanced load and the co-ax. feeder isunbalanced (with respect to earth) a balun (balancedto unbalanced line transformer) is used. The dipolemay be mounted on the vertical stabilizer or on astand-off mast, top-mounted on the fuselage.

The VOR/ILS receiver contains a conventionalsuperhet, a filter for separation of signiils and aconverter to provide the required outputs which aro

l. audio to AIS;2. bearing information to two RMIs;3. deviation from selected radial;4. TO/FROM signal;5. flag or,warning signal.

The RMI feed is the result of the automatic VORoperation. Since the pointer on the RMI moves to aposition giving relative bearing with respect to thelubber line the magnetic bearing (omnibearing) mustbe combined with heading information as previouslydescribed. The necessary differential synchro orresolver will be in the receiver or, in the case ofequipment conforming with ARINC 579,in the RMl.ln the former case magnetic bearing (mag.) is requiredby the receiver, this being obtained from the compasssystem via the RMI. The necessary switching fordisplaying VOR as opposed to ADF information, oneither or both of the pointers, is on the RMI.

The deviation and TO/FROM signals are the resultof manual VOR operation. These steering commandsare displayed on a course deviation indicator (CDI) anelectronic version of which is shown in Fig.4.7. TheCDI, however, may not be a stand-alone unit; it islikely to be part of a multi-function indicator knownby a variety of names such as horizontal situationindicator (HSl) or pictorial navigation indicator PNLIn the installation shown in Fis. 4.9 an HSI is used

63

DME 1

Comprssl1l o

Norm

S/by t ) Test

DME

Power RF

CaptainRMI

ight-:-|F----------,-----

No. 1 VOR/ ILS RxAudio ,.-----f Fl

ToFrom

DEV Flag

FlORMI

transfer relayIJ

oBsCaptain's VOR/ILStransfer relay

lentralnstrumenlrarning panel

rNs Captain's RAD/INSrelay

Captain's HSI

Fig 4.9 Typical VOR installation

Fig.4.l0 KPI 552 pictorial navigation indicator(courtesy King Radio Corp.)

il

Captain'scontrol panel

interohone

with remote course selection (o.b.s. - omnibearingselection) fitted, say, on an autopilot/f l ight directormode select panel. Figure 4.10 i l lustrates theKing KPI 552 PNI where the built- in o.b:s.,iniolporated (course knob), has been set to 335", afly-right command is being given by the deviation barand we have a TO indication (large arrow'head aboveaircraft symbol).

Various switching arrangements are possible; thoseshown il lustrate the captain's choice of VOR/lLS I 'VOR/lLS2 or inertial navigation system (lNS)information being displayed on his HSI. Switchingbetween VOR/lLS I or 2 is achieved by means of thetransfer relay (TFR/RLY) while VOR/lLS or INSswitching is by means of the radio/lNS (R.AD/lNS)

relay. The first officer (F/O) has a similararrangement at his disposal. Deviation signals fromnumber I system to F/O's HSI and from number 2system to captain's HSI may be via isolationamplifiers.

The flag signal is of vital importance since it gives

.ir*

warning of unreliable data from the VOR/ILSreceiver. It wil l be fed to all instruments selected todisplay VOR/ILS information and often to a centralinstrument warning system (CIWS). Should thedeviation signal be fed to the automatic flight controlsystem (AFCS), then obviously so must the flag orwarning signal.

Controls and Operation

The controller is not particularly complicated.Frequency selection is achievetl by roiation of twoknobs, mounted co-axially or separately, sodetermining the appropriate 2/5 code fed to thereceiver. It is normal for DME frequency selectionto be made from the sarne controller with DMgstandby, normal and test switching also provided(s-ee Chapter 7). Self-test switching faciities for theYOR/ILS are provided. Additiona] controls, otherthan those shown in Fig. 4.9 , may be provided,namely VOR/lLS on-off and audio vo-lume.

. The location of display switches and the courseselector has been tnentioned previously, as has theinterpretation of indications given to itre pilot.

Simplif ied Block Diagram Operation

Received signals are selected, amplified and detectedby a conventional single or doubie superhet receiver.The detected output is a composite signal which mustbe separated into its cornponent parts by means ofappropriate fi l te ring circuits.

. The audio signal, 1020 Hz identif ication, is routedvia an amplifier and possibly a volume contiol on thev.h.f. nav. controller to the flight interphonesub-system of the AIS. The associated audio filtermay be switchable to give a passband of 300-3000 Hzwhen the VOR system is being used as a ground-to-aircommunication link.^^ lh9_reference phase channel (CVOR) consists of a9960 Hz filter, a discriminator to deteci the 30 Hzl.m. and,- not shown, amplifier circuits. Limiting ofthe signal takes place before the discriminator to re-move unwanted amplitude variatipns. The 30 Hz ref-erence signal (R) then undergoes various phase shifts.

For manual VOR operation, as previouslymentioned, we need to shift R by the selectldcourse. This is achieved by the phase-strift resolver,the rotor of which is coupled tqthe course or OBSknob. A digital readout of the selected course isprovided. The phase-shifted R is now compared withthe variable phase sigrral. If they are in phase or lg0"

out of phase there is no lateral movement of thedeviation bar. Since it is simpler to determine whentwo signals are in phase quadrature (at 90.) a 90"phase shifter may be inclutled in the reference channelprior to feeding the phase comparator wheredetection of phase quadrature will give no movementofthe deviation bar. In the absenci ofeither or bothof the signals the flag will be in view.'To'or 'from' information is derived by comparingthe variable phase with the reference phase shifted bithe OBS setting plus l80o It follows-thar if thereference phase has been shifted by 90o beforefeeding the deviation phase comparator we onlyreqrrire a further 90" phase shift before feeding the'to/from'phase

comparator rather than a lgOdphaseshift as i l lustrated in Fig. 4.1 l. If the inputs to the'to/from' phase comparator are within, say, + g0o ofbeing in phase then a TO indication is given; if withint 80'of being in anri-phase a FROM intication isgiven; otherwise neither a TO nor a FROM indicationis given.

For automatic VOR operation the referencechannel is phase-shifted and compared with thevariable phase. If the two inputs are in phasequadrature there is no drive to the motor, otherwisethe motor will turn, changing the amount by whichthe reference phase is shifted until phase quadrature isachieved. The motor connections are arranged so thatthe stable null of the loop gives the requirei shaftposition which represents the magnetic bearing to thestation. Compass information is fed to a differentialsynchro, the rotor of which is turned by the motorfollowing station magnetic bearing. ThL differencesignal represents relative bearing which positions theappropriate RMI pointer via.a synchro repeater. TheRMI card is positioned by compass information.

Characteristics

The following characteristics are selected andsummarized from ARINC characteristic 579-1 . lishould be noted that there are radical differences inoutputs, between ARII{C 579-l and the olderARINC 547 with which many in-service systemsconform.

Frequency Selection160 channels, 50 kHz spacing, range 108-1 17.95 MHz.Standard 2/5 selection system.Channelling time less than 60 ms.

ReceiverSatisfactory operation with 1.5 pV sigral.

65

T o A I S

Ref. channel

Deviationbar

Flag

To/From

cDt/oBS

- - - - - - - l

Compass

Fig.4.l I Simplified block diagram' VOR

Nomore than6dBa t tanua t i ona t / . i | 7k i z ;a tana logues ig ra lp rooo r t i ona l t ope rpend i cu la r l i nea rreast 60 dB attenuatio n at f "t 31 '5 kHz'

" i"pr1""l'it'-i: "t:mmfll fri?:n|;Tfii:"'station is requtrt

outputs . ̂ ^n .nn , o*'' the spaiing of which is an analogue of the

Audio: at least 100 mW into a load of 200-500 Q ffiil; 'Tt;digh-leutl

output should give 2 V across

from a 3 pV input ,ignuirnoaututed 30 per ..nt'lt i ziio ii r*a.fo'?"ou'* deviation adjustable within

1000 Hz. I modulated

it

t:: -:- "' ;;;; t s to t lb nautical miles' rhe corresponding

omni-bearing: digital ouJput in fractional blli:'fo'* io*i'u'r ":f:;.'lfi1.Ti;:'%tti\;b"o*;l*"

#:'fi:iJffift,:ft1,;Tlsts":lT{:: :J"'i,*:::i1!";;;;;:;r'.'^'n3-n14'reverou'�pu'�voltages, one proportiori?,rr."rt. of the bearing, i;;; ;t used by the automatic flight control system

the other to its cosine. -ih.

.nulogu. ou,put i, ""'"'

Gat-l 19 moi.iocprt' In the event of loss of

desiened to feed "t OBS ;i;l''"'*irnu*'of t*o iistanct infotmation from the DME the deviation

purau.l connected nr'" friiJn'.;. not lnt"r.t uigruute o;tp"t should. automatically revert to an angular

*i*, Wts used with ARINC 547' .

'r^6-*-'- atui'tion *"d;;'f;;;;'n er-vi1g^z v for 10" off

Deviation: a trigh-levef ani'f-.*-i**f d'c' voltage "out"' (Note: iti"i to ARINC 579 deviation outputs

66

Fig.4.l2 Ttc T-308 VOR/ILS test set (courtesyTel-lnstrument Electronics Corp.)

represented angular displacenrent.)TO/FROM: ground referenced providing 2 mA foreach of two 200 Q loads in parallel. In addition alowlevel output of 200 gA may be provided to feedolder instruments.Warning: high level, 28 V d.c. valid, absent invalid.low level, between 300 and 900 rnV valid, less than100 mV invalid. The low level signal should becapable of driving frorn one to five 1000 Sl parallelloads. The VOR digital output should also includewarning bits.

Ramp Testing

Testing of VOR should always be carried out with aramp test set capable of being tuned to any VORfrequency, radiating sufficient energy to allowsatisfactory operation of the VOR and providing ameans of simulating various VOR radials. Most testsets include provision for testing ILS as well as VOR.Among those available are the Cossor CRM 555.IFR NAV4OIL.

The CRM 555 operates on any of the 160 VORchannels with a frequency accuracy of10.0035 per cent (--10'C to +30oC). Modulation ofthe carrier is such that the simulated bearing may beset to any reading between 0 and 360o with acalibration accuracv of t l" or mav be switched in45o steps with an u..uru.y of t 0.3". Carrier powercan be attenuated in I dB steps between 0 dBm and-120 dBm (0 dBm corresponds to an output ofI mW). A self-test facility is provided. TheNAV-401L offers similar facilities but is more 'state

of the art' and so offers Stightty more in the way ofperformance.

The TIC T-278 is part of the T-308 test seti l lustrated in Fig. 4.12. The facil i t ies are not as'extensive as either of the previously mentioned testsets but it has the advantage ofease of operation andless cost. It is FCC type accepted. Operation is on108'00 MHz radiated from a telescopic antenna.Bearings of 0,90, 180 and 270" canbe simulatedboth TO and FROM, alternately variation, 90-l l0'' to' or270-290o 'from', is available. A J lo switch

*:;

6'

gives a useful sticky needle check. set attenuator or moving the test set antenna furtherActual testing should be carried out in accordance away. Various bearings should be simulated (check

with the procedures laid down, but briefly it would whether they are 'to'or 'from' station), theinvolve correctly positioning the test set antenna and appropriate reading should be checked on the RMIradiating on sufficient frequencies to test frequency and the OBS operated so as to check the manualselection of the VOR. Sensitivity may be checked by mode of VOR.reducing the r.f. level received either by use of the test

68

5 lnstrument landing system

Introduction

In order to be able to land the aircraft safely undervisual flight rules (VFR), i.e. without any indicationfrom instruments as to the aircraft's position relativeto the desired approach path, the pilot must have atleast 3 miles horizontal visibility with a ceiling notless than 1000 ft. Although most landings are carriedout under these conditions a significant number arenot; consequently, were it not for instrument aids tolanding a considerable amount of revenue would belost due to flight cancellations and diversions.

One method of aiding the pilot in the approach toan airport is to use a precision approach radar (PAR)system whereby the air traffic controller, having theaircraft 'on radar', can give guidance over thev.h.f.-r.t. The alternative method is to provideinstrumentation in the cockpit giving steeringinformation to the pilot which, if obeyed, will causethe aircraft to make an accurate and safe descent andtouchdown. The latter, which may becomplemented/monitored by PAR, is the methodwhich concerns us here.

Early ILS date back to before World War II; theGerman Lorentz being an example. During the warthe current ILS was developed and standardized inthe United States. The basic system has remainedunchanged ever since but increased accuracy andreliability have resulted in landing-minimumvisibil i ty conditions being reduced.

The ICAO have defined three catesories ofvisibil i ty. the third of which is subdivided . Allcategories are defined in terms of runway visual range(RVR) (see ICAO Annex 14) and, except CategoryIII, decision height (DH), below which the pilot musthave visual contact with the runway or aboit thelanding (see ICAO PANS-OPS). The variouscategories are defined in Table 5.1 where thestandards are given in metres with approximateequivalents in feet (in parentheses). Sometimescategories IIIA and B are called'see to land'and'see to taxi'.

The ILS equipment is categorized using the sameRoman irumerals and letters according to its

Table 5.1 ICAO visibility categories

Category d.h. r.v.r.

IIIIIIAI I IBIIIC

8 0 0 m (2600 ft)l 200 ft)700 ft)l 50 ft)

400 m200 m3 0 m

Zerc

operational capabilities. Thus if the ILS facility iscategory II, the pilot would be able to land theaircraft in conditions which corresponded to thosequoted in Table 5.1. An obvious extension of theidea of a pilot manually guiding the aircraft with noextemal visual reference is to have an autopilot which'flies' the aircraft in accordance with signals fronr theILS (and other sensors including radio alt imeter) i.e.automatic landing.

Basic Principles

Drectional radio beams, modulated so€s to enableairborne equipment to identify the beam centres,define the correct approach path to a particularrunway. In addition vertical directional beamsprovide spot checks ofdistance to go on the approach.The total system comprises three parts, each with atransmitter on the ground and receiver and signalprocessor in the aircraft. Lateral steering is providedby the localizer for both front-course andback-course approaches; the glideslope providesvertical steering for the front course only whilemarker beacons give the distance checks.

[,ocalizerForty channels are allocated at 50 kHz spacing in theband I 08' 1 0- l I I .95 MHz using oniy those liequencieswhere the tenths of a megacycle count is odd; so, forexample 108'I 0 and 108. I 5 MHz are localizerchannels while 108.20 and 108'25 MHz are not.Those channels in the band not used for localizer are

60 m (200 ft)30 m (100 ft)

69

allocated to VOR. The coverage of the beacon willnormally be as shown by the hatched parts of Fig.5.1, but topographical features may dictate arestricted coverage whereby the ! 10" sector may bereduced to l8 nautical miles range.

Azimuth

Fig.5.l Localizer front beam coverage

The horizontally polarized radiated carrier ismodulated by tones of 90 and 150 Hz such that anaircraft to the left of the extended centre line will bein a region where the 90 Hz modulation predominates.Along the centre line an airborne localizer receiverwil l receive the carrier modulated to a depth of 20per cent by both 90 and 150 Hz tones. Deviationfrom the centre l ine is given in d.d.m. (difference indepth of modulation), i.e. the percentage modulationof the larger signal minus the percentage modulationof the smaller signal divided by 100.

The localizer course sector is defined as that sectorin the horizontal plane containing the course l ine

(extended centre line) and limited by the lines onwhich there is a d.d.m. of 0'155. The change ind.d.m. is l inear for I 105 m along the l ineperpendicular to the course l ine and passing throughthe IIS datum point on the runway threshold; thesepoints 105 m fronr the course l ine l ie on the 0'155d.d.m. l ines, as shown in Fig. 5.2. The beacon issituated such that the above criterion is met and thecourse sector is less than 6'. Outside the coursesector the d.d.m. ' is not less than 0 '155.

The ICAO Annex 10 specification for thelocalizer-radiated pattern is more complicated thanthe description above indicates, in particular in thevarious tolerances for category I, II and iII facil i t ies;however we have covered the essential points for ourpurposes.

The airborne equipment detects the 90 and 150 Hztones and hence causes a deviation indicator to showa fly-left or fly-right command. Full-scale deflectionis achieved when the d.d.m. is 0'155, i.e. the aircraftis 2-3o off course. Figure 5.3 shows a mechanical andelectronic deviation indicator both showing slightlyover half-scale deflection of a fly-right command.Provided the pilot flies to keep the command bar atzero, or the autopilot f l ies to keep the d.d.m. zero,the aircraft will approach the runway threshold alongthe course line.

In addition to the 90 and 150 Hz tones thelocalizer carrier is modulated with an identificationtone of 1020 Hz and possibly (exceptionally categoryIII) voice modulation for ground-to-aircommunication. The identif ication of a beaconconsists of two or three letters transmitted by keyingthe 1020 Hz tone so as to give a Morse coderepresentation. The identification is transmitted notless than six times per minute when the localizer isoperational.

GlideslopeGlideslope channels are in the u.h.f. band,

Bercon

0.155 DDM

Course sector < 6o

15O Hz > 90 Hz

150 Hz < 9O Hz

70

Fig. 52 Localizer ooursc,lelector

ILS datum point 0'155 DDM

Fig. 5.3 Electromechanical and electronic course deviatronindicators (courtesy Bendix Avionics Division)

speci f ica l ly 328.6-335.4 MHz at 150 kHz spacing.Each of the forty frequencies allocated to iheglideslope system is paired with a localizer frequency,the arrangement being that localizer and glidesiopebeacons serving the sarne runway. wil l have liequenciestaken from Table 5.2. pilot selection ol'the requiredlocalizer frequency on the controller wil l cause'bothlocalizer and glideslope receivers to tune to theappropriate paired frequencies.

Table5.2 Localizer/glideslopefrequency priring (MHz)

Localizer Glidepath

Lot'alizer ()li<lepatlt

l0e.-s0109.5 5l0e 70109.75109.90109.95I 1 0 . 1 0l 1 0 . 1 51 1 0 . 3 0I 1 0 . 3 5I l0 '50I 10 .55l 1 0 . 7 01 1 0 . 7 sI10 .90l 1 0 . 9 5l l l . l 0l l l ' 1 5I I 1 . 3 0l l 1 . 3 5I I 1 . 5 0l I 1 . 5 5I I 1 . 7 0I I 1 . 7 5I I 1 .90I l l . 9 s

332.60332.3s333.20333.05333.80333.65334.40334.25335.00334.85329.60329.45330.20330.05330.80

- 330.65331 .703 3 1 . 5 5332.30332'15332'90332.75333.s033 3 .3 53 3 l . 1 0330.95

1 0 8 . 1 01 0 8 . 1 5t08 .30r08.35r 08 's0r0u .55r08.70108.75108.90108.95109 '101 0 9 . 1 5109'30109.35

334.70-r,r4.553 3 4 . r 033.r.95329.90329.7 5330.50330.35329.30329.1533 r .403 3 1 . 2 5332.00331 .85

71

The principle of glideslope operation is similar to

that of localizer in that the carrier is modulated with

90 and 150 Hz tones. Above the correct glidepath

the 90 Hz modulation predominates while on the

correct glidepath the d.d.m. is zero, both tones giving

a 40 per cent depth ofmodulation. The coverage andbeam characteristics shown in Figs 5.4 and 5.5 aregiven in terms of the glidepath angle, typically2|-3'. Category I facilities may have asymmetricalupper and lower sectors, the figure of 0'0875 d.d-m-corresponding to an angular displacement of betweert0'070 and 0'140 0. By contrast a category III facil i tyis as shown in Fig. 5.5 with a tolerance of + 0'02 0 onthe 0'12 0 l ines.

Although d.d.m. = 0lines occur al2 0,30 and 4 0thev are not stable in the sense that if the pilot obeys

the steering commands he will not maintain the

corresponding angles of descent. The first stable nullo".un at S 0 wtrich for a glidepath of 3" is at l5o.

This is sufficiently different from the desired descent

angle to create few problems;however to avoid

confusion the glideslope beam should be 'captured'

from below.Once in the correct beam fly'up and fly-down

signals are indicated to the pilot in much the same*-uV ur with the localizer. Figure 5.3 illustrates a

fly-up command of just over half-scale deflection.The glideslope output is more sensitive than localizer

in that typically a |' off the glidepath will give

full-scale deflection (about 0'175 d'd.m.) comparedwith about 2t off the course line for full'scaledeflection.

Marker BeaconsA marker beacon radiates directly upwards using a

carrier frequency of 75 MHz. The modulating signal

depends on the function of the marker.An airways, fan or'Z'marker is a position aid for

en-route navigation located on airways or at holdingpoints. As such it is not part of ILS. The carrier ismodulated with a 3000 Hz signal which causes awhite lamp to flash in the aircraft while stationidentification in Morse code is fed to the AIS.

The outer marker is normally located 4f milesfrom the runway threshold. The carrier isamplitude-modulated by 400 Hz keyed to give twodashes per second which can be heard via the AIS andcauses a blue (or purple) lamp to flash.

The middle marker is located 3500 ft from therunway threshold. The carrier is amplitude'modulatedby 1300 Hz keyed to give a dot-dash pair 95 times per

minute which can be heard via the AIS and causes anamber lamp to flash.

The ILS marker beam widths are sufficiently wide

in the plane perpendicular to the course line to coverthe course sector.

Simplified Block Diagram Operation

Since the localizer and VOR frequencies occupy thesame band it is normal to have a v.h.f. navigationreceiver which selects,.amplifies and detects signalsfrom either aid, depending on the frequency selected.

Figure 5.6 illustrates the basic block diagram of alocalizer receiver. A conventional single or doublesuperhet is employed. A.g.c. is important since anincrease in the 90 and 150 Hz output signals by thesame factor would increase the magnitude of thedifference, so giving more deflection of the deviation

DDM : 0.0875

I ton'n I

Course line

Azimuth

Fi;. 5.4 Glideslope coverage

Fig. 55 Glideslopebeam characteristics

72

o.0875

3OO-3OOO Hz

Fig. 5.5 Localizer simplified block diagram

I Deviation indicator

Fig. 5.7 King KN 72 band pass filter, simplified

indicator for the same d.d.m. Sigral separation isachieved by three fi l ters: audio,90 and 150 Hz. Theaudio signal, identification and possibly voice, ispassed via audio amplifiers (incorporating a noiselimiter) to the AIS. The 90 and 150 Hz iignals arefull wave rectified, the difference between-therectif ier outputs driving the deviation indicationwhile the sum drives the flag out of view.

The 90 and 150 Hz fi l ters, together with therectif ien and any associated circuitry. are often partof the so-calied VORi LOC converter which may bewithin the v.h.f. navigation receiver or a separaie unit.A combined converter wil l usually enrploy activefilters which serve as either 30 Hz bandpiss filters forVOR operation or 90/ I 50 Hz band pass fi l ters for

r__ _ ___ :=___ __J

localizer operation. Figure 5.7 shows the circuit usedin the King KN 72 VOR/LOC converter. When aVOR frequency is selected the ILS Hi line is low, soturning off Ql which effectively disconnects R2 fromthe circuit, the centre frequency of 30 Hz is set byR3. Selection ofa localizer frequency causes the ILSHi l ine to go high, so turning on Ql and placing R2 inparallel with R3. R2 is set to give a centre frequencyof 90 or 150 Hz as appropriate.

The glidepath receiver converter block diaeram issimilar to that of the localizer except that the-audiochannel is not required. A separate receiver may beused or all navigation circuitry may be within thesame unit. In any event separate antennas are usedfor localizer and glidepath.

35:t

White

Blue

Amber

To AIS

Fig.5.8 Marker simplified block diagram

The marker is fixed tuned to 7 5 MHzand may

employ a t.r.f. (tuned radio frequency) or superhet

.r..iu.r. The detected audio is t-ed to three filters for

tone separation and also amplified and fed to the AIS'

The filtir which gives en output causes the

appropriate lamp-switching circuit to give an - .interrupted d.c. output to drive the associated lamp'

When sq'itched to Hi the sensitivity of the receiver

is such that it responds to airways marker beacons

even though the iircraft is at a relatively high

altitude. Wittr trigtr sensitivity there is a danger that

when at lower altitudes, for example when flying over

the outer and middle markers on approach, the lamps

may be l it for longer than the maximum of 10 s' It is

even possible for the outer and middle marker lamps

to be lit simultaneously. To avoid this, low sensitivity

is selected, whereby an attenuator (i0 dB) is piaced in

line with the receiver input. Switching may take

place at 10000 ft.

"l*': J'

Installation

in Chapter 4 an installation incorporating a

VOR/IiS receiver was discussed and illustrated

(Fig. a.9). In considering ILS we are interested in

itro"se ouiputs derived from the localizer' glidepath

and marker receivers. Localizer and glideslope

deviation (fly-leftifly-right, fly-up/fly-down

respectively) will be fed to a conventional or

Lt.it.onic ieviation indicator (Fig' 5 '3) and/or an HSI

(Fig. a.l0) and an attitude director indicator' ADI

ieii. s.sl. In the HSI the localizer drives a iateral

aril.tion bar right and left of the course arrow while

glideslope deviation is given by a deviation pointer

74

Fig.5.9 Typical attitude director indicator

and scale conventionally on the left-hand side of the

instrument. tn the ADI the localizer drives a rising

runway laterally tc display deviation (vertical .movement representing radio altitude) while the

glideslope drives a pointer over a scale, again on the

left-hand side of the instrument'

Localizer, glideslope and marker signals are also

fed to an autoland system when fitted' The localizer

deviation will be used to supply the appropriate

ciemand signal to the roll (aileron) and yaw (rudder)

channels. ihe pitch (elevator) channel wil l respond

to glideslope. As the aircraft approaches touchdown

thJresponie of the pitch channel to glideslope

deviatiln signals is progressively reduced; this

reduction is triggered by the outer marker and thencecontrolled in accordance with the radio altimeteroutput. A modern ILS will provide dual paralleloutputs for both localizer and glidepath deviation inorder that the AFCS. may accept information onlywhen the same signal appears on each feed of aparallel pair.

A general aviation installation is illustrated inFig. 5.10 incorporating King equipment. TheKX 175B is panel-mounted and contains a720-channel v.h.f. comms receiver, a 2O0-channelv.h.f. nav. receiver and all necessary controls withdigital readout of comm. and nav. frequencies on thefront panel. The KX 175B also provides tuninghformation for the DME and glideslope receiver.

F8. 5.10 Kirg general aviation comm./nav. system

The KN 72 and KN 75 are remote-mountedVOR/LOC converter and glideslope receiverrespectively. The KN 72 gles localizer deviation andflag signals (as well as VOR deviation, TO/FROM andflag), while the KN 75 gives glideslope deviation andflag. The KMA 20 is an audio control consoleproviding speaker/phone selection for seven receivechannels and mic. selection for two transmit channelsas well as containing a marker receiver plus its controlsand lamps. The indicator,Kl 206, shows localizerdeviation (vertical bar) and glideslope deviation(horizontal bar) as well as showing VOR deviationand TO/FROM indication if a VOR frequency isselected, the deviation relating to the OBS setting alsoon the KI 206. lf a Kl 204 is used instead of theKI 206 then the KN 72 may be omitted since aVOR/LOC converter is built in.

Typically a deviation indicator movement will beof 1000 S) impedance and require 150 pA forfull-scale deflection (f.s.d.), therefore the voltageacross the deviation output of the receiver should be150 mV for a d.d.m. of 0.155. I f the receiver

deviation output circuit has an output impedance of200 O and supplies the required current to fiveindicators in parallel then when less than fiveindicators are used the deflection will not properlycorrespond to the d.d.m. Consider a d.d.m. of 0.155,then 750 pA must be supplied for five loads inparallel from a generated deviation voltage of300 mV. Now consider four loads fed from a 300mV,200 Cl source, the total current will be300 x 103(200 + 250) = 666.7 ptA divided equallyamong the four loads so that each load has666'7 l4 = 166'7 pA, i.e. the indicators will over-readby about I I per cent. Unless the receiver output is aconstant voltage for a variety of loads (ARINC578-3) we must comp€nsate for loading variations.

Various methods have been used for loadingcompensation in the past. One possibility is tochoose different receiver output impedancesdepending on the number of loads;in this case thereceiver and the mounting rack should be suitablylabelled. Another possibility is to fit a shunt resistorin an aircraft junction box through which the deviationsignal is fed. With two indicators a 330 O shuntwould be needed giving a 330 O and two 1000 Qloads in parallel, i.e. total load of 200 (-). Finally,but not exhausting the possibilities, five separatebuffered outputs may be provided, each indicatorbeing fed from one of the buffer amplifiers. Similarconsiderations apply to flag circuits where usingfour 1000 Q loads in parallel is standard procedure.

Antenna arrangements vary between differenttypes of aircraft. Mention of combined VOR/

Phi l l ip3 hcad scrcw(16 pt.ccs)

Fig. 5.t I Boeing747 localizer aerials (note Bendix weatherradar scanner with spoiler grid on parabolic reflector formapping purposes - see Chapter 9). (Courtesy BoeingCommercia l Aeroplane Co.)

Lo[r l@rlirctantennt

75

localizer antennas has been made in Chapter 4 butthe glideslope and marker antennas will always beseparate. As an example of a large passenger aircraft,consider the Boeing 747. Three VOR/ILS receiversare installed fed by one V-type VOR antenna at thetop of the vertical stabilizer, two dual localizbrantennas in the nose and a total of six glideslopeantennas in the nose-wheel doors. One markerbeacon receiver is instalied, fed by a flush-mountedantenna on the bottom centreline of the aircraft.

The localizer antennas are mounted above andbelow the weather radar scanner. The lower antennafeeds receivers I and 2 while receiver 3 is fed fromthe upper antenna. Antenna switching between VORand localizer aerials is achieved by either solid state orelectromechanical switches mounted behind theVOR/ILS receivers.

The six glideslope antennas are split into twogroups of three, one group in each nose-wheel door.A non-tunable slot (track antenna) dual unit is

installed in each door leading edge while two tunablearrays (capture antennas) are mounted on the sides ofeach door. A total of four hybrid antenna couplerscombine the r.f. outputs of the glideslope antennasproviding suitable impedance matching.

Controls and Operation

Normally a combined VOR/ILS/DME controller isemployed (Fig. a.9). Such a controller is brieflydescribed in Chapter 4. The marker receiverswitching is likely to be remote from the combinedcontroller and its action has been described above.

In use the glidepath should be captured frombelow, approaching from a direction determined bythe approach procedures for the particular airfield.The marker sensitivity should be on low for theapproach. The appropriate selection should be madeon the audio control panel.

VOR/ILS receiver No. 3

Glide slopeantennahybrids8554 and 8555and att€nuators8582 and 8583

Trrck antennacoaxial cable

G/S trackantennaB 561

Antenna coaxialconnector D8561

Capture antcnnacoaxial cable

{ Coaxialcable9uard Antcnna coaxial

connectors D8558

recbiver No. 2

G/S capturcantenna 8558

Right aftnose gear door

See@rG/S captureantenna 8558

G/S capture antenna hyg49-{-\8560

c/S;;t';; --

\. .antenna array \\

t \t - Attachins

Tuning "trg, . ]\'*. - screws (5 places)

-.\\. . \ \ .

S rwo

Aft door shown in retracied position(6t\J

Fig.5.l2 Boeing ?47 glideslope aerials (courtcsy BoeingCommercial Aeroplane Co.)

76

Track antennas

Capture antonnas

Track aerials

Fig.5.l3 Boeing 747 simplified glideslope aerial couplingalrangements

Characteristics

The basis for the following is ARINC Characteristic578-3 although much of the detail has been omittedand not all sections covered.

Units Tlte receiver should contain all the electroniccircuitry necessary to provide deviation and flagsignals for both localizer and glideslope. The controlunit should provide for frequency selection of ILS,VOR and DME using 2/5 coding.

Antennas Separate localizer and glideslope antennasshould be provided covering the appropriatefrequency bands (108.00-l12.00 MHz and328'6-335.4 MHz respectively) and both havingcharacteristic impedances of 50 O with a VSWR ofless than 5: l .

Power Supply I l5 V, 400 Hz, single phase.

Localizer Receiver Forty channels at 50 kHz spacing108'00-l I l'95 MHz. Maximum channel time 60 ms.Selectivity is such that a carrier modulated 30 perc€nt at 1000 Hz should provide an output at least60 dB down when separated from tuned frequencyby t 3l'5 kHz; response should be within 6 dB when

carrier within I l2 kHz of tuned frequency.Sensitivity is such that the flag should clear with a5 pV 'hard' input sigral ('hard' pV: the output of asignal generator calibrated in terms of open circuitload). The receiver should be protected againstundesired localizer signals, VOR sigrals and v.h.f.comm. signals. The a.g.c. should be such that thereceiver output should not vary by more than 3 dBwith an input signal level range of l5-100 mV.

Gli<leslope Receiver Forty channels at 150 kHzspacing, 328'6-335.4 MHz. Channels to be pairedwith localizer channels for frequency selectionpurposes. The selectivity is specified in a similar wayto localizer but the 60 dB points are at + 80 kHzwhile the 6 dB points are at t 2l kHz. The flag shouldclear with a20 pY'hard'.sigral. Protection againstunwanted glideslope sigrals must be guaranteed. Thea.g.c. should be such that the input signal level to thetone filters should not vary by more 163n +| to-2 dB for an increase in input from 200 to20 000 pV and should not vary by more than *3,-2 dB thereafter up to an input of 100 000 pV.

Deviation OutputsLocalizer:.high-level2 V for 0.155 d.d.m., low-level150 mV for 0.155 d.d.m. Dual outputs in parallel for

Right door

77

AFCS. Output characteristics should not vary forloadS between 200 Q and no load. When 90 Hzpredominates the 'hot' side of all deviation outputs

should be positive with respect to the 'common' side;

in this case 'fly-left'is given.Glideslope: similar to localizer but high- and lowleveloutputs are 2Y and 150 mV respectively for0 ' 175 d .d .m .

Flag Outpu* Two highJevel warning signals (super

flag) and one lowlevel waming signal should beprovided by both localizer and glidepath receivers-The high-level flag characteristic is 28 V d.c. for valid

status with current capabilities; 25 mA for AFCSwaming; 250 mA for instrument wamings. Thelow-level flag should provide a voltage of between300 and 900 mV into up to five parallel 1000 Qloads.

MonitoringWarning sigtals when: no r.f., either 90 or 150 Hzmissing, total depth of modulation o1'composite90/150 Hz signal is less than 28 per cent, etc.

Ramp Testing

A radiating test set must be used with a basiccapability of simulating off-glidepath signals. Inaddition the test set should operate on one or moreaccurate spot frequettcies and provide facil i t ics fordeleting either of the modulating frequencies.

TIC T-308 This test set was mentioned in Chapter 4in connection with VOR testing. In addition to theVOR test set module we have the T-268, T-288 andT-298 for testing the marker, localizer and glideslopereceivers respectively. The T-268 provides at least7C per cent modulation for the 400, 1300 and3000 Hz tones. The T-28B operates on 108' I MHzand can s imulate 0 d.d.m.,0 '155 d.d.m. lef t and r ig ,ht(switched) or 0 to t 0'199 d.d.m. (variable). TheT-298 ooerates on 334'7 MHz and can simulateb a.a.m.,0.175 d.d.m. up and down (swi tched) or0 to t 0'280 d.d.m. (variable). Either the 90 or the150 Hz tones may be deleted with both the T-28Band the T-298.

Cossor CRM 555 Forty localizer and forty glideslopechannels may be selected, all crystal controlled.There are seven d.d.m. settings for localizer-simulateddeviation and five for glidepath, the d.d.m. switch

being marked in decibels, e.g.:

6'6 dB fly-right (+ 0'1549 d.d.m.)'4 '0 dB f ly- le f t ( - 0 '092e d.d.m.) '3 ' 76 dB f l y -up (+ 0 ' 175 d 'd .m ' ) ' e t c .

Further switch positions on the d.d.m. switch allow

for deleting one or other of the tones. ln addition a

variable 0 to 1 150 gA deviation is available. Stepped

attenuators provide output levels variable between

0 dBm and - 120 dBm in I dBrn steps, in order that

receiver sensitivity may be checked (test set aerial

positioning wil l affect this check). Modulating tones

of 400. 1300 and 3000 Hz are available fbr marker

checks. Finally, l0l0 l- lz modulation is available for

audio checks. As menticlned in Chapter 4 the

CRM 555 can also be used to check VOR.

IFR NAV402 AP Conttins a modtrlated signal

generator for marker, VOR, localizer, glideslope and

communicat ions test ing. The output <-r l ' the test set is

variable between 7 and - I l0 dBnr on all frecluencies

set by a var iable t )equency contro l (phase- locked at

25 kHt. on eaclt blnd except for glidepatlt whcre

irrterval is 50 kt-lz). The localiz-er deviatit ln can be

swi tched to 0 '09-1. 0 '155 or 0 '100 d.d.nr . whi le

g l i des lope d .d . t n . o f l e r s 0 ' 091 ,0 '175 and 0 '400 '

Tone deiet ion can be selected. Al l t l r rec r t tarkcr toncs

are av l i lab le. as is 1020 t lz . for lud io chcck '

htrcedure The prdredure for a lunctional check is

straightforward if the operation of ILS is understood

and full details of the test set are known. In practice,

the procedure wil l be l isted in the aircrl lt nlaintcnance

manual. Careful attention must be paid to test sct

antenna positioning if receivers with low sensitivity

are not to be passed as serviceable. Self-test facil i t ies

on both the test set and the aircraft installation should

be used if available.

WM,n*#]"F r c r 6 . ,

- E F f , G r , t + - ' ; - , ,Fig. 5.14 NAV402 AI ' tcst sct (cotrr lcsy l l rR l ' l lcctrontcs

l nc . )

78

6 Hyperbolic navigation systems

General Principles

The need for a co-ordinate system for navigationpurposes is self-evident, the most important being thegeat circle l ines of longitude and the l ines of latitudeparallel to the equator, itself a great circle. Figure 6. Iillustrates two alternative svstems suitable for use inndio navieation.

Fig.6.l Circular and hyperbolic co-ordinate systems

If two fixed points on earth have a sequence ofconcentric circles drawn around them, each circlerepresenting a particular range from the fixed centre,then points of intersection are defined but ambiguousexcept on the line joining the two points (base line)where they are uniquely defined. Such a systern iscalled rho-rho since two distance (rho) measurementsare involved.

We can use the concentric circles to definehyperbolic lines. Where any two circles intersect wewill have a difference in range defined; for example,the range to point A less the range to point B. I,helocus of points which have the same difference inrange wil l describe ahyperbola. Thus in Fig. 6.1 thehyperbolic l ine hh' is the locus of the point X suchthat AX - BX = constant. By plotting the l ines forseveral different constants we obtain a family ofhyperbolic lines. In the radio navigation systems to

be discussed we use the terms circular l.o.p. (lines ofposition) and hyperbolic l.o.p.-

The patterns considered are not suitable forposition fixing since two circular Lo.p. intersect attwo places whilst knowing the difference in range totwo points simply places one anywhere on one of twohvperbolic l.o.p. Knowing the starting position andsubsequently the track and ground speed (or headingand true airspeed) will make it possible to use therho-rho system, since a position caiculated by deadreckoning will identify at which of the intersectionsthe aircraft is. To use the hyperbolic l.o.p. we mustgenerate another family of lines by taking a thirdfi;red point, we then have the co-ordinate systemshown in Fig.6.2. A fix is given by the unique pointwhere two hyperbolic l.o.p. cross. Of course the useof three fixed points gives the possibility of arho-rho-rho system where three range circlesintersect at a unique point.

Fig.6.2 Hyperbolic navigation position fix (courtesy LittonSystems International Inc., Aero Products Division)

The co-ordinate patterns described above arecurrently used in three radio navigation equipments,namely Loran C, Decca Navigator and Omega.Predecessors of these systems include GEE, a BritishWorld War II hyperbolic system developed to navigatebombers on missions to Germany.

79

It should be clear by now that the requirement ofa hyperbolic system is that it can measure differencein range while a rho-rho-rho system must nleasureabsolute range. Two methods are in use: timedifference measurements for Loran C and phasemeasurements for Decca and Omega. These methodsdictate that Loran C is a pulsed system while theother two are continuous wave (c.w.).

A basic problem with phase measuring systems isthat range can only be determined if the wholenumber of cycles of e.m. radiation between theaircraft and the transmitting station are know. This isillustrated in Fig. 6.3. An aircraft at X measures thephase of the signal from station ,4 which is

Fig. 6.3 Received signal phase measurement

transmitting at a frequency of l0 kllz. Thewavelength, L, is given by C/ I 0 000 whe re C is thespeed of l ight; thus tr = l 6 nautical nri les. lf thephase nreasured is. say t)0", the distancc,4X is( l6// + 4) nautical rniles where N is the nurnber ofwlrole cycles occupying lhe space betwccn A and X.Wc- sav that thc lane width is l6 nautical rniles andthc l ircr:rl ' t is (N + i-) l:rnes l 'ronr A.

Continuous Wave Hyperbolic PrinciplesWit l r l hypcLtro l ic svstcnr wc arc concerned wi thdil ' lr 'rcrrcc in nrngc nrthcr than ubsolute rurge:consctyrrcrrtly t lrt ' uirlrornc' crluiirrnent rnust nleasurethc' rl i l ' l i 'rcrrcc in phlsc bt'twcen rrdio waves fronr twolnrnsrr r i t l i r rg grot rnt l s tat iorrs . l " ' igure 6.4 shows t l ta ttlrcrc rvil l br. ' zcrrr plrlse dil ' lert 'ncc bctwcensv rrchltlrt izctl t nrnsrrtissions r'vc ry lr l l [ ' l wavelength.

An lircrl l ' i nrcusuring a phlsc dil ' t i 'rcncc ol-0A d l | corr ld bc on uny o l ' thc drs l r rd l .o .p. . i .e . inany o l ' t l rc I lncs l rc tweerr l rarrsrr r i l tcrs A and l l . eachol 'which is h l l l 'a wuvclcngth widc at t l re b lsc l ine.Sincc 'cvcry lanc is i t lc r r t ica l to t l r r ' recc iver on theluircrl l ' t a lunt' corrnt nrusl bc cstlblishcd cit lrcr l l 'ornlhc l i rcru l ' t 's s t l r t i r rg poin l or , t lur ing l l ight . l ' ronr anindcpcndt'rrt position l ix. l i lch lane rrray be

80

'Race ivor

l0^-06l./LOPS

Fig. 6.4 Continuous uiave hyperbolic navigation(courtesr- Litton Systelns International Inc., Aero ProductsDivision)

subdivided into say. centi lanes ( | / l00th of a lane)and so determining on which l.o.p. the aircraft isl lying is sirnply a matter of lane and centilane countingliorrr sonre known point. A l ix requires a separatecount to be rnade of the lanes and centilanes betweenanother pair of transnlitters. onc transmitter nlay beconlrnor.r to the two pairs. The two l.o.p' wil lintersect ai the aircraft 's position.

The possibil i ty of lane slip exists; i.e. missing I lane

in the count. lf this happens the correct lane tttttst beestablished, this prt 'rcess being terrned laning.Obviously laning is easier wlten l iures are widcr.Suppose the frequency of transmitter A is l0 kHzwhile that of B is l5 kHz. t lten we have a dit ' ferencefrequency ot'5 kt{z which corresponds to a lane widthof 30 000 nr as opposed to l5 000 m for l0 kHz andl0 000 m for l5 kHz. In this way lane width can bemade wide without having to transmit impossibly lowfiequencies. While the use of wide lanes is ofinrportance for t lte purposes of laning. narrow lanesgive greater resolution and hence greater potentialaccuracy.

Measuring the dift-erence in phase between signalsfrom two transntitters wil l only be nreaningful if thetransnrissions ltave a knowtr and t ' ixed phaserelationship. Tyo possibil i t ies exist:

l. one of't l 're transntitters can be tlesignated themaster, the other the slave which. cln receivittgthe transnrission frotrt t l tc' ntaster. wil l ensulcits own transtnissiott is synchronized:

2. both transntittcrs are synchr-oniz-ed to sotrrestandard tinre scale such as provided by anatont ic c lock.

Prrlsed Hyperbolic PrinciPleslrr such systertts laning is.rrot a problcltt sitrcc'

,fiTg

'r" r"l

T = 2 t + dExtended - -base line Master

Fig. 6.5 Pulsed hyperbolic navigation

unambiguous l.o.p.s are obtained. Consider the twotransmitters at A and B in Fig. 6.5. One is designatedthe master; this transmits pulses of energy at a fixedpublished p.r.f. On receipt of the master pulse theslave will transmit, usually after some fixed delay, sayd ps. If the propagation time from master to slave isI gs then we c.ul see that an airCraft at the masterstation position, or anywhere on the extended baseline outward from the master, will measure a timedifference of (2t + d) gs when comparing the tinre ofarrival of the master and slave transmissions. Anaircraft on the extended base line outward from theslave would record a time difference of d ps. Shouldan aircraft be anywhere other than on the extendedbase line the time difference will be some uniquereading between d and (2t + d) ps.

Disadvantages of hyperbolic systems are that lanewidth varies with distance from base line and thatsignal geometry is important. With a hyperbolicco-ordinate system the angle of cut between two l.o.p.can be such that the tangents to the lines at theaircraft position are almost parallel; for other aircraftpositions the hyperbolic I.o.p. may cut almost atright angles. Of course if more than two l.o.p. areavailable the geometry problem is of littleconsequence since a most probable position can becomputed. Figure 6.6 illustrates the various l.o.p.geometries.

Continuous Wave Rho-Rho and Rho-Rho-RhoSystemsTo measure the phase of a received sigral a suitablereference must be available, generated within thereceiver. Let the phase ofthe reference signal be @r,the phase of the received signal be @" when theaircraft is at point A and fu when the aircraft is at

d < T < 2 t + d

Extended -- - - - - t : oSlave base line

L.O.P. = line of constant timedifference. T '

) . P . 2

accurate

L .O.P. IBest

Worst geometry pooraccuracy

Mult ip le L.O.P. goodaccuracy

due to redundancY

Fig. 6.6 Various geometrics for hyperbolic systenrs

point B. The airborne equipment wil l tncasurc tlredifference in phase between the receivcd signal andthe reference signal, i.e.:

Q ^ = Q r - Q r

when the aircraft is at point A, while:

Q m = h - Q ,

when the aircraft is at point B. The change in phascas the aircraft moves will provide a measure of thechange in range, we have:

@ . : ( 0 " - Q ) - ( 0 b - 0 ' )= Q a - Q b ( 6 . 1 )

These ideas are illustrated in Fig. 6.7.The above working has assumed that the reference

sigrral does not drift in the time it takes for theaircraft to travel from A to B. If the reference phaseis @r. and @16 when the aircraft is at point A and Brespectively, then equation (6.1) must be modified

L.O.P. 3

L.O.P.

geometry very

81

to account for this drift, it becomes:

Q ^ = ( 0 ^ - @ u ) - ( 0 ' " - 0 ' u ) (6.2)

Thus an equipment continuously monitoring thechange in measured phase in order to calculate changein range will be in error by an amount depending onthe reference oscillator drift.

At the moment of switch-on the reference signalon board the aircraft is not phaseJocked to thegound transmitter's frequency. Further, since thereis a signal phase shift due to the transmission path,there will be a phase or clock offset (@o) betweenreceived signal and the local reference. If atswitch-on both the transmitter and receiver positionsare known @o can be calculated, and if at subsequentaircraft positions this phase offset remains the samethen by measuring phase difference as describedearlier the change in range from the known startingpoint may be computed. Reference oscillator driftcan be considered as a change in phase offset'

Errors arising due to a change in phase offset canbe minimized in three ways:

l. usc difference in phase between synchronizedsigrrals from two remote transmitters, in whichcase any change in reference phase cancels out(this is the hyperbolic approach);

2. a precision reference oscillator of atomic clockstandard can be carried on the aircraft, in whichcas€ drift is negligible over the duration of theflight (this is the rho-rho approach);

82

+ 0 ^ : Q a - 6 u

Fig. 6.7 Change in measured phase with afucraft moveinnenl

A t B

3. estimate the phase offset throughout flight by

utilizing signals from more than twotransmitters (this is the rho-rho-rho approach)'

The operation of a rho-rho system is illustrated in

Fig. 6.8. As the aircraft flies from I to 2 the phase

changes in the signals received from transmitters A

and B are continuously measured; this allows theairborne equipment to count the number of range

lanes and centilanes traversed with respect to bothtransmitters. Equation (6.1) applies, since reference

oscillator drift is negligible. Thus if the aircraftposition at point I is known it can be computed at 2.

\ d I A - ( 6 A 2 - a L O 2 ) - l c l 1 1 - d s 6 1 ) l 6 M O ' l d 3 2 - O ! O 2 l - t o g l - C 1 g 1 ;

. t . ' 1 2 - 6 A 1 t - { 6 1 9 2 - c L g 1 ) - t c 6 2 - c a 1 l - 1 6 1 9 2 - { 1 6 1 1

rclE LGAL mnLUrOn OrrFt ' t6LO2 - 6LOtl

Fig. 6.8 Rho-rho navigation (courtesy Litton SystemsInternational Inc., Aero Products Divisio')

With a rho-rho.rho s),stenl two range (circular)l.o.p. give a position fix while a third can be used toeliminate error in phase offset, @". ln Fig.6.9 it canbe seen that a non-zero @, gives us the situation wherethree l.o.p. do not intersect at one point but-form atriangle within which the aircraft is positioned.

hyperbolic and rho-rho-rho methods require threeground transmitters for a fix but the geonretry of theaircraft and transmitters will degrade the accuracy ofthe hyperbolic system more so than either rho-rho-rhoor rho-rho. A hyperbolic systenr has the mostcomplex computer program, but is nevertheless

oTransmitter 2

,'\1x Transmitter

True posit ion

--- Calculatcd L.O.P.- True L.O.P.

Fig. 6.9 Rho-rho-rho navigation

Consideration of this position trianglc shou.s that theperpendieular d is tances l ior r r t l rc ' t rue posi t ion to thecalculated l .o .p. ure equal to e lch othr . - r lnd g ive anlcesurc of ' f " . Sul ' l ic icnt i r r t i r rnrat iorr is avai lablel ionr the three l .c l .p . to evaluate p" . assunt ine thatrr-fercnce oscil lator drift is the orrlv sorrrce of t 'rror.Should other errors contr ibute to the calculatedl .o.p. the perpendicrr lar d is tances l ' rom the t ruep,osi t ion to the calculate d l .o .p. wi l l no l nccessar ih, beL-qual and an approxinrate solution nrust be sought.

Cdmparison of SystemsThere is no clear-cut best system to enrploy, and infact all are in use as follows:

pulsed hyperbolic I-oran Cc.w. hyperbolic Decca Navigator, Omegac.w. rho-rho Omegac.w. rho-rho-rho Omega

It is interesting to observe that manufacturers ofOmega navigation systems have opted for differentmethods of calculating position, i l lustrating thatthere is no universally accepted best method.

The rho-rho method is certainly the simplest ofthe three, needing only two qround transmitters andemploying a relatively simple cornputer program.It does, however, have the costly disadvantage ofrequiring a very stable reference oscillator. Both

True posit ion

P t c - - -

P . P^ P . Kncwn points

probably the least costly since its local oscillatorstabil ity requirenrents are less stringent than even therho-rho-rho system.

Omega Navigation System (ONS)

Omega is a very low-frequency. c.w., long-rangenavigation system. Three time-multiplexed signals ofl0 '2 , I l '33 and l3 '6 kHz are t ransn-r i t tedomnidirectionally from each of eight stationsstrategically located around the world. Although theconcept was patented in 1923 it was not unti l themid 1960s that the US Navy established the firstexperimental stations. By 1968 it was establishedthat ONS was t'easible and the setting-up of aworldwide network commenced. The USA isresponsible for the stations in North Dakota, Hawaii,Liberia and a temporary station in Trinidad, whilestations in Norway, Japan, Argentina, La Reunionand, by 1980, Australia are the responsibil i ty ofnations which have established bilateral agreementswith the USA. Although the responsibility forco-ordination was originally allocated to the US Navyit has now been taken over by the US Coast Guard.

'The Omega Stations and Broadcast PatternsEach station has a transmitter Dower of l0 kW with

, \

83

Table 5.1 Sigral format, o.n.s.

Stations

{_- Ios1-2 s I '0 sH <'_}

0 ' 9 s<---'

1 '0 s<|*.'

l ' l sH

1.2 sH

I ' I s<----}

0 ' 9 se

NorwayLiberiaHawaiiNorth Dakota[: ReunionfugentinaTrinidad/AustraliaJapan

I 1 .3313.6 I I ' 3310.2 13.6

10.2

ABCDEFGH

r0.2 r 3 '6t0.2

I 1 . 3 313.6to.2

I 1 . 3 313.610.2

I l -3313.6ro.2

I 1 . 3 313.6to.2

I 1 .33r3.6 l 1 . 3 3

the exception of the temporary station in Trinkladwhich has a I kW transmitter. Radiation is from anomnidirectional antenna which takes the form eitherof a vertical tower, approximately 450 m high,supporting an umbrella of transmitting elements, or avalley span typically 3500 m in length. Equipmentredundancy ensures reliable operation 99 per cent ofthe time.

As mentioned abdve each station transmits threefrequencies in a time-multiplexed pattern which isunique and provides identification. The transmissionformat is shown in Table 6.1. It can be seen that atany one time only three stations will be transmitting,each on a different frequency. There are shortintervals (0'2 s) between transmission bursts. Thepattern is repeated every l0 s.

All transmitters are phase-locked to a nearlyabsolute time standard provided by the use of atomicclocks at each of the station locations. The result isthat the three frequencies in all transmitterssimultaneously cross zero with positive slope atprecise times every l5/ l Tths of a millisecond. Thisphase relationship is i l lustrated in Fig. 6.10. The netresult is that the timing error between stations is atmost I ps, leading to a maximum error in position fixof 300 m.

hopagationThe band of frequencies l0-14 kHz is an appropriatechoice for a phase-measuring navigation s1)'stem sincee.m. radiation at these frequencies can travelthousands of miles with predictable phase-changecharacteristics. A natural waveguide is formed by theearth's surface and the D layer ofthe ionosphere, thedimensions of which are suitable for. propagation ofthe ONS frequencies. This mode of propagationaccounts for the range of the signals and the

84

Fig. 6.10 Omega frequency relationships (courtesy Litton

Systems International Inc., Aero Products Division)

Fig. 5.1 I Earth -.Ionosphere waveguide (courtesy LittonSystems Interpational Inc., Aero Products Division)

predictability of the changes in phase.A requirement of the system is that four or more

stations can be received everywher€. Account mustbe taken of the attenuation of the signal which varieswith direction due to the rotation of the earth.Signals travelling in an easterly direction sufferapproximately 2 dB/1000 km attenuation, whilethose on a westerly path suffei approximately

1 6 N M 9 C Y C L E S

1 4 { N M l o c Y c r t s

4 dB/1000 km. North and south attenuation is thesame at 3 dB/1000 km. A further consideration isthat signals cannot be used close to the source sincethe phase variations are unpredictable in this region.The implementation of ONS with eight stations, whichare not equi-spaced around the world, leads to asituation where, under normal conditions, betweenfour and seven stations are usable depending on thereceiver location.

Fig,6.12 Typical usable coverage

Factors Affecting Propagation

l. Diurnal EffectThe height of the ionosphere varies by approximately20 km from day to night, being highest at night. Thephase velocity of the propagated wave will be greatestduring the day when the dimensions of the 'waveguide'

are least; this leads to phase variations whichfortunately are predictable and cyclic. Corrections tocompensate for diurnai effect may be implementedby means of a software routine. The entry of GMTand date at switch-on is required bv the routine.

2. Ground ConductivityThe different attenuating effects of the oceans andvarious types of landmass changes the phase velocityof the v.l.f. signal. The greatest loss of signal strengthoccurs in the ice-cap regions where the change inphase velocity is significant. Water has least effect.The effect of ground conductivity being well known

means that a conductivity map can be stored in thecomputer, so enabling a propagation-correction factorto be calculated for the path between the receiver andthe known station location.

A complication arises in that it is possible toreceive a direct sigral and one which has gone the'long way round', in which case we have a mutualinterference problem. Automatic deselection ofstations at ranges in excess ofsay 8000 nautical milesis used to minimize this effect.

3. Geomagnetic FieAThe earth's magnetic (H) field alters the motion ofions and electrons in the lower region of theionosphere, thus affecting v.l.f. propagation. Againthe equipment software may be used to applycorrections.

4. Norspheroidal EffectsThe computation of aircraft position must take intoaccount that the signal path from transmitting stationto aircraft receiver is not on the surface of a sphere.Further, pressure differences at various latitudeseffect the height of the ionosphere so compensationmust be made for the effect on phase velocity.

5. Modal InterferenceThere are various modes of propagation in theearth.ionosphere waveguide. If one mode isdominant the phase grid produced will be regular;however in practice a competing mode can be almostequal to the dominant mode in which caseirregularities appear in the phase pattern. The mostserious case occurs when one mode is dominant atnight and a second during the day. It follows thatduring sunrise and sunset the two modes will beequal. Some Omega receivers automatically deselectstation B (Liberia) at critical times since signals fromthis station are particularly susceptible to modalinterference at night.

6. Solar EffectsA solar flare gives rise to a large emission of X-rayswhich causes a short-term disturbance in a limitedpart of the ionosphere. Suih an event is called asudden ionospheric disturbance (SID) or a suddenphase anomaly (SPA) and may last for I h or more;l.o.p. in the affected regions may be shifted by up tosay 5 nautical miles. These SIDs occur about 7 to l0times per month, but during the peak of the I l-yearsunspot cycle a major solar flare may product a shiftin l.o.p. by up to 15 nautical miles. This latter eventis predictable, and warnings may be issued.

Infrequently large quantities of protons are

85

released from the sun, producing a so-called polar capdisturbance (p.c.d.). The effect of a p.c.d., which isto shift l.o.p. from say 6 to 8 nautical nriles, may lastfor several days. Only those transmission pathspassing over the poles are affected. Since the p.c.d. isof long duration navigation warning messages may bebroadcast.

Position FixingAs previously discussed ONS may use hyperbolic,rho-rho or rho-rho-rho methods, a root mean squareaccuracy of l-2 nautical miles being obtainable withall methods providing the computer software correctsfor predictable errors.

Whatever method is used the lane in which theaircraft is flying must be established. Lane widths forthe basic frequencies and difference frequencies aregiven in Table 6.2. It can be seen that the broadestlane for the direct ranging methods is 144 nauticalmiles while that for the hyperbolic method is 72nautical miles. If it is known which broad lane theaircraft is in then it is possible to resolve laneambiguity for the narrower lanes automatically, asshown in Fig.6.l3. In this example it is known in

Table 6.2 Frequencies and lane widths, o.n.s.(lane widths in nm)

Dircct ronging ltltperbolic

which 3.4 kHz lane the aircraft is flying. Phasemeasurement of the l0'2 kHz signal gives threepossible l.o.p. while the l3'6 kHz signalgives tburpossible Lo.p. Only one of the possible Lo.p. fromeach group is coincident, this being the unique l.o.p.on which the aircraft is positioned.

Rate AidingThe ONS transmission pattern extends over a periodof l0 s. If the phases of all usable signals aremeasured over this period and then l.o.p. aregenerated for position fixing. an error wil l result.since some of the phase information wil l be up tol0 s old. Aircraft direction and speed informationniay be used to update the phase inforrnation lorl.o.p. calculations, this process being known as rateaiding. In practice we can generate l.o.p. at less thanl0 s intervals, say every I s, thus ONS can beconsidered as a dead reckoning system withposition-fixing updates every second or so.

Direction and speed information can come from anumber of sources, for example compass heading andtrue air speed from an Air Data Conrputer or trackand ground speed tiom Doppler or INS. Some Omegaequipments generate track and ground speedinternally from computed position changes.

If for any reason there is a loss of signal deadreckoning, data on direction and speed inputs otlast-known internally generated track and groundspeed can be used to continuously calculate theaircraft 's position. so that on receipt of sufficientusable signals, lane ambiguity is easily resolved.Obviously if the internally generated last-known trackand ground speed are used during dead reckoningthen aircraft manoeuvre during tlr is phase.may causelaning problems when signals lre receiveti aglrin.

Basic

frequ encies

10.2 kHz 161 1 . 3 k H z 1 4 . 413.6 k l ' l z 123.4 kHz 4ti2.3 kHz '12

l . l k t l z 1 1 4

13 .6 -10 .2 =l3 '6-1 I 3 =I 1 . 3 - 1 0 . 2 =

87 . 262436i 2

13.6 KHZLOP

' 13 6 KHZ lanes Broad lane 10.2 KHZ lanes

Unique LOP

10-2 KHZLOP

Fig: 6.13 Resolving lane ambiguity (courtesy Litton Systemslnternational Inc., Aero Products Division)

86

In such a case aircraft approximate position wouldhave to be entered by the pilot.

Most Probable PositionThere is a redundancy in the Omega system in thatnormally more signals will be received than arenecessary to compute the two l.o.p. needed for a fix.ln this case, data from all receivable stations, and asmany frequencies as possible, may be used to generatea number of l.o.p. If all frequencies are received fromall stations there would be 3 X 8 = 24 phasemeasurements every l0 s, giving up to twenty-fourl.o.p. for a single fix. The multiple l.o.p. wil l notcross at a point but will define a small polygonwithin which the aircraft is positioned. The computerwill calculate the aircraft's most probable positionwithin this polygon.

In practice there will be far fewer than twenty-fourphase measurements available. Automatic deselectionwill take place for reasons of poor signal to noiseratio, poor geometry, susceptibility to modalinterference or outside usable range (too close or toofar). Manual deselection will be accomplished as aresult of pre-flight or in-flight information receivedconcerning station status or unusual ionosphericactivity.

Communication Stations, v.l.f.A worldwide high-power military communicationsnetwork operating in the band l5-25 kHz ismaintained by the US Navy. As a secondary purposeof the network is to provide worldwidesynchronization of time standards, the carrier signalsare precisely timed, and so may be used fornavigationpurposes. Since control of the stations is out of thehands of those bodies, either national or international,responsible for civil aircraft navigation, use of thenetwork for navigation can only be considered assupplementary to other forms of navigation.

Hyperbolic navigation is not suitable for use withv.l.f. comms stations since absolute phase differencesbetween two received signals cannot be determineddue to each station operating on an unrelatedfrequency. A further disadvantage is that the diurnalphase shifts are not as predictable for v.l.f. signals asthey are for ONS sigrals.

Several manufacturers offer equipment with v.l.f.and Omega capabil ity; in some cases v.l.f. is optional:In such equipment Omega signals provide the primarynavigation information while v.1.f. signals provideback-up should insufficient Omega signals be usable.

InstallationA typical simplified installatibn diagram is shown in

Fig. 6.14. The ONS consists of a receiver processorunit (RPU), control display unit (CDU) and antenna

. coupler unit (ACU). Such a break-down of 'black

boxes' conforms to ARINC Characteristic 599 butsome manufacturers choose to separate the receiverand computer and also the antenna and coupling unit.

The RPU is fitted in a convenient location. themost important consideration being coolingarrangements, a forced downdraught or integalblower being typical. The CDU must of course bemounted in view, and in reach, of the pilot;normally special cooling arrangements are notrequired.

The antenna used may be of H field or E fieldtype, the latter possibly employing a separate couplerunit with a supplied interconnecting cable. An Efield system is sensitive to precipitation staticdischarge, thus good bonding and sufficientstrategically spaced static wicks are essential. An Hfield system is sensitive to magnetic (a.c.) noisesources and a skin mapping should be carried out oninitial installation to determine the optimum locationfor the antenna which may be on the top or bottomof the fuselage.

Skin Mapping Detailed procedure is given inmanufacturer's literature, but basically the aircraftshould be parked away from all power lines, bothabove and below ground, and away from allobstructions. Ambient signal plus noise is theninitially recorded approximately 100 ft from theaircraft with a spectrum analyser set at l0'2, I l '3 andl3'6 kHz. Similar measurements are made with anACU secured by tape at various airframe locations.Comparison of ambient and airframe measurementswill identify several possible positions. The optimumposition(s) can then be found by repeating themeasurements under various on and off conditionsofengines, lighting, electronics and fans. The finalposition should be checked out for signal to noiseratio with engines running at 90 per cent minimum.

Brief Description of UnitsThe descriptions which lollow are based on the LittonLTN-2 I l; other systems have similar units which varyin detail.

Receiver hocessor Unit The RPU is the major partof any ONS. Omega broadcast signals from the ACUare processed together with inputs from other sensorsto give present position and guidance parameters asrequired. The major parts of a RPU will typically be:

r.f. circuitry;

cl

InstallationProgram

A t cSpeed Source

Fig. 6.14 Litton LTN-21I ONS installation (courtesy LittonSystems International Inc., Aero Products Division)

Autopilot

H.S. t .

F .D. t .

AircraftData Bus

AircraftInstrumentation

88

central processor for computing function;scratch-pad RAM for temporary data storage;special RAM in which pilot-entered data is saved

during power interrupt;ROM to store program which will incorporate

corrections;power supply assembly;analogue interface;digital interface;BITE;antenna switching;.chassis.

bnnol Display Unit T\e CDU provides theinterface between the flight crew and the ONS. Datatransmission between CDU and RPU is via twoone-way serial digital data buses. The RPU transmitsfour 32-bit words to the CDU while the CDUtransmits one 32-bit word to the RPU. The d.c.voltages for the CDU are provided by the RPU. TheCDU annunciators are driven by signals from theRPU.

Antenna Coupler Unit Two H field bidirectionalloop antennas are wound on ferrite rods arranged atright angles to each other. Pre-amplification of thesignal takes place in the ACU. Provision is made forthe injection of a test signal to each loop.

ONS Interface

Operator Inputsl. Present position latitude and longitude: entered

during initialization, i.e. during preparation of thesystem prior to take off;

2. waypoint latitude and longitude: rrp to nineentered as required during initialization; editingfacility available for in-flight entry;

3. Greenwich Mean Time/date: entered duringinitialization.

Extemal Sensor Inputsl. Speed: from air data computer (ADC) or Doppler

radar in a variety of sigral formats;2. heading: from compass system;3. drift angle: from Doppler radar, optional;4. speed valid sigral;5. heading valid signal;6. compass free/slaved input;7. oleo strut switch input;8. drift angle valid signal.

Other Inputsl. Frequency standard: rho-rho opti6n;

2. 26V a.c. 400 Hz reference: from externalequipment accepting synchro feeds from ONS;

3. aircraft data bus: interface with digital air datasyst€m (DADS), inertial reference system (lRS),flight muragement computer system, etc.

In s t alla tio n Pro grammingVarious receiver processor unit connector pins,termed 'program discrete pins' are grounded bymeans of a link to earth in order to select:l. Speed input format;2. frequency standard;3.magnetic/trueheading input/output;4. oleo strut logic;5. synchro output;6. gid mode: local, Greenwich or t$ro alternatives;7. antenna mount: top/bottom.

System Outputs

Analogue/Discretel. Track angle;2. cross track deviation;3. track angle error;4. drift angle;5. track angle error plus drift angle;6. true heading:7. desired track angle;8. track change alert;9. track leg change;

10. steering sigral (roll command);I l . To/ f rom.

Digital12. Present position (lat./long.);13. heading (mag./true);14. track angle;15. ground speed;16. distance to waypoint;17. t ime to GO;18. wind angle;19. wind speed;20. cross track distance:21. track angle error;22. drift angle;23. desired track.

Warning24. Cros track deviation failure;25. true heading warning;26. steering signal warning.

Figure l.l3 defines pictorially those outputs relatingto angle and distance.

89

C-ontrols and OperationFigure 6.15 shows the Litton LTN-21I CDU which issimilar to those of other manufacturers. A verycomprehensive range of controls and displayed data isavailable. Brief details only are given here.

The pilot is able to enter his present position andup to nine waypoints defining a great-circle navigationflight plan which can be updated during flight'Information displayed to the pilot helps him to flythe specified route from waypoint to waypoint or flyparallel offsets from the flight plan. If the autopilotis engaged steering information from ONS causes theaircraft to automatically follow the flight plan, inwhich case the display is used for monitoringpurposes.

The system also demands entry of GMT and date.A keyboard is used for all data entry which ischecked for operator error, in which case a codedwarning is given. System failure waping is given bythe WRN annunciator when malfunction and actioncodes may be displayed. Several other annunciatorsgive waming of track leg change imminent (ALR),system in dead reckoning mode (DR), synchronizationof system with transmitted signal format taking place(SYN),lane ambiguity (AMB) and manually ent;redtrue airspeed, magnetic heading or cross track offsetbeing displayed (MAN).

90

Alcrt ann(amber)

Dead reckoningann (amber)

Sync ann(amber )

Ambiguityann (amber)

Warn ann( red)

Manua l ann(amber)

Hold pushbutton(greenl

Clcar pushbutton(green)

The following information may be displayed byappropriate positioning of the display selector switch:

GMT/DAT Greenwich Mean Time and dateTK'GS Track angle and ground speedHDG/DA Heading and drift angleXTK/TKE Cross track distance and track angle errorPOS Present positionWPf Waypoint (selected)DIS.TIME Distance and time (to'go')WIND Windditection and velocityDTK/STS Desired track and status (malfunction)MH/TAS Magnetic heading and true airspeedSTA Station statusFROM/TO From and to waypoints for current leg.

System SoftwardThe major tasks for the software employed in arho-rho-rho Omega system are described brieflybelow.

Synchronization The transmissiori pattern must be'identified in order that the ONS will know when each

station is broadcasting. Since the Omega transmissionpatiern repeats every l0 s synchronization is attemptedby ampling l0 s of data in order to try and find thestait time (station A transmitting lO'2 kHz burst).

Dim controlRight numericaldisplay

From/to andwaypoint display

Waypointsoloctor switch

Track changepushbutton(green)

Mode switch

Entcr pushbutton

Data keyboardpushbuttons

Fig. 5.15 Litton LTN-21I CDU (courtesy Litton Systernslnternational Inc., Aero Products Division)

roi I ..ors rrME

64r;'l][\s11 -

rAS

. t / \ \f bursr = tan-r /r t'urg ) )- antenna phase shift HllT::lx'L: ilir:",,Ti::TlT;:ii'"illfo*'

\lJ l cos d // (6'J) ti,nitin.a in order to save storage and computation

R burst = (rrin o\' * /> .o, p\' (6.4) tim;' simplifications include:

\-N / \-T-/ l. increasing integation step size along path to say

If synchronization is not successful a further l0 speriod is sampled and so on.

Since before synchronization is complete signaldirection is not known, the antenna is set to anomnidirectional mode.

The idea of the synchronization routine is to lookfor correlation in phase measurements from sampleto sample for the three broadcast frequencies. Noisealone will of course appear with random phase, notcorrelated between samples.

Phase and Signal-to-Noise Measuremenrs The phasedifference between the received signal and a localreference is sampled at regular intervals throughoutthe burst. Forming sine and cosine sums of thesampled phase angles will allow a burst phasemeasurement (average of samples) and signal-to-noisemeasurement to be made. If no signal is beingreceived during the sample then no contribution willbe made to either the sine or cosine sums. We have:

where the summations are of the samples over theburst, @ is the phase angle and R, which lies between0 and l, gives a measure of sigral to noise ratio(s.n. r . ) .

The values of @ burst and R burst are fed to atracking filter in order to give smooth values @ and R.There are a total of twenty-four tracking filters(three frequencies X eight stations). Rate aiding isapplied to @ to compensate for known aircraftmotion. Each tracking filter is updated after theappropriate burst, i.e. every 10 s, rate aiding valuesare calculated every 0'l s.

Antenna Selection Every l0 s the bearing to theeight stations is computed and stored. Every secondthe difference between bearing and heading iscomputed and used to select the longitudinal loop,lateral loop or combination of loops to make theantenna directional, the main lobe being in thedirection of the station to be received..

Station Selection For Omega rho-rho-rho navigationthree stations must be received to calculate the threeunknowns (latitude, longitude and clock or phaseoffset). Various criteria are used in the selection ofstations to be employed:

l. eliminate manually deselected stations;2. eliminate stations for which the aircraft is not

within area covered (see Fig.6.l2);3. eliminate stations with known modal

interference problem at night in certain areas(Liberia);

4. eliminate stations on the basis of poors.n.r.;5. eliminate frequencies from particular stations

whose phase difference between computed andmeasured exceeds a certain figure.

All qualifying frequencies are used for positiondetermination. If less than minimurn number ofstations are available, the dead reckoning mode isentered.

hopagation Conection The computer must calculatea propagation correction 0p, the value of which willdepend on the path from station to aircraft, the timgof day and the date. Factors affecting propagationhave been discussed earlier and while complete

2. using coarse memory map, i.e. subdividing earthinto, say, 4" x 4" block and assigting aconductivity index to each corresponding toaverage conductivity in that area;

3. simplifying sub-routine which computes bearingof signal path to earth's magrretic field.

Computer comparisons of simplihed and moreaccurate models have been carried out and showexcellent agreement.

Cunent Least-Squares Enor Cabulttton Themeasured phase @ is corrected for propagation shifts,0p, and estimated clock offset, f., and then comparedwith the phase, @r, derived from the current calculatedrange between aircraft.and station to give A 0, wehave

L Q = ( Q - O p - Q J - Q r (6.5)

There is one A@ for each station frequency so therewill be at most twenty-four. If there is no error, thatis 0p, 0. and @, are all correct, then A0 will be zero.In practice eqrors will exist, so the purpose of theleast-squares error estimation routine is to findcorrections to @., and the computed position tominimize Ad for the stations/frequencies in use.

It

Since the most reliable information comes from thestrongest signal each A@ is weighted by its s.n.r.(smoothed R from equation (6.4) ). The squares ofthe A@ are computed to prevent cancellation in thesum. We have:

minimize >R(AO)2 (6.6)

where the sum is taken over all the stations andfrequencies in use.

The phase difference, LQ, can be expressed interms o(A@., AN, AE and B where the first threeterms are the corrections in clock offset. northposition and east position respectively while B is thebearing to the station. If we consider the signalreceived from station I on frequency J, we have:

A0(l,J) = Adc - AN . cos B(l) - AE . sin B(l)(6.7)

Thus equation (6.7) is used in (6.6), theminimization of the weighted sum of squares, giving aleast-squares estimate of the current error whichenables correction of clock offset and position.

The correction vector X = (A0., AN, AE)T issmoothed by clock and position filters, rate aiding ofspeed, resolved north and east about aircraft heading,being applied to these filters. The output of the filters

is wind (north and east),latitude and longitude. Thewind is not computed when the aircraft is on theground, as indicated by the oleo strut switch.

Summary The above notes on system softwave are byno means complete; some functions of the softwavehave not been mentioned although have been impliedelsewhere in the chapter. The major navigation tasksand their implementation are best summarized by aflowchart (see Fig. 6.1 6).

The ProgramThe actual program used in any ONS is proprietaryand will vary greatly depending on the type ofmicroprocessor used, the method of navigation andthe ingenuity ofthe author.

In general there will be a main loop which checksfor power interrupts, computes propagationcorrection, carries out self-testing, etc. The main loopwill be interrupted when 10.2, I l '3 or 13.6 kHzinformation is available for processing and also whenthe CDU is ready to input or output data.

In the LTN-21I the phase data interrupts for thethree frequencies occur regularly at 6.25 ms intervals.The I I '3 and I 3'6 kHz interrupt loops simply serveto read the appropriate phase data while the

Clock

Omegrsignal

Phase | 0 - lS in /Cos inedetector look up

Sin IC"t C

Burstprocessing

CouR -

'sr (1, J)

" i l ' J )

LOc

ClockoffsetestimatorQc

t(r, J) + -Trackingfi l ter

LeastA N A E

R (r, Jl ? estrmator6o

6,

Rate aidPropagationpredict ion

GMTLat./Long.

t

-t Lat./Long.+ -l LPosition

f i l terRange tostation

Expectedphase

Wnd

Rrtc rid Ratc aidnorth oast

Fig.6.16 Software flow chart

gi2

l0'2 kHz interrupt loop processes all phase data anddrecks to see if it is time to start loops which occurregularly and perform various computations anddrecks. Briefly u,e have:

lfl) ms loop: 'update

clockrate aid computationcheck synchronizationantenna selectionburst processingserve CDUhorizontal steering commandsleast-squares estimatorsave kev data

amplified and limited sigrals are compared in phasewith reference sigrals derived from a 4.896 MHzclock. A real-time interrupt is generated oncompletion of a phase measurement for each of theOmega frequencies. The l0'2, I l'3 and 13'6 kHzinterrupts are each generated 160 times per second toinform the computer that phase data is available for368 gs, during which time the appropriate interruptroutine is entered and the data read. The sensorphase data forms word I in a four-word, l6-bitdigital multiplexer. -

Heading and speed inputs enter the system in theform of three-wire synchro feeds. Scott-Ttransformers resolve this input into the sine andcosine components which are then demodulated andfiltered to provide d.c. signals to an analoguemultiplexer controlled by the computer. Afteranalogue to digital conversion the heading and speedsine and cosine components are multiplexed as word 2in the digital multiplexer. Words 3 and 4 containdata relating to various discretes, program pins,validities and source selectors.

ARINC 575 data from the CDU is converted to

I s loop:l0 s loop:

Hardware

C.omputo Inputs Figure 6.17 shows a simplifiedsystem block diagram of the LTN.2I l. Omega signalsfrom the ACU are fed via an antenira switching matrixto three narrow-band receivers. one for each of thethree Omega frequencies. Antenna switching isderived from the computed relative bearing of thestation being received at that particular time. The

l?u?*^ss'-sIl ; -

I

[**r**')

Fq. 6.17 Litton LTN-21 I block diagram (courtesy LfttonSystems International Inc,, Aero Products Division)

sTtiltNG rNTtttAct

POWTI SUPPTY

sl

TTL (transistor-transistor logic) levels and shifted intoa serial to parallel shift register. When the input datais ready an interrupt is generated and the contents ofthe register are read by the computer.

A digital interface card, part of the steeringinterface. is a link between the ONS and othersystems. There are four ARINC 575 receivers forDADS TAS (digital air data system true air speed),flight management, IRS (inertial reference system)and an inter-system b.c.d. or binary bus for interfacewith another ONS or possibly another type system.The address bus from the computer identifies therequired receiver which stores the particular inputword in a register and generates an interrupt. Thecomputer acknowledges this interrupt, so requesting atransfer ofdata onto the l6-bit parallel data bus via aserial-parallel tri-state register. Since the ARINC 575word is 32-bits long, transfer takes place in twosections.

Memory The navigation computer program is storedin a 20K X l6-bit word (K = 1024) UVEROM(ultraviolet erasable ROM) which can be programmedfrom cassette tape using a programming adapter. Thedata is retained in the WEROM for an estimated 100years unless erased by exposing all twenty chips toultraviolet light.

Additional memory is provided on thecomputer/processor card in the form of a 2K wordscratch pad RAM for temporary storage and a128-word data save memory used to store presentposition, time/date, waypoints and additional datarequired to resume normal operations after atemporary power failure. Back-up power for datasave is provided by a 4500 pF capacitor for at least7 min.

The Computer The computer uses the input andstored data referred to in the above paragraphs tocarry out the necessary navigation problemcomputations and to output the resultinginformation. The microprocessor employed is aTMS-9900, a l6-bit CPU (central processing unit)capable of addressing 32K words of length l6 bits.

Input/output (l/O) functions are treated in thesame way as memory for addressing purposes, theaddress map being split into sixteen sectio,ns, twelveof which are assigred to WEROM and two each toRAM, save data memory, steering interface I/O andother I/O. As an example F A 0 0 hexadecimal =

15 x 163 + l 0x 162 +0 x 16 r +0 x 16 ' =640001e ,

is the addres of the digitd multiplexer phase sensorword which will contain phase measurement datacorresponding to l0'2, I I '3 or l3'6 kHz depending

94

on which interrupt is being serviced. Not all theavailable addresses are assigned in the LTN-21l.

The computer can recognize sixteen interruptlevels with the highest priority level 0 and the lowestpriority level 15. An interrupt mask is contained in astatus register and continuously compared with thesystem-generated interrupt code. When the level ofthe pending interrupt is less than or equal to thecurrent-enabling interrupt mask level (higher or equalpriority) the processor recognizes the interrupt. Onrecognition the current instruction is completed,details of the position in current program storeo,the appropriate interrupt service routine started, andthe interrupt mask forced to a level that is one lessthan the level of the interrupt being serviced. Whenthe interrupt routine is complete the interruptedprogram continues where it left off. In the LTN-21Iseven interrupt levels are impiemented:

0 reset (power on)lI pending power fail or program cycle fail;2 lO'2 kHz sensor data input;3 l3'6 kHz sensor data input;4 1 l '3 kHz sensor data input;5 reserved for a sensor data input;6 CDU ARINC data ready;7 ARINC receiver data ready.

Cttmputer Outputs A l6-bit word is transmitted tothe receiver module for control of theantenna-switching matrix and the antenna-calibratefunctions.

Various output functions respond only to theaddress bus state and do not require specific data tobe placed on the output bus. These functions, withhexadecimal addresses are :

reset program cycle fail (F100), select analoguemultiplexer address (F2X0), start analogue todigital converter (F300), acknowledge c.d.u.ARINC Rx (F600) and ARINC interruptacknowledgement (CFAO).

The 'X' in address F2X0 can be any number from 0to 7 dependingon the funption, e.g. heading sine,heading cosine, speed sine, etc.

Sigrals associated with other systems andinstruments are output from the computer via thesteering interface which is divided into three mainsections: (l) analogue functions; (2) digitalcommunications; (3) discrete flag drivers.

The analogue interface card provides four three-wiresynchro outputs and both high- and lowlevel two-wired.c. cross-track deviation. Digital data is fed to theanalogue card on the data bus and converted to d.c:

in a digital to analogue converter. The analogue d.c.sipal is fed in parallel to sample and hold circuitsaddressed by the computer via a decoder. Each ofthe synchro channels has a modulator for the sine andcosine d.c. inputs followed by a Scott-T transformerwhich provides the three-wire synchro output. Theparticular outputs are determined by the state of twosynchro output select program pins, ground or open.In this way synchro number I will give heading ortrack, synchro number 2 will give 1 drift angle ortrack angle error, synchro number 3 will give trackangle error + drift angle, track angle error or desiredtrack angle while synchro number 4 will give aircraftsteering or track angle error.

The digital interface card has been referred to abovein connection with its input function. In additionthere are ARINC 575 and ARINC 561 serial digitaloutputs, two-wire b.c.d./binary for the 575 andsix-wire (clock, sync and data) for the 561. TheARINC transmitters are selected when theappropriate data is on the data bus by suitableaddressing from the computer. The output word is32 bits long, and so must be entered into a register intwo parts under the control of the address bus.Parallel to serial conversion takes place when theword is assembled in the register.

Flag signals at TTL levels are output from thecomputer and latched via drivers. The signals are thenbuffered, scaled and/or level shifted befoie output.

Characteristics .

Much of this section on ONS has been based on theLitton-21 I which is an ARINC 599 system.'Thisbeing so, what follows is a particularly briefsummary of the ARINC Mark 2 ONS since detailszuch as input and output have already been covered.

The system comprises three units: a receiverprocessor, control/display and antenna/coupler unitwhich, together are capable of receiving andprocessing Omega ground station signals (v.l.f. notprecluded) so as to provide minimum functionalcapabilities ofpresept position readout andhorizontal track navigation. The system shouldoperate worldwide with a present position error ofless than 7 nautical miles.

The power supply for the system is I 15 V 400 Hzsingle phase fed via a circuit breaker. In addition a26 V zt00 Hz reference in accordance with ARINC4134 will need to be supplied via a circuit breakerfrom the appropriate instrumentation bus forexcitation of synchros.

Rmp TestingUttle needs to be said here since the computing po*er

of an ONS is such that comprehensive monitoring andself-test routines may be incorporated in the systemsoftware. Monitoring of system performance takesplace virtually continuously during flight. Inaddition, operator error detection aids smoothoperation and a reduction in reported defects due to'finger trouble'. With a malfunction code readout andself-test display, turn-round delays for ONS.installation defect investigation and repair areminimized.

The Decca Navigator

IntroductionDecca navigator was invented in America byW. J. O'Brien but f irst used by the Brit ish in theclosing stages of World War II. Since then a numberof marks of the equipment have emerged from thecontinuous development of this, the most accurate ofall the radio navigation aids. The system came secondin the two-horse race for adoption by the ICAO asthe standard short-range navigation system. That itsurvived is a credit to the Decca Navigator Companywhose confidence in the basic merits of the systemwere such that it continued its airborne developmentprogram despite the setback in 1949.

Decca is a low-frequency c.w. hyperbolicnavigation system. The service is provided forsuitably equipped aircraft, ships and land vehicles bychains of transmitting stations. Each chain comprisesa master station and normally three slave stations, allat known geographical locations (typically 70 milesapart), radiating phase-locked signals. The choice offrequency could give a ground wave coverage ofl0O0 nautical miles but c.w. operation prevents theseparation of ground and sky wave signals so thewable range is limited to about 240 nautical milesby night and about twice that by day. There arechains in various parts of the world, in particularnorth-west Europe and the northeast seaboard ofNorth America.

Phase differences between the master station andeach of its slaves are displayqd to the pilot on threephase meters or Decometers. The observed phasedifferences identify hyperbolic lines marked onspecially prepared charts. By noting at least twophase readings the pilot can plot his position as theintersection of the corresponding lines. For ease ofuse the charts are printed with the three differentfamilies of hyperbolic lines in purple, red and green,hence the red slave station and the green Decometer,etc. Decometers are still used but for airbornesystems, automatic and computer-based methods

s

are uzually found, with aircraft position being shown

on a roller map (flight log display)'

The Radiated SignalsEach chain is assigned a fundamental frequenry / in

the range 14-14'33 kHz. The stations each radiate a

harmonic of /, namely 6/from the master znd 5f,8f

and 9/from the purple, red and green slaves

respeitively. Thus with f = 14kHz we have radiated

frequencies of 6f = 84 kHz, 5f = 70 kHz, 8/ = l.l2'ktlz

andg/= 126 kHz. Using different frequencies for

the stations in a chain allows separation in the

airborne receiver.Decca chains are designed by an alphanumeric

code. The basic codes are 08, lB, . . ., l0B' the

corresponding fundamental frequencies being

separated by a nominal 30 Hz (separation is 29'17,

30 or 30'83 Hz). A subdivision of this basic

allocation is provided by the so'called 'half

frequencies' 0E, lE, . . ',98 which have a nomind

spacing of l5 Hz from the B fundamental frequencies.Additional subdivision is achieved by the use offrequencies 516Hz above and below the B and E

frEquencies tb give groups of six frequenciesdesignated by the appropriate number and the letters

A, B, C, D, E, F. An examPle of a grouP offrequencies is given in Table 6.3 showing code,fundamental and master (6/) frequencies; the slavefrequencies are pro-rata.

Table 5.3 Decca frequencies - numerical group 5

Fig.6.l8 Zone patterns (courtesy the Decca Navigator Co'

Ltd)

fundamental frequency of the chain.The zones are subdivided in two ways' depending

on the method of phase comparison used in the

aircraft. The transmitted signals cannot be compared

in phase directly since they are of different

frequerrcies; frequency multiplication or division may

be used to bring the signals to a common frequency'

Since the transmitted frequencies are related to the

fundamental by multipliers 5,6,8 and 9 (purple'

master, red and green respectively) comparison can

take place at the l.c.m. of any two of the multipliers

which include 6, Thus purple and mastertransmissions can be'phase compared at 30/, i'e' at

between 420 and 429'9 kHz. Similarly the red and

green comparison frequencies are 24f and l8f

respectively. A lane is defined as the region between

hyperbolic lines with zero phase difference, at the

comperison frequency, i.e. every half'wavelength(pig. O.+). Thus there are 30 purple ,24 ted and

i8 gtt.n lanes per zone with baseline widths of

app-roximately-352 m,440 m and 587 m respectively'

Lane numbering starts from the master and runs

0-23 for red,3O'47 for green and 50-79 for purple'

Decometers can be read to one or two hundredths of

a lane. Figure 6.19 illustrates a position fix in terms

oflanes: the red Decometer reads zone I (bottom

window), lane l6 (outer scale), lane fraction 0'30

Chain Fundamentalcode lf (Hz)

Master6f (Hz)

5A5B5C5D5E5F

14 165.83t4166.67l4 167.50l4 18c.8314 181.67l4 182.50

84 99585 00085 0058s 08s85 09085 095

Position FixingAs with any hyperbolic system two hyperbolic linesof position must be identified, the fix being given bywhere they intersect. With Decca the hyperbolicpatterns are divided into zones and lanes. Zones aredesignated by letters A to J starting at the master endof the master/slave base line. The sequence of lettersrepeats as necessary to cover the whole pattern(Fig. 6.18). Along the base line, zones have aconstant width of between 10'47 and l0'71 km,corresponding to haff a wavelength at the

96

at';

z InEiaEcTtoIl ot ndttotmox LtxEs ts./ .

at oF Potntox / - osl

DEccA co ORDTNAT€ DECQA CO-OnDttrATCRtD I ro i0\ dar:l O rs.O

FB 6.19 Plotting a position fix from Decometer readings(courtesy the Decca Navigator Co. Ltd)

(inner scale) while the green Decometer reads zoneD, lane 35,lane fraction 0.80. Thus the Deccaco-ordinates are I 16.30 and D 35.80, intersecting as$town.

The accuracy obtained by using frcquencyrnultiplication is often not required for air navigation;furthermore a better s.n.r. can be achieved bydividing the received sigrals down to the fundamental.Since phase comparison is at / the zones are the'lanes' for a dividing type receiver. Fractions of azone are measured to a resolution of I llO24, i.e. justover l0 m.

Resolving lane AmbiguitiesSince each lane appears to be the same, in so far asphase difference measurement is concerned, the pilot$rould know where he is, to within half a lane, inorder to initially set the appropriate Decometer byhand. Thereafter, since through gearing the lanefraction pointer drives the lane pointer and zone read-out, the Decometer will record the correct co-ordinateby an integratipn process. Any interruption inreception would require a resetting of the Decometers.

Although the dividing type receiver does notmeasure lanes there is still ambiguity within a zonecaused by the division process. For example dividingthe master signal by six gives rise to an output whichcan start on any of six cycles, only one of which iscorrect. The ambiguous cycles are known as notches.The resulting ambiguity is the same as described in theprevious paragraph since, for example, an error of*lnotch in the master divider output gives an error inthe zone fraction reading of 1/6 while an error of -l

notch in the red divider output gives an error in thezone fraction reading ofl/8. The net error in the redzone fraction reading would be ll6 - l18 = ll24 zoneor I lane.

To resolve lane ambiguities most Decca chainsoperate in the MP (multipulse) mode (an older Vmode will not be discussed here). Each station intum, starting with the master, transmits all fourfrequencies (5f,6f,8f ,9f) simultaneously. As€quence of transmissions lasts 20 s during whichtime each station transmits the MP signals for 0'45 s.ln the receiver the four frequencies are summed,producing a composite waveform which has a

97

5f

6t

predominant spike or pulse occurring at thefundamental rate (Fig. 6.20).

In a receiver employing the multiplying method,with readout on Decometers, lane ambiguity isresolved by feeding a lane identification meter suchthat one arm of a six-armed vernier pointer, identifiedby a rotating sector, indicates the correct lane. Thesector is driven in accordance with the phasedifference between l/6 of the master transmission(remembered during the MP transmission by aphase-locked oscillator) and the fundamental derivedfrom an MP transmission. The vernier pointer isdriven in accordance with the phase differencebetween the master transmission and six times tnefundamental derived from an MP transmission, thedrive being through I : 6 gearing. During the masterMP transmission the lane identification meter shouldread zero and may be adjusted to do so if in error.

Fig. 6.20 Multipulse transmission - summatibn (courtesy

the Decca Navigator Co. Ltd)

Fig. 6.21 Mk 19 Decca Navigation System (courtesy the

Decca Navigator Co. Ltd)

98

As each MP trafismission occurs the appropriateDecometer lane reading should correspond to thelane identification meter reading and may be adjustedif necessary. The current lane identification may beheld to assist checking but will only be valid for a fewseconds due to aircraft movement.

ln a receiver employing the dividing method theMP transmissions provide a reference frequency /with which the phase of each divider output can becompared. The phase conrparison and subsequentcorrection takes place automatically within thedivider circuits. This process, known as notching,removes ambiguity from the zone fraction data.

Resolving Zone AmbiguityWe have seen how the particular lane (within a zone)in which the aircraft is f lying can be identif ied but theporsibil i ty of an incorrect zone reading sti l l existssince zones, l ike lanes, are indistinguishable by thenormal phase nreasurements. A zone identif icationmeter resolves the zone ambiguity, not completelybut to *' i t lr in a group of f ive zones, i.e. a distance ofover 50 km on the baseline.

An 8'2/signal is transmitted with the MP signalsfrom each station in turn. A beat note between the8f and 8.2f components of an MP transmission isproduced having a frequency of fl5. The master beatfrequency is 'remembered' and compared with eachslave beat t iequency in turn. The resulting hyperbolicpattern has zero phase differences on lines five zonesapart, giving the required resolution.

hstallationDifferent options are available two of which arestrown in Figs 6.2 I and 6.22. The Mk 19 receiver iscapable of driving Decometers (or digital ieadout)and/or a l1ight log; the multiplying method is used todrive the Deconreters while the dividing method isused to drive the flight log through a computer unit.The Mk l5 receiver uses the dividing method onlywith readout on a flight log.

Where space is at a premium, a Dectrac positionfixing unit (PFU) may be installed in conjunctionwith either a Mk 15 or Nlk l9 receiver. The DectracPFU contains one indicator with four scales and asingle pointer, effectively replacing the Decometersbut not allowing the same degree of accuracy inreading. although this may be recovered by theaddition of another small unit. A zone identityreading can be taken from the single indicator.

A capacitive type antenna is used, comprising acopper mesh within a fibreglass plate mounted flushwith the aircraft skin. The mesh is at least 2 ft2 inarea. A pre-amplif ier/matching unit allows a long

Fig.6,22 Decca Mk l5/Danac navigation system (courtesythe Decca Navigator Co. Ltd)

feeder run. The antenna should be mounted as nearto the centre of the aircraft as possible, either aboveor below the fuselage. If below a fixed I 80" phaseshift is applied in the pre-amplifier.

Mk lS/Danac Block Diagram OperationFigure 6.23 shows a sirnplif ied block diagram of aMk l5i Danac installation. A significant feature ofthe system is the degree of automatic controlachieved.

The receiver output is fed to the computer as fourpulse trains representing the received master, red,green and purple signals divided down to thefundamental frequency ,f. .The master/slave phasedifferences at f are digitally measured in the computer,thus giving three hyperbolic l ines of position eachderived as a l0-bit binary number representingll l}2aof a zone.

The computer converts the Decca co-ordinates intoX and Y demand signals for the servos driving thelaterally moving stylus and the vertically moving chartrespectively. The major computing effort is carriedout oft'-line on a more powerful computer whichcalculates constants to be used in the hyperbolic toX-lz conversion. These constants are written on a

99

zln"i"J",Fast/normal lock

Decca Navigator Mark 15 Receiver

Fig.6.23 Decca Mk 1S/Danac navigat ion system block

diagram (courtesy the Decca Navigator Co. Ltd)

non-visible part of the chart, among other data, in theform of a black and white ten-track digital Gray coderead by a l ine of ten photoelectric cells.

The )/ servo position feedback signal is in the formof a 9-bit word derived from nine of the digital tracksreferred to above. The X servo position feedbacksigrral is a 9-bit word derived from printed circuittracks read by wiper contacts mounted on the styluscarriage. The servo drives are, of course, thedifferences between the demand and feedback sisnals.

Selection of the correct Decca chain isautomatically achieved by including the coderepresenling the frequency among the constants readby the photoelectric cells. Other data among theconstants are the zone values for one or twocheckpoints on the chart (to which the stylus goesinitially) and the chart scale.

Setting up is largely automatic. If the requiredchart is in view the following sequence takes placewhen the system is switched on:

l. pushbutton lamps light for checking purposes,and the chart constants are read into thecomputerl

2. the stylus moves to a check point and receiverlocks on to required signals;

100

Low s igna lwarning

F l igh t log

Warning

P/E cclls

lamp

3. zone fraction computation takes place usingMP transmissions for notching;

4. stylus takes up a position within a zone on thechart corresponding to the aircraft positionwithin the true zone. If the zone is correct theOP button is pressed and thereafter the stylusand chart should move so as to foilow theaircraft 's movements. lf the stylus is in thewrong zone it may be manually set by pressingSET and operating the pressure-sensitiveslewing control.

The correct zone is known from the zoneidentification indicator and the pilot's knowledge ofhis position to within five zone widths. When theaircraft 'flies off'-the current chart and on to the next(on the same chart roll) the stylus goes straight to theaircraft position on the new chart except undercertain conditions, e.g. chain change, in which casethe above initial procedure relating to zoneidentification and slewing is cariied out.

The pilot may bring another chart into view bypressing LOOK AHEAD and operating the slewingcontrol. Pressing the LOOK AHEAD button a secondtime causes the new chart to remain in view, otherwisethe stylus is returned io the aircraft's present position

#r; ffil I control Ir|*'r L- l |Irl \ ./ a

\v_-/

Danac control ler

(still calculated during LOOK AHEAD) on theprevious chart.

Pressing the INT button causes the system to gointo the integration only mode where.the MP andzone identif ication facil i t ies are switched off. It maybe desirable to select INT when flying in a fringe areasince spurious or imperfect notching signals maycause the warning lamp to come on, indicating adiscrepancy between displayed position and receiveroutput. On some charts covering fringe areas orchains without MP transmissions, INT is selectedautomatically by a suitable chart constant.

The LOCK button has several functions, one ofwhich is to put the receiver phase-locked oscillatorhto a fast lock condition, providing the warning lampis on. It may also be used to init iate the automaticsetting-up routine when a new chart, brought intoview by LOOK AHEAD, does not have the samecolours as the previous chart.

The stylus may be prevented from marking thechart by selecting WRITE off, otherwise the track ofthe aircraft will be traced out on the ghart. TheTEST-DIM-BRIL switch is the only control notpreviously mentioned, it may be used for lamp testor to select the brightness level of the lamps.

The above description is sketchy to say the least, butI hope it will give the reader an idea of how one DeccaNavigator system configuration performs its function.

Loran C

lntroductionl.oran A was proposed in the USA in 1940, hdd trialsn 1942 and was implemented over much of the northand west Atlantic in 1943. Since then coverage hasbeen extended to many of the oceanic air routes ofthe world, but some time in 1980 the last Loran Atransmitter should be switched off. Since theimplementation of Loran A the family has beenextended to B, C and D. Loran B was found to beimpractical and Loran D is a short-range,low-altitude system intended for use where l ine-of-sightsystem coverage is inadequate

Loran C is a long-range" pulsed hyperbolicnavigation aid with accuracy approaching that ofDecca under favourable circumstances. It wasintroduced in 1960 and now provides a valuableservice in rpany parts of the world, in particular thenorth and east Pacific and Atlantic. The system is.used by many ships and aircraft and would appear tohave an indefinite future.

(hain LayoutA transmitter, designated the master, has associated

with it, up to four slave transmitters designated W, X,Y and Z. The master occupies a central positionsurrounded by the slaves so far as the geographyallows. Base lines are of the order of 500-1000nautical miles over sea but are reduced over land.

The range of the system is about 1000 nauticalmiles (from master) using groundwaves and up toabout 2000 nautical miles using skywaves. Theaccuracy depends on the geometry of the chain butmay be in the order of about 400 ft at 350 nauticalmiles range to 1700 ft at 1000 nautical miles rangeprovided groundwaves are used. With skywaves theaccuracy would be in the order of l0 nautical milesat 1500 nautical miles range.

The Radiated SignalsPulses of 100 kHz r.f. are transmitted from allstations. The slave transmissions are synchronizedwith those of the master either directly (triggered bymaster groundwave) or by use of atomic clocks. Thedelay between the time of transmission of the masterand each slave (coding delay) is fixed so thatwherever the aircraft receiver is located in the areacovered, the slave signals will always arrive in the sameorder after the master.

Since all chains transmit the same r.f., mutualinterference must be avoided by use of different pulserepetit ion periods (p.r.p.) for each chain. There are atotal of six so-called basic rates, each of which haveeight specific rates as given in Table 6.4. The chainsare identif ied by their p.r.p., thus chain SS7 (Easternseaboard of North America) has a basic rate period of100 000 ps (SS) which is reduced by 700 ps since thespecific rate is 7, hence the period betweentransmissions from the master (and from each slave)is 99 300 1,rs, i.e. fractionally over l0 transmissions per

Table 6.4 Basic and specific rates forLoran C

Basicrepetitionpeiod( t ts)

Specificperiods( subtract )( t ts)

30 00040 000s0 00060 00080 000

100 000

0 0I 1002 2003 3004 4005 5006 6007 7008 800

HLSSHSLSS

101

second. Not all the basic rates are in use and indeedsome may never be used since 6 X 8 = 48 chains areunlikely to be needed.

Groups of eight pulses of r.f. are transmitted fromeach station once during a repetition period. Withsynchronous detection in the receiver the eight pulsesare combined to give a much better s.n.r. than onewould obtain with a single pulse. The spacingbetween pulses within a group is I ms. The mastertransmits a ninth pulse in its group, 2 ms after theeighth, for identification.

Some types of interference (e.g. skywavecontamination) can be discriminated against by use ofphase coding. The r.f. of certain pulses within a grouphas its phase reversed; unless this is properly decodedin the receiver synchronous detection will give a lossof sigral power. Additionally, since master and slavephase coding is different for a particular chain,decoding can be used to separate the received mastersignals from the slave signals.

To measure the time difference between master andslave transmissions corresponding'events' must beidentified in each. Obviously from Fig. 6.24 it isimpractical to measure from leading edge to leadingedge or even to use the lagging edges, consequentlyone of the cycles must be chosen in master and slavetransmissions and the time between them measured.Such a process is known as cycle matching orindexing.

From the point of view of s.n.r. the eighth cycle isthe obvious one for indexing; however it may besubject to skywave contamination and thereforedifficult to identify. The minimum difference inpropagation times between skywave and groundwaveis 30 prs, so up to and including the third cycle thepulse is clear. For this reason the third cycle isusually chosen for indexing, particularly in fullyautomatic equipment.

An automatic receiver would select the third cycleby looking for the unique change of amplitudebetween the second and fourth cycle; in this way theindedng circuits are able to lock on. The transmissionof the first eight pulses must be accurate andconsistent since an error in indexing of one cyclewould give a l0 ps time difference error.

If indexing is carried out manually using a c.r.t. todisplay the pulses, on time-bases of decreasingduration as the process proceeds, use ofup to theeighth cycle may.be possible with a skilled operator.

InstallationA Loran C system may consist of up to five units,namely antenna, antenna coupler, receiver, c.r.t.indicator and control unit. A c.r.t. display is usedwhere the indexing procedure is manual or where,if automatic, it is thought necessary to provide theoperator with monitoring of the procedure. On somesystems indexing is manual but thereafter the thirdcycle is tracked automatically.

Fig 6.24 Loran C pulse and pulse format

Figure 6.25 shows the Decca ADL-S1 Loran C/DThe pulse duration is approximately 270 ps, i.e. a receiver and control indicator; an aerial and coupler

total of about 27 cycles of r.f. in each pulse. To would be needed to complete the installation. Theradiate a pulse of short rise-time leads to problems in ADL-8 I is fully automatic providing digital timefrequency spectrum spreading and transmitting antenna difference readouts with a resolution of 100 ns on thedesign at the low carrier frequency involved. In fact control indicator and 50 ns via a computer interface.99 per cent of the radiated energy must be in the band Synchronization provides third cycle

-indexing in

90'l l0 kHz, hence the slow rise and dbcay time goundwave covdr and optimum cycle indexiig duringillustrated \n Fig. 6.24 (in which the signal format, iky*au. working. Three time-difierences arem:Nter and three slaves, is also shown). The maximum computed, two of which may be displayed. Tunableamplitude occurs by the eighth cycle. automatic notch filters provide rejeition of the

strongest interfering signals. Overall systemPrinciples of Operation . performimce checks may be performed using built inThe basic principles of a pulsed hyperbolic iest equipment (BITE).navigation aid have been given earlier in the chapter.

102

The antenna is usually a capacitive type, sometimes

Fig. 6.25 ADL-81 Loran C/D (courtesy the Decca NavigatorCo. Ltd)

serving both ADF sense and Loran, in which case anantenna coupler would provide the necessaryimpedance matching and isolation lor the tworeceivers served. A pre-amplifier may be included forthe Loran feed.

Block Diagram OperationThe received sigrals are separated into n:aster andslave groups by the phase decode circuits, the groupsbeing fed to the appropriate master and slave phaselock loops (p.l. l .). In Fig. 6.26 only one slave p.l. l. isshown bqt in practice there will be a minimum of twoto provide the two hyperbolic l.o.p. required for a fix.Three slave p.l. l.s would enable an automatic systemto select the two which gave the best angle of cut,although the calculations involved would probably be

carried out by a computer to which the threetime-difference readings would be fed.

Gate pulse formers feed the p.l. l .s with a series ofeight pulses spaced by I ms and of, say, 5 ps duration.The object of the p.l.i. is to provide a signal to theappropriate oscillator,.driving the corresponding gatepulse former, so that the phase of the oscillator, andhence timing of the gate pulse, is altered such thateach gate pulse is coincident with some specific pointon the third cycle in any received pulse. The rate atwhich the gate pulse groups are generated is set toequal the rate of the required chain.

The basic idea of indexing, as carried out by thep.l. l.s, is as follows. ln Fig. 6.27 l ine I is arepresentation of the leading edge of a received pulse.Line 2 is a representation of the leading edge of a

,

103

Fig.6.26 Loran C simplified block diagram

O 10 20 30 40 5O rrs

Fig.6.27 A method of indexing

received pulse after it has been delayed by l0 ps andamplified by a factor of 1.5. It can be seen that linesI and 2 cross at a time 30 ps after the pulse leading

104,

edge. Line 3 is a representation of the result ofsubtracting the delayed and amplified received pulsefrom the received pulse. Since the crossover point ofthis difference signal is at 30 ps it can be used as aservo signal to set the gate pulse timing forcoincidence with the tlfrd cycle. The futl bandu,idthof 20 kHz is required to preserve pulse shape duringthe indexing process, although initial signalacquisit ion may take place with a restrictedbandwidth in order to improve the s.n.r.

Once phase lock is established, t ime-differencercadout is easily achieved b1i starting and stopping acounter with the master and 4ppropriate slave gatepulse group respectively.

Acquisit ion of the received fraster and slavepulses. i.e. the init ial aligrment to a point where thep.l. l. can take over, may be carried out by theoperator or automatically. With manual acquisit ionboth received pulses and gate pulses are displayed ona c.r.t.; a slewing control allows the operator to alignthe gate pulses with the third cycle of the receivedpulses by use of diff'erent time-base selections for theA-type display (time-base horizontal deflection, signalvertical deflection). With automatic acquisit ion thegate pulse groups must be slewed or sweptautolnatically unti l the p.l. l . can track successfully.

7 Distance measuring equipment

Introduction

Distance measuring equipment (DME) is a secondaryradar pulsed ranging system operating in the band978-1213 MHz. The origins of this equipment dateback to the Rebecca-Eureka system developed inBritain duringWorld War II. International agreementon the characteristics of the current system was notreached unti l 1959 but since then implementation hasbeen rapid.

The system provides slant range to a beacon at afixed point on the ground. The dift-erence betweenslant range and ground range, which is needed fornavigation purposes, is srnall unle5s the aircraft is veryhigh or close to the beacon. Figure 7.1 shows therelationship between slant range, ground range andheight to be:

s2 = G2 + (r46oso)2 (7.1)

igroring the curvature of the earth. To see the effectof this consider an error in range of I per cent, i.e.S = l'01G. Substituting for G, rearranging andevaluating we have:

s + /t8s3for a I per cent error. Thus at 30 000 ft if the DMEreadout is greater than about 35 nautical miles theerror is less than I per cent, while at 5000 ft greaterthan about 6 nautical miles readout will similarlvgive an error less than I per cent.

Giving range, DME alone can only be used forposition fixing in a rho-rho scheme, three readingsbeing needed to remove ambiguity. With theaddition of bearing information, such as that derivedfrom VOR, we have a rho-theta scherne; DME and

VOR in fact provide the standard ICAO short-rangenavigation system. A DME beacon may also belocated on an airfield equipped with ILS, thus givingcontinuous slant range readout while on an ILSapproach, such use of DME is l imited at present.

I'ACAN is a military system which gives bothrange and bearing with respect to a fixed beacon.The ranging part of TACAN has the samecharacteristics as civil DME. There are, however, morechannels available with TACAN since it utilizes anextended frequency range of 962-1213 MHz. Thus acivil aircraft equipped with DME can obtain rangemeasurement from a TACAN beacon provided theDME can be tuned to the operating frequency of theTACAN concerned. Many civil aircraft carry a DMEwhich covers the full frequency range.

Transponder lnterrogator

Frg.7.2 The d.m.e. sYstem

Basic Principles

The airborne interrogator radiates coded r.f. pulsepairs at a frequency within the band 978-1213 MHzfrom an omnidirectional antenna. A groundtransponder (the beacon), within range of the aircraftand operating on the channel to which the interrogatoris selected, receives the interrogation andautomatically triggers the beacon transmitter after afixed delay of 50 irs. The omnidirectional radiationfrom the beacon is coded r.f. pulse pairs at a frequency63 MHz below or above the interrogation frequency.This reply is received by the suitably tunedintenogator receiver and after processing is fed to the

G n m

Fig. 7.1 Slant range/ground r?nge triangle

105

range circuits where the round trip travel time isccmputed. Range is given by:

R = (r - so)lt2'3s9 (7.2)where: R is the slant range distance in nauticalmiles to or from the beacon; I is the time inmicroseconds furs) between transmission of theinterrogation and reception of the reply. Theconstants in the equation are 50 prs correspondingto the fixed beacon delay, l2'359 prs being thetime taken for r.f. energy to travel I nautical mileand return.

Both beacon and transponder use a singleomnidirectional antenna shared between transmitterand receiver in each case. This is possible since thesystem is pulsed, and diplexing is simple since thetransmit and receive frequencies are different.

Once every 30 s the beacon transmits its identitywhich is detected by the pilot as a Morse code burstof three letters at an audio tone of I 350 Hz. It shouldbe noted that the r.f. radiated from the beacon durineidentif ication is of the same lorm as whentransmitting repiies, i.e. pulse pairs. The differenceis that when replying there are random intervalsbetween transmissions whereas during identif icationthe intervals are constant at l/ l350th of a second.

Further Principles and TerminologyBy now the reader may have identif ied severalproblems with the principles of system operation asdescribed. With DME, many aircraft wil l be askingthe beacon 'what is my range?', the beacon wil l replyto all of them, the problem being how each is toidentify its own reply. Another problem is how toprevent the airborne DME interrogating anout-of-range beacon since this would be wasteful ofequipment l i fe.

It is obvious that the DME operation must be in atleast two phases since we cannot expect aninstantaneous readout of the correct range themoment we select a beacon. There must be someperiod when the DME is acquiring the range followbdby a period, hopefully much longer, during which theindicator continuoush displays the correct reading.In this latter period rve must consider the eventualityof a temporary loss of reply such as occurs during thetransmission of the identif lcation (ident) signal by thebeacon, or perhaps during'manoeuvre'when allsignals might be iost.

We have assumed that the r.f. anergy will travel ina straight line from aircraft to beacon and back. Thisof course will be the case unless there are anyobstructions intervening; however, it is possible thatthere may be more than one path to or from the

106

beacon if there are reflecting objects inconvenientlyplaced with respect to the aircraft and the beacon.This possibility arises since the antennas at both endsof the l ink are omnidirectional. Should such a 'dog

leg' path occur. the round-trip travel t ime Z inequation (.7 .2\ may be that for the long way roundand thus lead to a readout in excess of true shortrange.

To describe the way in which the system designcopes with this it is necessary to introduce severalnew terms which are defined and explained below.

Jitter Dellberate random variation of the timeinterval between successive interrogations. Eachinterrogator pr t rd l lgs5 a J i t ter ing pulse (pai r )repetit ion l-requency (p.r.f.) 'which, over a period ofseveral interrogations, describes a unique patternsince thc variations are random. With aninterrogation rate of, say, 100, the average intervalbetween interrogations wil l be l0 ms, with anyparticular interval being between say 9 and I I ms.The unique iuterrogation pattern enables the DME torecognize replies to its own interrogation byst roboscopic techniques.

Automatic Stattdby Often rei'erred to assignal-activated search. When the aircraft is out ofrange of the beacon to rvhich the airborne DNIE istuned, no signals wil l be received. This state inhibitsinterrogations unti l such time as the aircraft is withinrange and signals are received.

The implementation of this feature determineswhether interrogations commence as a result of meansignal level exceeding some predetermined level or therate of signals being received exceeds somepredetermined rate. The two alternatives are equivalentas the aircraft approaches the beacon from beyondmaximum range, and typically interrogationscommence when the received signal count is in excessof 300-400 per second. They are not equivalent whenthe aircraft is close to the beacon since the meansignal level will be raised due to signal strength;consequently the required rate is much reduced forthe former altei'native. This is of littie consequencewhen the aircraft is well within range; one would notexpect the DME to be on auto standby. Whengound testing, however, an auto standby circuitwhich monitors mean signal level-can give unexpected,but not unexplainable, results since the test set(beacon sirnulator) wil l normally outputconstant-strength signais regardless of range simulated.

Squitter The auto standby circuit will not allowinterrogations to commence unti l i t detects signals

from the beacon. When a sufficient number ofintenogating aircraft are within range of the beaconthere is no problem, since another aircraft comingwithin range will receive all the replies and thus biginto interrogate. If, however, we consider the beaconhaving just come on line or the first flight, after aquiet period, approaching the beacon, we have a-chicken-and-egg situation: the beacon will not replyunless interrogated; the interrogator will notinterrogate unless it receives signals.

From the explanation thus far there are in factsigrals available, namely ident, but this means anaircraft may have to wait 30 s, perhaps more in weaksignal areas, before coming out of auto standby. Thisis unacceptable; consequently the beacon is made totransmit pulse pairs even in the absence ofinterrogations. Such transmissions from the groundbeacon are known collectively as lsquitter' todistinguish them from replies. When the randomsquitter pulse pairs are received the airborneequipment starts to interrogate.

A beacon must transmit randomly distributedpulse pairs at a repetition rate of at least 700; thisminimum rate includes distance replies as well assquitter. Beacons which supply a full TACAN service,i.e. range and bearing, must maintain a rate of 21.00pulse pairs per second. In order to achieve this duringident an equalizing pair of pulses is transmitted100 ps after each identity pair. A range-only DMEbeacon at a constant duty cycle of 27OO puise pairsper second is not ruled out.

Ifwe consider the case of a beacon with a constantduty cycle in a quiet period all transmitted pulse pairsare squitter, apart from during the dots and dashes ofthe ident signal transmission. With one aircraft usingthe beacon interrogating at a rate of, say, 27 then thenumber of squitter pulse pairs will be2700 - 21 = 2673 s-r while the reply pulse pairs willnumber 27 s-t. Two aircraft would Gad to isquitterrate of 2646 s-t and a reply rate of 54 s-r and io onuntil we arrive at a condition of beacon saturationwith a nominal maximum of 100 aircraftinterrogating. We can see that all the squitter pulse.pairs have become synchronized with receivedinterrogations. From the interrogator's point of viewall received pulse pairs appear to be squiiter exceptthose identified by the range circuits as being ,rp-limto its own interrogations.

Maintaining a constant duty cycle for the beacon isachieved by varying the receiver sensitivity. When nointerrogations are received sensitivity is sufficientlyhigh for noise to trigger the beacon modulator 2700times per second. As interrogations are received thesensitivity decreases so maintaining the duty cycle.

Should more than 2700 interrogations per second bereceived the sensitivity is reduced still further, thusmaintaining the service for those aircraft closest rothe beacon.

In fact the nominal madmum of 100 aircraft isexceeded since interrogation rates on track (seebelow) are considerably less than twenty-seven formodern equipment, and further the interrogator doesnot need 100 per cent replies in order to maintarnreadout ofrange. The beacon capabil ity of 100aircraft may be reduced if peak traffic is much lessthan this figure.

Search During search the range-measuring circuits ofthe interrogator have not recognized those pulsesarnongst the total received which have the samejittering pattern as the interrogation. Theinterrogation rate is high so as to decrease search time,the maximum rate allowed being 150 s-r. The searchtime in a modern equipment is typically less than I s.A p.r.f. of 135 is avoided since it may causeinterference with the bearing measurement functionof TACAN. The readout will be obscured by a .flag'if of the mechanical type, or will be blanked ifelectronic. The counter drums of an electro-mechanical indicator can be seen to be rotating whenthe interrogator is searching; an electronic indicatormay have a lamp or Le.d. which illuminates duringsearch.

It is an ICAO recommendation that if after l5 000pairs of pulses have been transmitted withoutacquiring indication of distance then the p.r.f. shouldnot exceed 60 until a change in operating channel ismade or a successful search is completed. In practiceuse of automatic standby circuits and search p.r.f.s aslow as, say,40 in modern equipments makes thisrecommendation redundant.

Track Dunng track the range-measuring circuits,having acquired the reply pulses, follow their early orlate arrival as the aircraft moves towards or away fromthe beacon. Continuous range readout is given withthe'flag'out of view. The p.r.f. is low. In order tooptimize beacon capability a maximum average p.r.f.of 30 is laid down. This assumes that 95 per -ent ofthe time is occupied by tracking, thus:

9sr+ ss < 3000 Q3)where: 7is the track p.r.f. and S the search p.r.f.

In practice modern equipments may have track p.r.f.sof less than 10.

In some equipments the transition from search totrack, during which the range measuring circuits checkthey have in fact acquired the correct signals, is

107

known as acquisit ion. lt is convenient tt l identitythis event by a separate ternt since it takes a l lnite.thoupilr short. t ime and the equipntcnt is neithersearc l t i r rg nor t rack ing.

Mennry If replies are lost an interrogator wil l notimmediately revert to search or auto standby but wil lenter its memory condition; this rrtay be one of twotypes, e i ther s tat ic or ve loc i ty . Wi th s tat ic menlorythe readout is maintained steady, whereas withvelocity memory the readout continttes to change atits last known rate. Mentory time wil l norrnally l iebetween 4 and 12 s.

If, during memory, replies are re-acquired, theequipment wil l continue tracking: thus the pilot wil lhave been spared a false warning. At the end ofnremory, if t l tere are ho signals at all being received,auto standby wil l ensue;otherwise the equipment wil lcommence searching.

Echo Protection The possibility of the interrogatortracking replies which have suffered reflection mustbe guarded against, both on the ground, for theinterrogation path, and in the air, for the reply path.

On the ground, depending on the geography of theterrain, the reflected or echo interrogation wil l arrivea short t ime after the l ine-of'sight interrogation' Thusif the ground receiver is suppressed for long enougha{'ter reception of an interrogation the echo wil l nottrigge.r a reply. Normally a suppression period, ordeacl t ime, of up to 60 gs is sufficient; exceptionallyup to 150 ps may be necessarY.

A similar situation exists in the air but a differentsolution is normally employed. The line-of-sight andthe echo replies will both exhibit the same jitteringp.r.f. as the interrogator; however, the l ine-of-sightreply arrives before the corresponding echo' Toachieve echo protection the interrogator is arrangedto search outbound. If the search commences at zeronautical miles and moves out, then the first set ofreplies satisfying the range circuit's search for thejitter pattern wil l be those corresponding to the trueiung.. To guarantee echo protection on changingchannel or before commencing search after memoryor auto standby, the range circuits should bereturned to the zero nautical miles condition. This is

done in some equipments where the reversemovement towards zero may be known as a reciprocalsearch, although no interrogation takes place. Inother equipments, where search is outbound from the

last reading, echo protection is likely but notguaranteed. In this latter situation use of the self-test

svitch or button will give full echo protection since

virtually all interrogators have a self-test facility which

108

simulates a range of zero, or near zero, nautical miles.'lhus

after self-test the outbound search commencesfiorn at or near zero.

Pcrcentage Repll' rNe can see from the above that notall interrogations wil l give rise to replies even if theaircraft concerned is well within range. lt mayhappen that an interrogation arrives during theground receiver dead tirne. Other causes of loss ofreplies are ident transmission from the beacon andsuppression of the interrogator recbiver by otherairborne L band equipment. Every time an L bandequipment, i.e. ATC transponder or DMEinterrogator, transmits, a suppression pulse is sent ona common line to all other L band equipment. Thismay well be when a reply would otherwise have beenreceived.

Ignoring, for the moment, ident transmission fromthe ground and suppression due to ATC transponderreplies we can calculate a worst-case percentage replyfigure. Assuming a beacon dead time of 60 ps andmaximum capability operating conditions of 21OOinterrogations per second, we have a total dead time

of 60 X 270O = 162 000 ps s-r ; i .e . dead t ime

constitutes 16'2 per cent of total t ime. The

maximum p.r.f. (average) of No. 2 DME is 30 with

a suppression pulse width of not greater than 60 ps;

thus No. I DME will be suppressed for at most30 X 60 = l80O ps s-r ; i .e . 0 '18 per cent of the t ime.

Thus we are left with 100 - l6'38 = 83'62 per cent

as the reply rate exPectation.The ident transmission occurs once every 30 s

when the total key-down time wil l be less than 4 s.

The code group transrnitted consists of dots and'dashes of t ime durat ion 0 ' l -0 '125 s and 0 '3-0 '375 s

respectively. The time between dots and dashes is

uuullubl. for replies. We have the situation where

about three replies will be lost during a dot, and

about ten during a dash, assuming a track p.r.f. of

about 2l . For a modern equipment with a lowerp.r.f., reply losses will be even less. Under these

circumstances it is not sensible to calculate the

exp'ected percentage reply since the effect of the

ident is possibiy to make the interrogator go into

memory, particularly when a dash is transmitted'

Since the memory time is at least as long as the

total key-down time the momentary switch between

track and memory and back to track will not be

noticed bY the Pilot.It should be noted by the maintenance engineer

that in simulating ident durlrg a ramp test the identsigpal wil l be continuous, rather than keyed, as longas the appropriate switch on the test set is held on.

Thus if ident is simulated for longer than the memory

time the interrogator will start to search, This is

useful since operation of one switch on the test set

allows the checking of ident tone with its associated

volume control, memory time and search p.r.f.

The ATC transponder produces replies, and hence

suppression pulses, only when interrogated. If an

aiiciaft is wilhin range of one interrogator it will only

be interrogated about thirty times per sweep' With a

sweep rate of say 12 r.p.m. and a beam width of say

5o these thirty interrogations will occur during a time

interval given by the product of 5/360 and 60112,

i.e. aboui 0'07 s. For thirty interrogations the p.r.f '

would need to be 30/0'07 - 430 which is close to the

maximum p.r.f. of 450. We have a similar situation to

the reply loss during ident transmission, i.e' the

o..uti.n". is relatively infrequent; for example 0'07

in 5 s. If the aircraft is within range of more than one

interrogator the total interrogation time in, say, 5 s is

increased.Considering the effect on DME only during the

time the ATC transponder is replying we have,

assuming an ATC interrogation rate of 450 and a

suppression pulse duration of 30 ps, percentage

suppression i ime = 450 X 30 X 100/ I 000 000 = l '35

per cent. If we also take into account the worst'case

percentage reply for the DME system of 83'62 per

..nt *. have during this short time 83'62 - 1'35,

82 per cent replies. In fact the DME interrogator

stroutO .op. *ith this and remain on track'

The above is not quite the whole story. The

intention is to allay the fears of students who, on

finding out how many ways replies can be lost, wonder

how on earth DME works at all. The few simple cal-

culations given show that the situation is in factquite satisfactory. It can, however, be worse than sug-

gested since the ICAO specification only requires that

i it. OVf beacon have a 70 per cent reply efficiency;

one of the reasons, not previously mentioned, being

that t ime must be allorved for self-monitoring' Even

so most DME interrogators wil l cope with percentage

replies as iow as.or lower than 50 per cent'

Ground Speed and Time To Station The interrogatorcontinuously monitors the slant range to the beaconwhich, of course, wil l change as the aircraft f l ies

towards or away from the beacon. Measurement of'

the rate of change of slant range gives. the speed of

approach or departure to the beacon. Suchmeasurement is carried out by most airborne DMEsand presenteci as so-called ground speed. tt isimportant that the pilot realizes that the readout canonly be considered as ground speed when the aircraft

is flying directly towards or away from the beaconand is some distance from it. An aircraft flying a

circular path centred on the beacon would register a

ground speed of zero on the DME indicator!If the airborne equipment has calculated gound

speed, it is a simple matter to give time to station(beacon) since TTS = DST/KTS where TTS is time to

station. DST is slant range and KTS (knots) is the

gound speed. Again this is only a useful indicationwtren ttrJ aircraft is on course to/from the beacon and

some distance from it.The time constant of the ground speed measuring

circuit is long but can cope with aircraft acceleration'

ln ground testing. however, one must wait some time

foithe ground speed reading to take up the simulated

value of velocity selected on the ramp test set, slnce

in switching-in a velocity one is simulating an infinite

acceleration.

Interrogation

The full TACAN interrogation frequency range is

1025- l 150 MHz wi th 1 N{Hz spacing. T}rus the

interrogation wil l be one of 116 possible frequencit 's

tlepending on the channel selected. The r'f is keyed

by pulse pairs. The timing, which is dcpendenl 'rn

cirannel selection, X or Y' is i l lustrated in Fig' 7'-1'

3 '5 t rs- 1 l l xJL=--------*lI l 2 i r s I

Fig. 7.3 Interrogation pulse spacing

The p.r.f. is dependent on the nlode of operation of

the DME:

Search 40- I 50 AverageTrack l0-30 -Average

The actual p.r.f. depends on tl-re equipment design

and may be lower than minimutrl l igures given. There

will be a small variation in the average p.r.f. due tojitter. The average p.r.f., assuming that 95 per cent of

the time is spent on track' must be less than 30' The

radiation is omnidirectional with vertical polarization'

Reply

The r.f. at one of 252 frequencies between 962 and

r09

l2l3 MHz isteyed by pulse pairs the timing of whichis similar to that given in f ig. I .5, the differince beinge1l f channel spacing is 30 prs, not 36 pr. ffrcradiation is omnidirectional with vertical poiarization.

X and Y Channel Arrangements

Jh...rr 1.-..lZ!^i1t-elryCation and 252 reply frequenciesin the full

IA!{I-fr.guency range. The-repty _.._.trequency is 63 MHz above or below the intiriogatingfre.quency, as shown in Fig.7.4. fne chann.ispacingis I MHz for both interrogation and reply. The'TACAN channels are numbered lX,ly,'. . . l iOX,r26Y.

Using Fig. 7.4 we see that channel 20X , say,corresponds to an interrogation at 1044 M;Hz';nd areply at 981 MHz, while channel I16I,, say,corresponds to an interrogation at I140 MHz and areply at 1077 MHz.

conjunction with VOR and, largely as a futurerequirement, ILS. To achieve ttris, OUe Uru.on, "r.co-located wirh VOR or ILS beaconr, in1r" i.j"gprescribed maximum separation fi.its 1a"nr"'i tothe convention on International Civil A'v",j"rl.Where we have coJocation constituting a sinelefapility the two systems should ;;;il ""'"ii""o"rafrequency pairing (Table 7.1).r, i rr."r1nii r""' 'associated identity signal.

Table 7.1 Frequency pairing

v.h.f. nav. freq. v.h.f. allocation TACAN channel

Eeaconreply Beacon

replyX

r08.00108.05r08 .10108 .1 5

ii1 lol I 1 . 9 5I12 .00l 1 2 . 0 5n 2 . r o

VORVORILSILS

19tILSVORVORVOR

l7xt 7 Yl8xt8Y

.'?{56Y57X57Y58x

Aircraftinterrogation

63Y

IYr26Y

64Y

--'--'t'-

-'f-'-r-

-*__\l\_

-_|.__\a

12131't511150

1c|aRIr50t088

1087

10251087

1025 to24

962

d4x 112.25r12.30

126X

63Xt x

t17'95 VOR t 2 6 y

VORVOR

59Y70x

Fig, 7.4 .Y/y channel arrangements

For civil DME beacons the 52 channels l-16, Xand nd 60-69, X andy,are avoided i"it*.reasons. Firstly DME is meant to be used inconiunction with VOR and ILS, which occupy 200channels rather than 252. Secondly, having h'fty_tworedundant channels, the gaps are chosen to-overlaplle lTC transponder fre[uencies "f l0i0;;; '1090 MHz to avoid any p-ossible interference,although different codes and ,nr,u.i ,"iprJrJion "r"also used for this purpose.

The use of_the fifty+wo missing channels is, how-ever, not precluded by the ICAO;they may be alloc-areo on a national basis. The fact that civii aircraftrnay wish to use TACAN beacons means thai manyDME interrogators have the full ZSi.f,.""rfr.'

The Link With v.h.f. Navigation

As stated previously DME is meant to be used in

1 1 0

With standard frequehcy pairing the need forseparate DME and v.h.f. nav. contiol units iseliminated. It is normal practice for u .o.Uin"Ocontroller to be used, the selected frequencyindication being given in terms of tne v.h.f. nav.frequency. Thus a selection of 10g.05 Uffr-woufOtune the v.h.f. nav. receiver to that frequency und theDME to the paired channel l7y.

. Some equipments have a hold facility whereby,when engaged. a change in the selectea ".fr.f. "u".lrequency will not cause the DME channel to change.W.hen.using hold, range and bearing info.rn"iion i,given but not to a common point. This could lead topilot navigation error, to .uoid ttris a warninf tr*nr r,illuminat ed when hold is sele cted ; n.*rtf,.i."rr,'ror.national authorities frown upon the availabilit ofsuch a facil iry.

In Table 7.1 the frequency pairing arrangementslt1:Lo*n. The frequencies shown .i Uring"uiio.ut.Oto- ILS are,ofcourse, localizer frrqu.n.irrit. t igtrrtof which is I I I .95 MHz. The gfi.iepatfr/ioJirelfrequency pairing is not affectel tV IUE p.iri"g.

Those TACAN channels not paired with v.h.f. nav.

dnnnels may nevertheless still be required. In thiscase the pairings for channels lX to I 6Y are134.40-135.95 MHz and for channels 60X to 69Y are133'30-134'25 MHz solely lor the purpose ofselection on combined controllers. Selection of oneol these chinnels would only give range informationro an aircraft not equipped with full TACAN.

Associated identity is the term given forqynchronization ofthe ident signals from co-locatedbeacons. Each 30 s interval is divided into lbur orrnore equal parts with the DME beacon identtransn'ritted during one period only and the associatedv.h.f. facil i ty ident during the renraining periods.Associated identity would also be used with a Vortacbeacon which provides bearing and range informationto both civil and military aircraft. A TACAN (orDME) beacon not co-located with VOR would usendependent identity. .

Instattation

The DME interrogator comes in rrrany forms; airlinestandard equipment is rack-mounted whereas generalaviation interrogators rriay be panel-rnounted withintegral controls and digrtal readout. King have goneone better with their KNS 80 integrated nav. systemsince one panel-rnounted box contains the DMEinterrogator: v.h.f. nav. recejver and converter,glideslope receiver, RNAV computer plus integralcontrols and readout of range, ground speed, andtime to station (see Fig. I .10).

Figure 7.5 shows a single DME installation with acombined v.h.f. nav./DME controller, an output to an

Fig. 7.5 DME installation with RNAV tie-in

RNAV computer/controller and with slant range andground speed or time to station displayed on an HSI(Fig. 7.6). All larger aircraft would have a dualinstallation, possibly with changeover relays for HSIfeeds. A combined controller is usually found, but itis possibie (not advised) to have separate DME and

v.h.f. nav. controllers. The RNAV facility (seeChapter l2) nray not be available, in which case slantrange would be fed direct to the HSI or often, aseparate DME indicator in which speed and time iscomputed. wirh a DME indicator fitted the HSI maystill act as a repeater for slant range.

Fig. 7.6 KPI 533 pictorial navigation indicator (courtesyKing Radio Corp.)

Co-axial cables are used for antenna feeder andsuppression. With a dual ATC transponder and dualDME installation all four sets will be connected inparallel for suppression purposes, so that when onetransmits they are all suppressed. The antenna ismounted on the underside of the fuselage in anapproved position. Sufficient spacing between allLband equipment antennas must be allowed to helppreven t mutual interference, although suppression,different frequencies, p.r.f.s and pulse spacing allcontribute to this.

Tuning information to both DME and the v.h.f.nav. receiver is likely to be 215, although b.c.d. andslip codes may be found. Screened cables, preferablytwisted and screened, are used for transfer ofanalogue or digital data and also for audioidentification to the audio integrating system. Theaudio may be routed through the controller if avolume control is irrcorporated in the system. Othercontroller/DME interconnections are for self-test, off,standby and on.

1 1 1

Controls and Operation

A drawing of a combined controller is shown in

Chapter a (Fig.4.9). Controls for DME are minimal.Frequency selection is usually by rotary click stopknobs, the digitat readout of frequency on the

controller being the v.h.f. nav. frequency, e'g.

108'00 MHz. The DME on/off switching mayincorporate a standby position. Usually 'standby'

indicates that VOR/ILS is on, while DME is on

standby i.e. transmitter disabled. Such a switch is

often marked 'off'-v.h.f. nav, and DME off;'receive' - v.h.f. nav. on, DME standby;'transmit' - both v.h.f. nav. and DME on. A self-testswitch will be provided on the controller or, rarely,be panel-mounted. Further switching takes place onthe indicator for ground speed (KTS or SPD) ortime-to-station (TTS or MIN). A hold switch mayalso be found (see previous note on'hold').

Operation is simple; just switch on, tune torequired beacon and ensure lock-on after a briefsearch. lf the indicator employs a mechanically drivendigital readout a flag will obscure the reading duringsearch, whereas with an electronic digital readout the

display will be blanked. When tuning to a differentbeacon the ident signal should be checked to ensurethe correct channel has been selected. Also if theDME is of a type which searches out from itslast-known reading the self-test must be operated toreturn the dials to near zero so that an outboundsearch will result in lock'on to a line-of-sight replyand not an echo. Even with DMEs whichautomatically search out from zero af'ter a channelchange the self-test should be operated occasionally'

Simplif ied Block Diagram Operation

The bl<rck diagram of Fig. 7 .7 can be used to explainthe operation of virtually all DME interrogators.Naturally variations will occur when comparing typesof DME; in particular the range'measuring circuits wil lreflect the ingenuity of the designer and further. asone rvould expect, have in recent years made thetransition from analogue to digrtal techniques.

A jitter generator divides the p.r.f. of a timingoscillator output by a variable divisor. For examplewith a basic p.r.f. of 400 a divisor of approximately20 would provide a track p.r.f' of 20, while a divisorof approximately 4 would provide a search p.r.f. of

100. Of vital importance to the operation of DME is

that the divisor varies randomly, so that if on trackthen between, say, l5 and 25 timing pulses may occurbetween successive output pulses /s from the jitter

generator.

112

The pulses ts are fed to the modulator and thusdecide the time of transmission. The modulatorproduces pulse pairs of the appropriate spacing which

in turn key the transmitter power amplifiers. The r.f.is generated by a frequency synthesizer the output ofwhich serves as receiver local oscillator as well astransmitter master oscillator. The amplified r.f. is fedto and radiated from an omnidirectional antenna.The peak power output of a modem airline standardDME will be about 700-800 W nominal.

Received pulses are fed to the receiver mixer via a

tuned preselector which gives image rejection andsome protection from the transmitted signal. Inaddition duplexing action will normally be employedto ensure receiver mixer protection duringtransmission. Since the transmit and receivedfrequencies are always 63 MHz apart, the frequencysynthesizer can be used as described above and the i.f.

amplif ier is tuned to 63 MHz. A dual superhet maybe employed. The receiver output will be thedetected video signal.

The decoder gives an output pulse for eachcorrectly spaced pair of pulses. The decoder output

consists of replies to all interrogating aircraft plus

squitter or pulses at the identif ication p.r.f. of1350 Hz, in which case a band pass fi l ter gives a1350 Hz tone output to the audio integrating system.

The auto standby circuit counts the pulses comingfrom the decoder and if the rate exceeds apredeternrined figure (say 400 per second) euables thejitter generator. If the rate is low there wil l be no

modulator trigger and hence no interrogation. A third

decoder output is fed to the range gate.

The zero time pulses /o are effectively delayed and

stretched in the variable delay which is controlledeither by the search or track circuits' The output of

the variable delay, often termed the range Satewaveform, opens the range gate ?'ps after every

interrogation. If a reply or squitter pulse is received

at a time when the range gate is open. a pulse is fed to

the cr.rirrcidence counter' Assume the DME issearching with an interrogation rate of 100, and

further, assume the range gate wavelorm gating pulses

are 20 ps in'duration, then on average during a period

of l/100 = 10000 prs the range gate wil l be open for

only 20 ps, i .e . l /500th or 0 '2 per cent of the t ime'

Now squitter and unwanted replies occur randomly

so the chance of full coincidence at the range gate is

roughly I in 500 for each of the decoder outputpuises. Since there are 2700 received pulse pairs per

second we wil l have, on average' 2700/500, i 'e.

5-6 pulses per second from the range gate.

During search the variable delay is continuously

increased at a rate corresponding to anything from

Suppressionpulse gen.

t -II

r ll l

_ l lIIIIIII

replios

PRF change

*-oirt"-nJlto ind.

Range .measunngcircuits

Rx supprcssion

Frg.7 .7 Interrogator block diagram

liming gen.o/P i l i l i l l i l i l i l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

Jittcr gcn.otP r t lRengo gateYVtVCtOrm

DodcrotP

r---1F3 7.8 Stroboscopic principle

HI t3

l|nec g!r.otP

20 to 400 nautical miles per second, depending on thevintage of the design. While in search the range gate,output rate, as detected by the coincidence counter,is low. When the delay lps is equal to the roundtrip travel time plus the 50 ps beacon delay the rangeBate output rate increases by a sigrificant amount.Assuming as above a search p.r.f. of 100, and also a50 per cent reply rate, then the output.of the rangegate will jump from say 5 pulses per second to 50pulses per second. This is the situation shown inFig. 7.8. When this easily detected increase in rateoccurs the mode control circuit will: (a) enable thetracking circuit; (b) inhibit the search circuit;(c) send a p.r.f. change sigral to the jitter generator;and (d) lift the indicator blanking or flag asappropriate.

During track the variable delay is controlled by thetracking circuits so as to keep each wanted reply inthe centre of the corresponding range gate waveformpulse. Should the aircraft be flying towards thebeacon, successive replies will appear early within thegate pulse, so causing the delay to be reduced. Theopposite occurs when the aircraft is flying away fromthe beacon. The variable delay represents the slantrange and so a signal proportional to or representingthis delay is fed to the indicator and/or RNAVcomputer.

Ifwanted replies are lost, the coincidence counteroutput registers a zero tate and hence the modecontrol switches to memory. With static memory thetracking circuits are 'frozen', whereas with velocitymemory the tracking circuits continue to change thevariable delay at the last known rate.

Range Measuring and Mode Gontrol

AnalogueTypically in an older analogue DME the variable delaytakes the form ofa phase shifter resolver, the rotor ofwhich is fed from the timing oscillator and ismechanically coupled to a distance measuring shaft.The tracking circuits in such equipment often employa ramp generatoi. Figure 7.9 illustrates a blockdiagram and waveforms which may be used to explainthe operation of such a DME but is not meant torepresent any particular equipment.

The timing generator output is sinusoidal and somust be fed to a pulse former (zero crossing detector)before the jitter generator. The timing signal is alsofed to a phase shift resolver where it is phase-shifted(delayed) by an angle depending on the position ofthe distance-measuring shaft which also drives thereadout. Pulses coincident with the positive-going

1 1 4

zero crossings of the delayed sine wave turn on(Q = l)a bistable which is turned otr (Q = 0) by thezero time pulses from the jitter generator. Thebistable output is connected to the positive-goingtrigger input of a monostable; in this way theresulting 30 ps pulses occur at a time determined bythose delayed timing pulses which occur I ps aftertransmission. The elapsed time I represents therange readout which will be obscured'by a flag duringsearch.

In the logic employed in Fig. 7.9 a low rate outputfrom the range gate will give a logic zero output fromthe coincidence counter, so enabling the searchcircuit but disabling the early and late gates. WhenZps corresponds to the actual slant range ofthebeacon the range gate output rate is high, hence thesearch circuit is disabled and a logic one is fed to theearly and late gates. The other inputs to the early andlate gates are the decoded pulses and a rampwaveform symnietrical about zero volts and coincidentwith a 30 ps range gate waveform pulse. The rampinput to the late gate is inverted so that the late gateis open for almost all of the latter half of the 30 psperiod, while the early gate is open for almost all ofthe first half. The slope of the ramp waveform ischosen so as there is a period (equal in duration to the -

decoder output pulse width) when neither early norlate gate is open. Thus when on track the wantedreplies are steered to the decrease or increase rangecircuits, depending on whether the replies arrive earlyor late within the range gate waveform pulserespectively.

The motor drive circuits supply the motor so thatwhen in search the readout and delay progressivelyincreases. While in track the motor will turn in adirection dependent on which of the decrease andincrease circuits gives an output. It can be seen thatin track we have a servo system which maintains thewanted replies in the centre of the range gatewaveform pulses.

The memory circuit is enabled with the early andlate gates when it clears the flag. Subsequently, shouldthere be a loss of replies, search will be inhibited andthe motor held (static memory) or made to continuerotating with the same sense and speed (velocitymemory) for the memory time.

DigitalWhat follows is an explanation of the principles of afirst-generation digital DME based on, but notaccurately representing, the RCA AVQ 85. Currentlythe trend is to use a special-purpose l.s.i. chip toperform the range measurement and mode controltasks.

\ - l ll

Delayed I ttiming ; \pulses i____

MoToRDRIVE

Ranoeoat"" Enable

- - - - l

I

IIIIII

roln Iming Ien. i

II-'l I

t l- J - - - . . i

r o

Delayedtlmrngpulses

Bistableo

r r l+ i T e ' I

t ' � l

Mono^stabre h j* so r,s ho l

Monostableo

Ramp gen.

The AVQ 85 has a search p.r.f. of 40, a track p.r.f.of l2 and a maximum range of 400 nautical miles,which corresponds to a two-way travel time of5000 ps. During this time the nuntber of pulse pairsreceived from a beacon wil l be, on average:

5 0 0 0 x 1 0 - 6 x 2 7 0 0 = 1 3 . 5 .

Of the thirteen or fourteen pulse pairs received onewill, hopefully, be a wanted repiy.

During the search mode the elapsed time betweenf6 and the time arrival of a particular decoded pulse

Decoder o/P Earlv n Late

Fig. 7.9 Analogue range measuring and mode control blockdiasram

is measured. If rn is the time measured after thenth interrogation then /1111 is the time to the firstdecoded pulse to arrive such that tn+1) tr, wherer,l = 0, L . ., and /o = 0. When we have equality, i.e.tn+L = /r, then /r, is, subject to further checking, theround trip travel t ime to the beacon. It can be seenthat if the aircraft is at rnaximunl range we shall need,on average, l3-14 successive interrogations to completethe search time- At a search rate of 40 this will takesay l3'5/40 s, i.e. about one-third of a second. Theacquisit ion time of the AVQ 85 is quoted as less than I s.

1 1 5

Int.rrogatiorl

lst mcesurornont

2nd measuremart

3rd measurement

4ttl melsurc|tranl

Sth mcaermont

6th measuremetrt

7th- moasurotnctrt

Fig.7.l0 Digital rangemeasurement - search

With the above operation only one timemeasurement needs to be stored in a register. With amodest amount of memory the wanted reply couldbe identified within two successive interrogations,provided that such a reply was received after each ofthe interrogations. If we assume a 50 per cent replyrate then four interrogations would be needed.During a 5000 ps interval less than eighty-four pulsepairs will be received, assuming a minimum spacing of60 ps between beacon squitter transmissions andallowing for a 60 ps dead time. Each timemeasurement would need l2 bits if a resolution ofone-tenth of a nautical mile is required. Thus'a fastersearch time could be achieved if a RAM of12 X 84 X 4 = 4032 bits were provided. A practicalcircuit would consist of 4 X lK bit (lK = 1024\RAMs, the pulse arrival times, expressed as distances,dfter each of four interrogations being recordedsuccessively in each RAM chip. The first chip wouldthus record the arrival times after the first, fifth,ninth, etc. interrogations, similarly for the second,third and fourth chips. Of course only one 4K bitdrip is needed, provided it can be organized into fourlinear arrays of l2-bit words. With a search p.r.f. of40 there is a period of 30000 ss (= l/40) less5000 ps in which to check for equal arrival times

1 1 6

which, when detected, signal the end of search.In Fig. 7.1I we return to the one measurement per

interrogation situation. Initially the distancemeasuring circuit counters and registers are cleared.Time measurement from /6 is carried out by thedistance counter which counts 809 kHz clock pulses,thus giving a range resolution of one-tenth of a mile.The sequence of events following the (r + I )thinterrogation of a search cycle is as follows:

l . t e + 2 0 p sDistance countgr ciears.Blanking counter loaded with contents ofdistance storage register = rn.

2. ts + 47 ttsBlanking bounter starts to count down.

3. ls + 50 r/sDistance counter starts to count up towardsmaximum range.

4 . t o + t nBlanking counter reaches zero and henceenables blanking gate and triggers range gatewaveform generator.

5 . t s + t r a lA decoded pulse arrives and passes throughenabled blanking gate to stop distance counter

Validreply

tlccodcd

Rangegatewavaformgen.

pulsos

Distanceto lnd8O9 kHz

809 kHz

F!. ?.ll Digital rarge measuring and mode control blockdiagram

and trigger transfer of data to distance storageregister; n becomes n + I and circuit waits fornext to.

The above sequence is repeated after eachinterrogation. Within, on average, fourteeninterrogations the time to a wanted reply will becounted and the distance storage register will containthe number of tenths of a nautical mile actual range.After the next interrogation the blanking counter willenable the blanking gate 3 prs before the arrival ofthewanted reply, since the blanking counter start is47 ps after re while the distance counter start is50 ps after fe. It therefore follows that the distancecounter will record the same d'.stance, subject toaircraft movement. thereafter.

The pulse from the range gate waveform generator

is of 6 ps duration, its leading edge being X ps after tewhere X is the time of arrival of the previouslymeasured decoded pulse less 47 ps. This gating pulseis fed to the range gate together with the decoderoutput. Coincidence indicates that the two latestpulses to be measured have arrived with the same time

delay t 3 prs with respect to /s and are thus probablywanted replies. The percentage reply checking circuitthen checks that two of the next eight interrogationsgive rise to a decoded pulse within the track gate andif so the mode switches to track. On track the p.r.f. isreduced and the indicator gives a readout ofthe rangeas measured by the distance counter. After switchingto track, at least four of any sixteen successive

PRF Indchang€ enable

Decodedpulsss

interrogations rrtust give rise to a range Sate output;failure initiates a switch to memory. Five secondsafter memory is entered the mode will revert to

search, subject to auto standby, unless the four-from'sixteen check indicates success, in which case trackresumes.

Characteristics

The following summary is drawn from the ARINCCharacteristic 568-5 for the Mk 3 airborne DME, it isnot complete and does not detail all the conditionsunder which the following should be met.

Channels252 channels selected by 215 switching.

hise SpacingInterrogation l2 r 0'5 ps mode X;36 x 0'5 prs mode XDecoder output if lZ ! 0'5 1rs mode X; 30 i 0'5 ttsmode Y.Decoder: no output ifspaeing of received pulse pain

more than I 5 ps from that required.

Range0-200 nautical miles with override to extend to300 nautical miles.

TmckW Sped0-2000 knots.

sToP

717

Acquisition TimeI s or less.

Memory4-12 s velocity memory.

r.f. Power Output> 25 dBW into 50 O load.

Fig.7.l2 TIC T-24A (courtesy Tel-Instrument ElcctronicsCorp.)

1 1 8

Intenogation Rote :r i : i: -

Overall less than 30, assuming on track 95 per cent oftime, searching 5 per cent of time.

Auto StandbyAt least 650 pulse pairs per second received beforeinterrogations allowed.

Tx Frequency StabilityBetter than t 0.007 per cent.

Rx Sensitivity-90 dBm lock-on sensitivity.

Suppressiort Pulse Durat ionBlanket: l9 prs mode X;43 ttsmode LPulse for pulse: 7 gs.

Antenna v,s.w.r.l '5: I over 962-1213 MHz referred to 50 O.

Antenna Isolotion> 40 dB between L-band antennas.

Outputsl. Digital: 32-bit serial b.c.d. word at least f ive times

per second, resolution 0.01 nautical miles.Buffers in utilization equipment.

2. Analogue: pulse phirs 5-30 times per second withspacing, in ps, 50 + l2'359d (d being slantrange). Each load l2K in parallel with less thanI 00 pF.

3. Range rate pulse transmitted for each 0.01 nauticalmile change in range.

4. Audio ) 75 mW into 200-500 Q load.5. Output impedance < 200 st.6. Warning flag ( I V d.c. for warning, 2i.S y d.c.

satisfactory operation.

Range Output AccuracyFrom t 0.1 to t 0.3 nautical miles, depending onsigral strength and time since acquisition.

Ramp Testing

A DME installation should be tested using a ramp testset which will test by radiation, simulate variousranges and velocities. operate on at least one spotfrequency for mode X and mode I/, and provide forsimulation of identification. Two such test sets arethe TIC T-24A (Fig. 7.12) and the IFR ATC"600A(Fig. 8.23) .

TIC T.24AA battery-operated, one-man test set operated from

*.*tF d* in$

-trf:l;l:a{{dtir-ri l @ *rt t lt

/ ! * ,

' k " r | - , ' a .

a

- E E - I

*$ - . ** e?.d

ciil,.,") ;

tlq . t. *.r& nfr_ft i*+ffi 4,frwa.:r. s , : , a i : d ; : i {

e a

, . . . : ; , . .fr:

T Et ,

t ^tld

{ i * . nl ' ^

* { r t f * @ r f

: 4*. .. :a.r/:{ . a

Fig. 7.13 TIC T-50A (courtesy Tel- lnstrument ElcctronrcsCorp.)

' ! 1 9

the cockpit and testing by radiation. Channels lTXand 17Y are available (108.00 and 108.05 MHz VORfrequencies) with range simulation ftom 0 to 399.9nautical miles in 0.1 nautical mile increments. Thevelocity, inbound or outbound, can be selected inlO-knot increments from 0 to 9990 knots. Squitteris selectable at 700 or 2700 pulse pairs per second.Identity is available as 1350 or equalized 1350 pulsepairs per second. An additional pulse pair l0nautical miles after the reply pulse pair can beselected, to enable a check of echo protection. Thep.r.f. meter has two ranges 0-30 and 0-150. Finallythe percentage reply may be selected in l0 per centincrements from l0 to 100 per cent.

ATC 600AThis test set does not have all the facilities oftheT-24A but does offer comprehensive testing abilityfor the ATC transponder (Chapter 8). Like theT-24Athe ATC 600A operates on channels l7X andl7Y. The range can be set from 0 to 399 nauticalmiles in I nautical mile steps. Twelve differentrrelocities may be simulated in the range

50-2400 knots inbound or outbound. The identity isequalized 1350 pulse pairs per second. The percentagereply is either 50 or 100 per cent by selection.Features of the ATC 600A not available with theT-24A are an interrogator peak r.f. power readout,accuracy t 3 dB (t 50 per cent) and interrogationliequency check.

Bench TestingVarious test sets exist for the bench testing of DME,one of these is the TIC T-50A (Fig. 7.13) which alsoprovides facilities for ATC transponder bench testing.This is not the place to detail all the features of such acomplex test set; suffice it to say that the test set ismade up of optional modules so that the customercan choose the most suitable package. One featurewhich must be mentioned is the ability to measurethe pulsed r.f. from the DME interrogator with aresolution of l0 kHz. TIC have found that manyunits change their output frequency, sometimesbeyond allowable limits, when a change in pulsespacing occurs; i.e. X to I/ mode or vice versa.

120

I ATC transponder

lntroduction

With the rapid build-up of international and domesticcivil air transport since World War II, control of airtraffic by means of primary surveillance radar (PSR)and procedures is not adequate to ensure safety in theair.

?---=__mFig E.l Primary surveillance radar

A PSR does not rely on the active co-operation ofthe target. Electromagnetic (e.m.) radiation is pulsedfrom a directional antenna on the ground. Providedthey are not transparent to the wavelength used,targets in tine with the radiation will reflect energyback to the PSR. By measuring the time taken, andnoting the direction of radiation, the range andbearing of the target are found. Display is by meansof a plan position indicator (see Chapter 9). Such aqystem has the following disadvantages:

l. Sufficient energy must be radiated to ensure theminimum detectable level of energy is receivedb! the p.s.r. after a round trip to a wantedtarget at the maximum range. Range is

,' proportional to the 4th root of the radiatedenergy.

2. Targets other than aircraft will be displayed(clutter). This can be much reduced by usingDoppler effect (see Chapter l0) to detect onlymoving targets.

3. Individual aircraft cannot be identified exceptby requested manoeuvre.

4. An aircraft's altitude is unknown unless aseparate height-finding radar is used.

5. No information link is set up.

Recognition of these disadvantages, in particularNo. 3, led to the development of a military secondarysurveillance radar (SSR) known as identificationfriend or foe (lFF). With this system only speciallyequipped targets give a return to the ground. Thissystem has since been further developed and extendedto cover civil as well as military air traffic; the specialequipment carried on the aircraft is the air trafficcontrol (ATC) transponder.

Basic Principles

Secondary surveillance radar forms part of the ATCradar surveillance system: the other part being PSR.Two antennas, one for PSR, the other for SSR, aremounted co-axially and rotate together, radiatingdirectionally. The SSR itself is capable of givingrange and bearing information and would thus appearto make PSR redundant: however we must allow foraircraft without ATC transponder fitted or a possiblefailu re.

We can briefly explain SSR in terms of Fig. 8.2.The SSR transmitter radiates pulses of energy from adirectional antenna. The direction and timing of theSSR transmission is synchronized with that of thePSR. An aircraft equipped with a transponder in thepath of the radiated energy will reply with speciallycoded pulsed r.f. provided it recognizes theinterrogation as being valid. The aircraft antenna isomnidirectional.

The coded reply received by the ground is decoded,and an appropriate indication given to the air trafficcontroller on a p.p.i..display. The reply wil l giveinformation relating to identity, altitude or one ofseveral emerElency messages. Figure 8.3 shows atypical data presentation. As can be seen, a variety ofsymbols and labels are used to,ease the task of thecon troller.

InterrogationOne interrogation consists of a pair of pulses of r.f'energy, the spacing between the pulses being one offour time intervals. Different modes of interrogation

721

Fig 8.2 Secondary surveillance radar

Fig. 8.3 Typical data presentation

lre coded by the different time intervals, each modecorresponding to +different ground-to-air .question'.

For example mode A - 'what is your identity'?Figure 8.4 illustrates the modes of interrogation.

Many transponders have only mode A ind Ccapability; this is sufficient to respond to aninterrogator operating on mode interlace wherebymode A and C interrogations are transmitted insequence, thus demanding identification and altitudeinformation. Mode D has yet to be uti[zed.

122

The maximum interrogation rate is 450 although,in order to avoid fruiting (see below - False Targets),the rate is as low as possible consistent with eachtarget being interrogated twenty to forty times persweep. The pulses of r.f. are 0.8 ps wide and at afrequency of 1030 MHz (L band) this being the samefor all interrogations.

ReplyA transponder will reply to a valid interrogation, theform of the reply depending on the mode ofinterrogation. A valid interrogation is one receivedfrom the interrogator mainlobe (see below,- SideI-obe Suppression), the time interval between pulsesbeing equal to the mode spacing selected by the pilot.

^ - In every reply two pulses of r.f. 1090 MHz, spaced20.3 ps apart are transmitted, these are the frame orbracket pulses, Fl and F2. Between Fl and F2 thereare up to twelve code pulses designated and spaced asshown in Fig. 8.5; a thirteenth pulse, the X pulse,may be utilized in a future expanded system.

The presence of a code pulse in a reply isdetermined by the setting of code selector switches onthe pilot's controller when the reply is in response toa mode A (orts) interrogation. If the interrosation ismode C the codc pulses transmitted are autoriaticallvdetermined by an encoding altimeter.

A pulse 4.35 gs after F2 may be transmitted. Thisis the special position indicator (Spl) pulse. otherwiseknown as the indicate position (I/p) or simply identpulse. lf the reply is in response to a mode Ainterrogation the SPI pulse is selected by aspring-loaded switch or button on the pilot's controller.A brief depression of the switch will cause the Splpulse to be radiated with every reply to a mode Ainterrogation received within l5-30 s. Some oldertransponders will transmit a SpI pulse in reply to amode C interrogation when the reply code contains aD4 pulse; this corresponds to an altitude in excess of30 700 ft.

' i

:ij'i

- 3

3I:.:

o!o=

ldontity

ldentity

Altitude

Unassigncd

Fig 8.4 Interrogation pulse spacing

Fr cl A1 c2 A2 C4 A4 'X'� 8r _Dr 82 d2 84 D4 F2

Fig 8.5 Reply train format

Coding: IdentificationThe code pulses transmitted depend on four codeselector switches, each of which controls a group ofthree pulses in the reply and may be set to one ofeight position,0-7. The code groups are designatedA, B, C and D, the pulses within each group havingsuffixes 4,2 and I (see Fig- 8.5). The resulting codeis binary coded octal, the most significant octal digitbeing determined.by the group A pulses, the leastsignificant by the group D.

Selection of the A pulses gives the familiar binarycode, as shown in Table 8.1. Similarly with selectionof the B, C and D pulses.

The number of possible code combinations iseasily arrived at since we have four octal digits giving8a = 4096. Some of the code combinations are sivenspecial significance; we have:

'7600 Radio failure 77OO Emergency

There is also a special code for hijack.

Coding: AltitudeThe flight level of the aircraft referenced to a pressure

Table 8.1 Group A code selection (similarly forgroups B, C and D)

AI SelectionA2A4

0000

00II00II

0I0I0I

: 0I

0I23456

il ,o,o

o'o'Fig. 8.6 Examples of pulse trains lbr particular selectedcodes

l*z.s r,t-l

123

of 1013.25 mbar (29.92 inHg) is encodedautomatically in increments of 100 ft, the code usedbeing laid down by the ICAO. The maximum encodedrange is from -1000 to 126700 ft inclusive. With100 ft increments this requires 1278 different codecombinations, with 4096 available we see there isconsiderable redundancy. It is impractical to use moreof the available codes since the aciuracy ofbarometric alt imeters is such that it is not sensible tohave, say, 50 ft increments; in any case the objectiveis to indicate fl ight levels which are in hundreds offeet .

To accommodate the redundancy the Dl pulse isnot used and, further, at least one C pulse istransmitted but never Cl and C4 together in a singlereply. Thus for each eight possible A groupcombinations of pulses we have eight B group, f ive Cgoup and four D group, giving 8 X 8 X 5 X 4 = 1280possible code combinations; two more than necessary.The extra two. if assigned. would correspond to

I 100 and -1200 f r .The C pulses form a unit distance reflected binary

code giving the 100 ft increments. As shown inTable 8.2 the reflected pattern, starting atCl = C2 = 0, C4 = l , begins rvhen Modle(A) = 8;i.e. when the renrainder on dividing the altitude (inhundreds of feet) by 10 is 8. Thus to find the Cpulses in the code for. say,25 400 ft we haveA= 254, Mod16(A) = 4, so Cl and C2 are in the reply .

Table 8.2 100-Foot increment coding

Mod,o(A) cr c4

890I2

Reflection

The A, B and D pulses form a Gray code giving atotal of 256 increments of 500 ft each, i.e.128 000 ft, commencing at -1000 ft. In order offrequency of bit change we have 84,82, Bl, 44, A2,Al , D4, D2; thus 84 changes every 1000 ft whereasD4 does not enter the code unti l 30 800 ft and D2until 62 800 ft. Needless to say aircraft in the generalaviation category wil l not need to employ encodingaltimeters giving D4 and D2 selection.

To find the A, B and D pulses we can useTable 8.3. Since the entries in the table commencewith zero, whereas the altitude commences at

c2

000I

II000

:rl

000II

I

II000

0 1 1 0 0 1 1 0 0 r 1 0 0 1 t 00 0 1 1 1 1 0 0 0 0 t r 1 r 0 00 0 0 0 . I r t 1 1 1 I r 0 0 0 00 0 0 0 0 0 0 0 1 r t I I I I I

-

4)61

i,:iI

:;

{

B4B2B1A4

D2 D4 AI A2

0000

- 0o00I

I

II

It

0000

0000

II0000

00

00

0 1 2 3 4 s 531 30 29 28 27 26 2s32 33 34 35 36 37 3863 62 61 60 s9 58 5764 55 66 6't 58 69 709s 94 93 92 9 l 90 8996 97 98 99 100 101 t02 I

t27 126 125 t24 t23 122 t2t It28 129 130 131 r32 r33 134 I159 158 r57 156 155 154 153 I160 161 162 163 t64 165 166 Il 9 l 190 189 188 187 186 185 It92 193 t94 195 196 r9't 198 I221 222 22r 220 219 218 217 2224 225 226 227 228 229 230 2255 2s4 253 252 25t 250 249 2

0II00II00II00II0

7 824 2339 4056 557 t 7 2 .88 8703 1042 0 l l 935 135s 2 1 5 l67 16884 18399 20016 2t53 1 2 3 248 247

9 l 0 l t t 2 1 3 1 4 1 522 2 t 20 19 l 8 t 7 1641 42 43 44 45 46 4754 53 52 5 l 50 49 487 f 74 7S 76 77 78 7986 85 84 83 82 8t 80

105 106 107 108 109 l l 0 l 1 ll l 8 t t 7 1 1 6 l 1 5 1 1 4 l 1 3 t r 2137 138 139 ,140 l 4 l 142 r43150 t49 148 t47 146 145 t44169 170 l7 l 172 r '13 174 t ' � t5182 r8 l 180 t79 178 t77 t7620t 202 203 204 205 206 2072r4 2t3 212 2tr 210 209 208233 234 23s ?36 23't n8 239246 245 244 243 242 241 240

Table 8.3 500-Foot increment codins

124

-1000, we must add 1000 to the altitude beforeentering the table. The table records the altitude inincrements of 500 ft. The following algorithm willgive the required code:

(i) add 1000 to the altitude;(ii) enter table with the integer part of the result

of (i) divided by 500;(i i i) read the code: row, column.

As an example we will find the complete code forI l0 200 f t :

( i ) 1 1 0 2 0 0 + 1 0 0 0 = l l l 2 0 0 ;( i i ) In t . ( l l l 200/s00) =222;( i i i ) 10110001 = D2 D4 Al A2 A4 Bl 82 F.4.

To find the C pulses:

Modls (l102) = 2, therefore 100 = Cl C2 C4.Complete code is l0 l 10001100.

False TargetsThere are several causes of unwanted returns beingdisplayed on the air traffic controller's p.p.i., one ofwhich is interrogation by side lobes. This is discussedin some detail below under the heading side lobesuppression.

Since the transponder antenna is omnidirectionalthe reply pulses nreant for one interrogator may alsobe received by another, providing it is within rangeand its antenna is pointing in the direction of theaircraft concerned. Such unwanted returns wil l notbe synchronized with the transmission of theinterrogatc'r suffering the interference and wouldappear as random bright dots on the p.p.i. This typeof interi-erence, known as fruit ing. carr be dealt withby making use of the tact that differentinterrogators work on difl-erent interrogation rates,

replies may thus be sorted on this basis.A reply from a transponder lasts for a period of

20'3 ps, thus the transmitted pulse train wil l occupy adistance of 163 000 X 20.3 X l0-6 = 3.3 nauticalmiles in space (speed of propagation being163 000 nautical miles per second). As a consequenceany two aircraft in line with the interrogator, andwith a difference in slant range of less than 1.65nautical miles, will transmit replies which overlap inspace and consequently mutually interfere at theinterrogator receiver. Such replies are said to begarbled. Secondary surveillance radar is most usefulwhen traffic densities are greatest. Thesecircumstances, of course, give increased garbling.

Reflections of the transmitted energy, eitherinterrogation or reply, from mountains, hil ls or largestructures wil l give an indicated reply at an incorrectrange. Since the direct path is shorter than thereflected path echo protection mav be used; i.e. thereceiver may be suppressed or desensitized for alimited period on receipt of a pulse.

Side Lobe Suppression (SLS)When a reply is received its angular position on thecontroller's p.p.i. is determined by the direction ofthe main lobe radiation from the interrogator. If thereply is due to an interrogation tiom a side lobe thenthe indicated bearing wil l be incorrect. Two systemshave been designed to suppress replies to side lobeinterrogation: they are the ICAO 2 pulse and theFAA 3 pulse SLS systems. Some older transpondershave circuitry applicable to either system; howeverthree-pulse SLS has advantages over its virtuallyobsolete rival and is the only system considered,

The polar diagram for the interrogator antennasystem is shown in Fig. 8.7. Pl and P3 are theinterrogat ion pulses spaced at 8,17 ,2 l or 25 psradiated from the directional antenna. P2 is the SLScontrol oulse radiated from an omnidirectional

r*'JP1, P3

Fig. 8.7 Three-pulse s.l.s.

I 2 r s f -

125

antenna. The field strength for P2 is such that anaircraft within the Pl/P3 main lobe will receive P2 ata lower amplitude than Pl/P3 whereas elsewhere P2will be greater. The condition for a reply/no replyare:

n >Pl no replyP2<Pl -9dB reply

otherwise mry reply

In the grey region the probability ofa reply increaseswith decreasing P2 amplitude relative to Pl.

Probabilityof reply

lnstallation

Figure 8.9 shows a typical transport category aircraftdual installation. Two transponders are mounted sideby side in the radio rack mating with back plateconnectors in the mounting tray junction box. Twoencoding altimeters and the control unit have theirconnectors routed through the back plate to thetransponders while direct connections to the front ofthe transponders are made from two L-band antennasand the suppression line interconnecting all L-bandequipments. Power supplies are routed through theback plate.

The encoding altimeters may be blind (no altitudeindication) or panel-mounted. On larger transportaircraft encoded altitude information may be derivedfrom a static pressure-activated capsule mechanism ina central air data computer (CADC).

The transponders are fitted on a shock-mountedtray. Cooling is by convection and radiation. Mosttransponders designed for the general aviation marketcombine transponder and control unit in one box,which is panel-mounted. To ensure adequate coolingsufficient clearance must be allowed around each unit.

A common co-axial cable interconnects thetransponders and DME interrogators. A transmissionby any one 'black box' will cause the receivers of the

Supply: 115 V, 4OO Hzand/or 28 V d.c.

l l lumination

, Grey I

Fig. 8.8 Probability ofa reply

S0ppression inlout

Fig. 8.9 Dual transponder installation

126

n-P2(dBsl

:ijl

J

{

.s..{E,i

3!I

:

EE

@EE

@,rrl/{o*-(/

other three to be suppressed for a period dependingon the_source; a transponder suppression puir. will beabout 30 ps.

control unit is, of course, panel-mounted inthe cockpit and provides the piloiwith.o*fi.t"control over the transponders. Only one transponderat a time is in use while the other iJon standbv.

Controls and Operation

Function Switch Off-Standby-A-B-C_D. Mode Coperation will depend only on the position of thealtitude reporting (a.r.) switch; hence if the functionsvitch has a'C' position such a selection would serveto switch modes A, B and D off but have no effect onthe selection of mode C. Many transponders haveonly A and C capability in which .ur. th. functionswitch might have positions Off-standby-A.

?!:.S*:!rhes Rotary switches mounted co-axiailym palrs;thumbwheel switches are a common

alternative. The code setected appears in a windowabove the switches and will determine the codepulses present in the reply in response to a mode A(or.B) in_terrogation. Some transpona.r, t .u" "facility for remote automatic teying fo, moOes e anaB; this may be selected by setting tlie code ,rit.h.,to 8888. In fact the necessary extra equipment hasnever been introduced.

I/P.Syvit-ch Spring-loaded to_off pushbutton or toggles'uritch for selection of the SpI puise. May be labelledSPI or ldent.

Lo Sense Switch When selected to .on'reduces the

transponder receiver sensitivity by l2 dB. Thisfeature was introduced as an inteiim measure forreducing side lobe response. Subsequentdevelopments have made this facility unnecessary.

A.R. Switch On-off selection for altitude reporting.

Test (T) Switch Spring-loaded to off pushbutton or

Spikeel iminatorpulse widthl imiter

Visual monitor

Fig. 8.10 Transponder btock diagram

727

toggle switch which energizes the self'test circuitry.Indication of a successful self-test is given by a green

visual monitor lamp which also illuminates when areply is sent in response to a valid interrogation. The

self-test switch and visual monitor may be repeated

on the transponder unit for the use of themaintenance engineer.

Tlansfer Switch Selects either transponder No' I orNo. 2 in a dual installation.

Pilot work load with transponder is minimal, there

being no indication other than the monitor lampwhile control switch changes are initiated on ATCs

instructions either by r.t. or through standardprocedures.

Simplified Block Diagram Operation

The interrogating pulses ofr.f. energy are fed tcl thereceiver by way of a 1030 MHz band pass filter' Thepulses are amplified, detected and passed as a videoiignal to the spike eliminator and pulse-wihth limitercircuits; these only pass pulses greater than 0'3 gs induration and limit long pulses to less than thatduration which will cause triggering. The decoderexamines the spacing of the interrogation pulses Pland P3, if it is that for the mode selected an outputwil l be given to the encoder. If the Pl-P3 spacing is2l prs an output wil l be given regardless of the modeselected.

The encoder. on being triggered by the decoder.produces a train of pulses appropriate to the requiredreply which is determined by code switches (mode A)or encoding altimeter (mode C). The encoder outputtriggers the modulator which keys the 1090 MHz

transmitter. Radiation is by an omnidirectionalantenna.

The side lobe suppression circuits are fed with thereceiver video output. In the event of an interrogationby side lobe, the receiver will be suppressed for about

35 ps commencing at a time coincident with the P2pulse, thus blocking the passage of P3.

The suppression circuit is triggered whenever there

is a reply. The suppression pulse so produced is usedto suppress the receiver and is also fed to otherL-band equipment for the same purpose. Thesuppression feature between L-band equipments is

mutual.Automatic overload control (a.o.c.) otherwise

known as group countdown, progressively reducesthe receiver sensitivity after the reply rate exceedstypically 1200 groups per second. Assuming I 5 pulse

replies,0'45 ps pulse widths and a maximum reply

rate of 1480 groups per second we have a typical

airline requirement of a I per cent duty cycle to

handle rnultiple interrogations.Self-test facil i t ies are provided. On being activated,

a test signal is injected into the front end of thereceiver. A successful self-test is indicated by thevisual monitor lamp which lights whenever there is an

adequate transmission. Some transponders have an

audio monitor facil i ty in addition to the visual

monitor.

Block Diagram Details

Transmitter ReceiverThe r.f. sections wil l employ normal u.h.f. techniquessuitable for processing signals in the region of1000 MHz. Interconnecting internal co-axial leads

. , iiIjj

i.{?{*I1:qi in

!':i!

. t

$i

l.;jll

I' i

{nI:

To vidcost6gos

Fig. 8.1I Logarithmic i.f. amplifier

128

must be of a length laid down by the manufacturer asthey play a part in the circuit action; in particular thelead to the transmitter will be of a leneth such thatreceived sigrals will see the transmitteifeed as a highimpedance, thus assisting in the duplexing action.

In flying from maximum range towards theinterrogator a large dynamic range of signal strengthis received, at least 50 dBs from minimum triggeringlevel. Providing adequate amplification for the weaksignals can result in saturation of the last stage(s) ofthe i.f. amplifier when stronger signals are received.Such saturation may give rise to the suppression ofvalid interrogations by the SLS circuit since thestronger Pl main lobe pulses will be limited.

Successive detection of the video sigral canovercome the saturation problem. Figure 8.11 showsa logarithmic amplifier using successive detection.fusuming a gain of A for each stage and a maximumstage input signal Z before saturation of that stage,an input to the i.f. amp. of VlAi wrll cause stage 4 tosaturate. In this event the detected output to thevideo stage will be (VlA') + (VIA) + (V) + (AV)which is approximately equal to AV for'a reasonablegajrnA. Table 8.4 illustrates the difference between aconventional and logarithmic amplifier.

DecoderMany transponders still in service use multi-tappeddelay lines in both decoder and encoder; however allmodern transponders employ monostables and shiftregisters, in the form of integrated circuits, which willbe the only type of transponder considered here.

The detected video signals are applied to the spikeeliminator and pulse width limiter in series. Theaction of the 0'3 ps delay and AND gate is to preventany pulses or noise spikes less than 0'3 ps in duration

Table 8.4 Comparison of conventional andlogarithmic amplifiers

Input signal Output signal

Conventional Logarithmic

AV

2AV

3AV

4AV

being passed to the decoder. Long pulses whichmight cause a'reply are reduced in duration by thepulse-width limiter monostable, the output of whichis a pulse about 0'5 ps in duration regardless of thewidth of the input pulse.

The decoder input monostable ensures that onlythe leading edge of Pl (positive-going t) triggers themode A and C delay circuits each of which consists ofa monostable and a differentiating circuit. The modeA and C AND gates will give an output if the delayedPl pulses from the differentiators are coincident withthe undelayed P3. An output from either AND gatewill give a trigger ('T') output from the OR gate.

Encoderln the example of an encoder shown, two lO-bit shiftregisters are used (e.g. Signetics8274). The operating

AV

AV

AV

AV

VI3

VT2

VT

v

Pulse widthlimiter

Odprrt

Spike eliminator

fig. E.f 2 Spike eliminator and pulse width limiterwavcforms

O ' 5 p s

Output

O'3 l rs

129

.{

r,{

J{.!:::.:

Pulsewidthlimiter

Fig.8.l3 Decoder

Input

30 trs mono output

8 lrs mono- output

21 1rs mono output

Differentiator I O/P

Differentiator 2 O/P

'A' output

'C' output

T'output

Fig, 8.14 Decoder waveforms - mode A intertogation

mode is controlled by inputs S0 and Sl, hold, clear,load and shift being selected by the input values$rown in the truth table (Fig.8.l5). These twocontrol waveforms are produced by two monostables,one triggered by the positive-going edge of the Toutput from the decoder, the other by thenegative-going edge. This anangement gives us the

130

sequenoe load, sh i l t , c lcar ,hok l . ' l hc

S0 waveform isa . lso used t ( ) ga tc a c lock gcr rc ra to r , pc r iod 1 .45 gs .

Load l 'ha lcading cdgc of ' l

tr iggcrs thc control l ingmonostab lcs so t l ra t S( ) = l ,S l . , ( .1 and t l rc sh i f treg is te r e le rncn ts w i l l bc loadcr l w i t l r t l r c b inarvinformation present on t lrc input l i rres. Since i l an<l

n nt t t l| Ln-]P'I P2 P3

l . l-

I l r s

nt l

n

nl l

$lBo,, os x o

10 bitshift re$ster

10 bitshift register

I Pi lot: code

switch

Encodingalt imeterswitchr-

Fig 8.15 Encoder

F2 are required in all replies, the appropriate inputsare high (+5 V). The one time-slot between .A4 andB1 and the two time-slots between F2 and SPI arenever used; consequently their input l ines, togetherwith the two spare after SPI. are low (earth). Thedynamic input lines Cl to D4 will be high or lowdepending on the code switch selection (mode Ainterrogation) or the altitude (mode C interrogation).Assuming a valid mode A interrogation has beenreceived there will be a pulse present on the 'A' linefrom decoder to encoder coincident in time with thepuise on the 'T'l ine. After inversion this A pulse is.applied in parallel to one of twelve NOR gates, theother inputs of which are connected individuaily tothe twelve pilot code switches. Those code pulsesselected result in a ground on the appropriate NORgate input. Since the other input in each case is also

low for the dur4tion of the load mode, the NOR gateoutputs corresponding to selected codes go high andthus set the appropriate shift register element. Similaraction.occurs whenever mode C interrogations arereceived although Dl selection is not involved. Noisefi ltering is employed (r.c. networks) within thetransponder for each of the altitude informationinput leads. SPI loading occurs when the A pulse iscoincident with the l5-30 s output pulse from theSPI timer produced on depression of the SPI switch.

Shift The trailing edge of pul5e T triggers the S 1monostable thus S0 = Sl = I and the shift registersare in the shift operating mode. Shift ing occurs witha high to low transition of the clock pulses, thus inthe l'45 1ls after the first transition the shift registeroutput is high (Fl); after the next transition the

ct

D4

131

s1

so

:S/R outoooo

:l::I

l:

cLx

S/R out7777

Differentiatoro|rt

c2 A2 C4 A4 F2D4B4D2

AND outoooo

AND out7777

Fit. 8.16 Encoder wavefolms

output will be high or low depending on hqw the Clelement was loaded, and so on.

Aear and Ho;ld S0 and Sl pulses are each ofapproximately 30 ps duration, thus S0 goes low ltrst

$ving S0 = 0, Sl = I when all the elements clear.ln fact during shift all elements will have cleared sothis clear mode of operation is not vital. What isimportant is that S0 goes low first, since we do notwant to load again until just before the nexttransmission. At the end of S I we have S0 = S I = 0,the hold mode which is maintained until the nextvalid interrogation.

The output ofthe shift register contains the codedinformation but not in the form required, i.e. a trainof pulses. Differentiated clock pulses are fed to anAND gate which is enabled by the shift registeroutput, thus positive pulses appear at the output ofthe AND gate in the appropriate time-slots. The final

132

monostable is to provide pulses of the correctduration to the modulator.

Encoding AltimeterAs the aircraft ascends, the decrease in static pressure

causes expansion of the capsule and consequentmovement of a position-sensing device, an optical or

magnetic transducer (Fig. 8.17). The resultingservo-assisted drive, through appropriate ge aring,

drives the altitude display and encoding disc. Thepressure reference for the indicated altitude can be

lhanged by the baroset control. It should be noted

*rat this does not affect the encoding disc position

which is always referred to 1013'25 mbar (29'92 in'Hg)

the standard mean sea-level pressure' As a result all

aircraft report altitude referenced to the same level;

an essential requirement for ATC purposes.Variations on the above are encoding altimeters

which give no indication of altitude, blind altimeters,

Tach.gen.

i l

i ' ---@II

fu--ffi-{ no,o,

I T-l ffi' t--:

- - - - -

III

Posisensdevi

Liohtrngce

Lightsourcc

sonsitivedevices

\-^-NCapsule

Encodingdisc

I *1.- Porver -l switchingtscrr(urts clrcul

Static connection Supply To

Fi& 8.17 Servo enoding altimetet

and those which do not have servo-assisted drives butdo employ a vibrator to give smooth movement ofpointer and encoding disc.

Altitude EncodingA transparent disc, usually glass, is divided into tracksand segments. Each concentric track represents oneof the code pulses, the outer track being C4, while asegment represents a oarticular altitude. An opaquepattern is formed on the disc so that on a particulars€gment the area of intersection with each track willbe either opaque or transparent depending on thecode assigned to the altitude represented by thatsegment.

The disc rotates between a light source andphotosensitive.devices, one for each track, alignedalong the 'reading line'. As the disc is driven by thebarometric alt imeter the appropriate segment is read.

Referring back to Tables 8.2 and 8.3 we see thatthe code changes by I bit for each 100 ft increment.This use of a l-bit change or unit distance code

ffi Barowr

transponder

minimizes the error when the reading line is alignedwith the junction of two segments.

In the simplified encoder circuit shown inFig. 8.20 with A/R switch ON Zener diode VR2 isshorted and emitter of Q2 returned to earth. Whenlight from LED VR1 falls on photo transistor Ql, viaa transparent part of the encoding disc, current isdrawn through R switching on Q2. Thus collector of

Q2 falls to a low value, i.e. input 2 to NOR gate islow. If input I is driven lqw by an output from thedecoder a high output lrom the NOR gate is availablefor loading into the encoder shift register.

If no light falls on Ql we have no volts drop acrossR, thus Q2 is off and input 2 to NOR gate is high.Under these conditions a'zero' wil l be loaded intothe appropriate shift register element. The circuitrydescribed is repeated for each track of the disc.

Side Lobe SuppressionOf the several possible ways of desigring an SLScircuit one is i l lustrated in Fig. 8.2 l, with the

133

ft---*| |-.+lF -+l l -> Toll-- transpondcrt l - +

ll-*t=

\ . , rn,sensitivcdevices

Fi& E.lt Encoding disc

Encodingalttmeter

Fig. 8.20 Simplified altitude encoder circuit

134

Fig .8 . l9 Segment fo r12300f t ,code0 l0 l l0 l0o . SegmentsEncoding altimcter range - l000-32 700 ft

Tracks

Mode Cload

f- +v

tlUI A/R onloff* - 1

j

-L=

ransponderT

R1

-v2

lnput

in, d

GI

M2, O

M3, O

Junctionc1 R2

D2

Ditchdigger

t - _ _

Fig.8.2l Side lobe suppression circuit

associated waveforms in Fig. 8.22. Monostable Ml

and AND gate Gl separate the Pl pulse from P2 and

P3 so that M2 will be triggered by Pl only and will

not be triggered until the next interrogation. M2 and

M3 provide a gating waveform about I ps wide in the

P2 pulse position.Th" input pulses are also applied to a'ditch-digSer'

circuit. Prior to Pl, Dl is forward biased and thejunction of Cl/R2 is low. Both inputs to G2 ate

low. The leading edge of Pl causes Dl to conductcharging Cl rapidly. The lagging edge of Pl causesDl to cut off, since the junction Cl/R2 falls by an

amount equal to the amplitude of Pl . Cl nowdischarges through Rl and R2. When P2 arrives Dl

will conduct providing the amplitude of P2 is

sufficient. The time constant ClRlR2 and the biasvoltages Vl and Y2 ue chosen so that if P2>PlAND gate G2 will receive an input via D2 which will

be coincident with the gating waveform from M3'

Thus the SLS pulse generator M4 will be triggered if

and only if Y2> Pl, the subsequent suppressionpulse being used to inhibit the receiver video output

to the spike eliminator.

Characteristics

The following summary is drawn from ARINCCharacteristic No.572.1 for the Mk 2 ATCtransponder. It is worth pointing out that severalfeatuies on the Mk I transponder are not required forthe Mk 2, however the engineer will find manytransponders still in service which have some or all ofthe following:Fig. 8.22 Side lobe suppression waveforms

135

l. two-pulse SLS;2. SLS countdown - receiver desensitized when

the number of SLS pulses exceeds a limitingfigure;

3. low sensitivity selection;4. receiver video signal output socket;5. remote automatic keying;6. external transmitter triggering position;7. audio monitor:8. transmission of SPI pulse whenever D4 is one

bit ofthe altitude reporting code.

Receiver

Minimum Ttiggering Level (MTL)-77 to -69 dBm at antenna or-80 to -72 dBm at transponder.

Dynamic RangeMTL to 50 dBs above m.t . l .

Frequency and BandwidthCentre frequency 1030 MHz.-3 dB points at 1 3 MHz.-60 dB points at ! 25 MHz.

Decoding FacilitiesDecoder output lbr pulses spaced 8, I 7 and 2 I pstolerance t 0.2 gs on spacing. Automatic mode Cdecoding regardless of mode selection switch.Space provision for 25 ps decoding.

Side Lobe Suppression FacilitiesPl > P2 + 6 dBs should give 90 per cent reply rate.6 dBs rather than ICAO 9 dBs ensures adequatemargin to allow for performance rundown in service.

SLS Pulse Duration25-45 ps.

Transmission

Tlansmitter Frequency10901 3 MHz.

Minimum Peak Power500 w.

Reply hIttlse Charac teristicsDuration 0.45 1 0'l ps measured between 50 per centamplitude points.0'05-0'l ps rise time, l0-90 per cent.0'054'2 ps delay time,90-10 per cent.

136

Reply Delay3 t 0 '5 ps.

Reply Rate Capabiliry1200 replies per second.

Reply Pulse Interval Tolerancet 0'l gs for spacing of any pulse, other than SPI. rvithrespect to F I ; t 0'15 ps for spacing of any pulsc' withrespect to any other except Fl: t 0.1 prs for spacingof SPI with respect to F2.

Mutual SuBpression P.ilse25-33 1s duration.

Manitor LampTo light when five replies are dctected at a rate greaterthar 150 replies per second. To stay i l luminated fbrl5 s af ter last reply detected.

AntennaPolarization: vertical.v .s .w.r . : bet ter than l .4 l : I a t 1030 and 1090 Ml tz .

Ramp Testing

A transponder can be tested ril situ using one ofseveral por table test sets. A sui table rantp test setwil l test by radiatitrrr._bc. ceplble of interrogating onat least mcldes A and ( . be capable rlf simulating aside lobe interroXptlon. display the transponder replyand provide a ffeans of measuring the transpondertransmitter frequency.

ATC 5OOAA popular test set is the IFR ATC 6004 illustrated inFigure 8.23. A reason for its popularity is the factthat it can test both DME and ATC transponder witha comprehensive range of checks, making it suitablefor functional tests on the ramp or bench.

Pl, P2 and P3 pulses are genefated and used to keya crystal-controlled 1030 MHz oscil lator. The intervalbetween Pl and P3.is switched to simulate a modeAy'C interlace, two mode A interrogations beingtransmitted for each mode C. The followingcharacteristics may be varied by front panel controls:

l. Pl-P3 intervd - to check decoder;2. P2 amplitude - to check SLS;3. Transmitter power output - to check MTL.

The reply is displayed by a bank of lamps, one foreach code pulse and oire for the SPI pulse. There is

x:l * t* * )l l r

l

r*iI

Fig. 8.23 ATC 600A (courtesy tFR Electronics lnc.)

also a numerical readout which shows either the pilotcode or the altitude in thousands of feet. In additionto this basic information the following can be checked:

l. F2 timing;

power output of transponder (1 50 per centaccuracy);frequency of the' transponder transmitter;percentage reply;invalid alt itude code, i.e. no C pulses or C I andC4 together;absence of code pulses in reply to nrode Cinterrogat ion.

Supply is by rechargeable battery or a.c., batteryoperation is l imited by a timer. Further l 'eatur.es aredirect connection to the transponder via an external34 dB pad, self-testing of display, lamps and batteryand direct connection ofencoding altirneter.

TIC T-33B and T-438The TIC approach to ramp testing is to use separatetest sets for L band equipments, the T-338 andT-43B being those for ATC transponder.

#ffi "n[f*

# '$$ffi

'tl*

Fig. 8.24 TIC T-438 (courtesy of Tel-Instrument ElectronicsCorp.)

i . ';*'!,r,".t'-'.1;

t+

)

6 .

3 .4 .5 .t

r t

u, : - :

137

Specifications for the two test sets are identicalexcept for the added facil i ty of direct connection ofan encoder which is available on the T'438.

The capabil it ies of these test sets and the ATC6O0A are similar in so far as ATC transponder ramp

testing is concerned, in that they both meet the FAA

requirements. Differences are largely due to the use

of the ATC 6004 as a bench test, although it shouldbe noted that a particular ATC 600A is best used aseither a bench test set or a ramp test set but not both.

To detail the differences, the TIC test sets do not

have facil i t ies for continuously varying Pl -P3 spacingor strobing the F2 pulse, and do not indicate invalid

or 'no altitude' information or transmitter power.

Features of the TIC test sets not available on theATC 600A are provision of all mil itary and civilmodes of interrogation and change of scale forpercentage reply meter (0-10 per cent SLS on:0-100 per cent SLS off). There are other minordifferences, and one other major difference. in that

the TIC test sets are designed ftlr use in the cockpiton the ground or in fl ight, the antenna beingmounted on the test set as opposed to the ATC 600Awhere the antenna is mounted on a tripod near theaircraft antenna- The antenna arrangements l 'or theTIC test sets necessitate the use ol direci connectionto the tr;ulsponder for receiver sensitivity checks.

::.J

, t<<"

138

I Weather avoidance

In t roduct ion

Weather forecasting is by reputation and, until theintroduction of satellites, in fact, notoriouslyunreliable. Even with modern techniques rapidlychanging conditions and lack of detailedinformation on the exact location and severity of badweather results in diversions or cancellation, of fligt t,where the forecast is the only available information.What is required is an airborne system capable ofdetecting the weather conditions leading to thehazards of turbulence, hail and lightning..

Attention has been concentrated on developinssystems which wil l 'detect' turbulence. tf an alrcr-aftpasses through regions ofsevere turbulence it isobviously subject to rnechanical stress, which maycause damage, possibly leading to a crash. Oncommercial flights, passenger comfort is alsoimportant since the number of customers would soondecline if the discomfort and sickness whichturbulence may bring became commonplace.Unfortunately the phenomenon of rapidly andrandomly moving air currents is not amenable todetection by any currently realizable technique;however, some progress may be made by utilizing apulsed Doppler system.

As a consequence of our inability at present todetect the turbulence directly, systems have beendeveloped which detect either water droplets orelectrical activity, both of which are associated withconvective turbulence in cumulonimbus clouds.Clear air turbulence has no detectable associatedphenomena which can give a clue to its presence.

To detect water droplets or raindrops aconventional primary radar is used with frequencyand special features chosen to optimize thepresentation of signals which would, in a normal searchradar, be unwanted. Weather radar has been used formany years, and is mandatory for large aircrift.There has been a steady move into the general aviationmarket by radar manufacturers but here a relativelyrecent innovation is the Ryan Stormscope, a patenteddevice which detects electrical activity.

The maximum diameter of a water droplet which

can remain in a cloud without falling to earth isdependent on the speed ofthe up-draught of air.In the appendix to this chapter it is shown that thesignal in a weather radar is proportional to the sixthpower of the droplet diameter, so strong signais areassociated with a rapid up-draught. If a small volumeof the cloud results in strong signals from one partand weak from an adjacent part we have a steep'rainfall gradient', most probably due to a down- andup-draught close to one another. The region aroundthis vertical wind shear is likely to be highlyturbulent.

The corresponding argument for associatingelectrical activity with turbulence goes as follows.A wind shear will result in the separation of positiveand negative electrical charges in the air due to thefriction of the moving air currents. An electricaldischarge occurs after sufficient charge of oppositepolarity has accumulated in distinct parts of thecloud. These discharges occur repetitively, mostbeing hidden from view but occasionally seen aslightning. Each discharge is accompanied by a largeburst of e.m. radiation which can be received at somedistance. In most radio applications the 'noise'

received due to lightning is a nuisance but itsassociation with turbulence is put to good use in aRyan Stormscope in a way similar to that in which aweather radar uses ithe 'nuisance' signals of weatherclutter.

We can summarlze and combine the abovearguments by saying: convective turbulence occurswhere we have large shear forces which imply:(a) an up-draught supporting large raindrops formedfrom the water vapour in the warm moist air risingfrom ground level; (b) a nearby down-draught whichcannot support large raindrops; (c) frictional forcesgiving rise to charge separation; and (d) electricaldischarge due to charge separation arid a saturatedintervening medium.

Cause and effect are very much bound up iri thisargument, the various phenomena beinginterdependent. However reasonable the theory, theultimate justif ication for the association betweenturbulence, steep rainfall gradient and electrical

139

activity is recorded correlation during many flights.Weather radar is certainly well proven with manyyears in service, while the Stormscope, although onlyavailable since 1976, has been independentlyevaluated and shown to be a useful aid'

Weather Radar

Basic PrinciplesWeather radar operation depends on three facts:

l. precipitation scatters r.f. energy;2. the speed of propagation of an r.f. wave is

known;3. r.f. energy can be channelled into a highly

directional beam.

Utilizing these facts is fairly straightforward inprinciple. Pulses of r.f. energy are generated by a

iransmitter and fed to a directional antenna. The r.f.

wave, confi4ed to as narrow a beam as practicable,wil l be scattered by precipitation in its path, some of

the energy returning to the aircraft as an echo. The

elapsed time between transmission and reception is

directly proportional to range R, in particular

R = ct 12 where c is the speed of propagation(= 162 000 nautical miles per second); t is theelapsed time; and the divisor 2 is introduced sincetravel is two-way. The direction of the target issimply given by the direction in which the beam is

radiated.Since the pilot needs to observe the weather in a

wide sector ahead of the aircraft the antenna is made

to sweep port and starboard repetit ively, hence we use

the term scanner for a weather radar antenna. Any

storm cloud within the sector of scan wil l effectively

be sliced by the beam so that a cross-section of the

cloud is viewed.Display of three quantit ies for each target is

requirid: namely range, bearing and intensity of echo'

.l ptan position indicator (p.p.i.) display is invariably

utid tinc. this allows the simultaneous display of the'

three quantit ies and is easy to interpret.A cathode ray tube (c.r.t.) is used in which the

beam of electrons is velocity modulated in

accordance with the received signal strength'

Wherever the beam strikes the'phosphor coating on

the back of the viewing screen a glow occurs, the

intensity of which is dependent of the velocity of the

electrons. Thus a strong signal is associated with a

bright spot on the screen; hence the term intensity

modulaiion. The beam is made to sweep across the

screen in synchronism with both the time of

transmission and the antenna position'

140

In a conventional (rho'theta) display the beantstrikes the screen at bottom centre (origin) at the

instant the transmitter f ires. Subsequently the beamwill be deflected across the screen in a direction

dependent on the scanner azimuth position, e.g. if the

scanner is pointing dead ahead the beant is deflected

vertically from the origin. In this way a time'base line

is traced out on the screen and is made to rotate in

synchronism with the scanner. '

The duration of the time-base, i.e. the length of

time it takes to tiaverse the screen, depends on the

range selected by the pilot. Every microsecond of

round trip travel t ime corresponds to a range of

0'081 nautical miles, thus for a selected range of

20 nautical miles the time-base will be about

250 ps in duration. An echo received 125 ps after

transmission would cause a bright spot to appear

half-way up a 250 ps.time-base, so indicating to the

pilot that the range of the target is l0 nautical miles'

The net result of the above is that a cross-section

of the targets within the selected range and scanned

sector of the radar are viewed in plan. The position

of the bright patches on the screen relative to the

origin is representative of the position of the targets

relative to the aircraft. Figure 9.1 illustrates the .-

situation.

Choice of Characteristics and Features

FrequencyThe higher the frequency (smaller the wavelength)

the larger is the backscatter cross-section per unit '

volume of the target (see A9.12) hence the greater

the echo power. However, high frequencies suffer

more atmospheric absorption than do low, and

further cannot penetrate clouds to the sarne extent'

Thus the choice of frequency is a compromise' An

additional consideration is the beamwidth; for a

given scanner diameter a narrower beam is produced

with a higher frequencY'Practically, taking into account availabil ity of

standard compqnents, the choice comes down to

either about 3'2 cm (X'band) or 5'5 cm (C-band)'

The majority of radars in service and currently

manufactured are X-band.

Pulse WidthThe volume of the target giving rise to an echo is

directly related to the pulse width (see A9.8) thus use

of long pulses will give improved range'There are two arguments against long pulses:

l. Since there is only one antenna and a common

Time-base

Fig. 9.1 Display principles

frequency for transmit and receive, the antennamust be switched to the transmitter for theduration of the pulse; thus the pulse widthdetermines minimum range. For a 2 ps pulseno return can appear for the first 2 ps of thetime-base, giving a minimum range =2c X tO-612 t one-sixth of a nautical mile.

2. Range resolution deteriorates with increasingpulse width. A pulse of 2 ps duration occupiesabout 2000 ft in space. If two targets are on thesame bearing but within 1000 ft of one anotherthe echo from the nearest target is sti l l beingreceived when the leading edge of the echo fromthe furthest target is received. The result is thatboth targets merge on the p.p.i. display. Therange of the targets does not affect the resolution.

--^-\-1

()

\

Since range resolution and minimum range are notcrit ical in a weather radar, pulses tend to be longerthan in other radars, say 2-5 ps. A shorter pulse width,say I ps, may be switched in when a short displayedrange is selected.

A technique is available which realizes theadvantages of both long and short pulses. Thetransmitted pulse can be frequency modulated so thatthe r.f. increases over the duration of the constantamplitude pulse. The frequency modulated return ispassed through a filter designed so that the velocityof propagation increases with frequency. Thus thehigher frequencies at the trailing edge of the echo'catch up'with the lower frequencies at the leadingedge. In this way, the duration of the echo iscompressed. It should be noted that the bandwidth

-,\\( , \ lL_ \",r\-l c\',l \

)

a{

r\\__

141

Target I Target 2

Har fa | |wavelength k-= IOOO' i +

t-\-\l Il - - -

- - - t

Incident pulse (2 1rs)

Reflected pulse

Fig. 9.2 Range resolution

requirements are increased by using frequencymodulation of the pulsed r.f., this being the penaltylbr obtaining better range resolution. This techniqueis known as pulse compression but, so far as theauthor is aware, is not used on existing airborneweather radars.

hrlse Repetition Frequency, p.r.f.Changing the p.r.f. will affect the number of pulsesstriking agiven volume of the target in each sweepand hence change the display integration factor(see A9.17). However in order to maintain a constantduty cycle (pulse width X p.r.f.) an increase in p.r.f.must be accompanied by a decrease in pulse width, sokeeping average power and hence heat dissipationconstant. Alternatively an increase in p.r.f.accompanied by a reduction in peak power will alsokeep heat dissipation constant. The net result is thatfor constant average power a change in p.r.f. does not,in theory, affect the maximum range of the radar.

Limits are imposed on the choice of p.r.f. since ifit is too low the rate at which information is received

142

+ 2

+ 3

time (7rs)

[DK3

- - - - - - t

+ 4

+ 5

Rang€

Fig.9.3 Second.trace echoes

is low, while if it is too high the serious problem ofsecond trace echoes may arise. If the characteristicsof the radar are such that the maximum range fromwhich echoes can be detected is, say, 200 nauticalmiles then the round trip travel time for a target atmaximum range would be about,2500 ps. In such asystem a pulse repetit ion period p.r.p. (= l/p.r.f.) of2000 gs would mean that the time-base start wouldoccur 500 ps before the return of an echo of theprevious transmitted pulse from a target at 2OOnautical miles. This second trace echo would appear

to be at about 40 nautical miles range. It follows thatin the above example the maximum p.r.f. would be400 and in general p.r.f. ( c/2R where R is themaximum range.

A popular choice for older radars was a p.r.f. of4O0 synchronized to the supply frequency. Withimproved performance leading to increased range, asubmultiple of the supply frequency, e.g. 200, wasused. In modern systems internal t iming isindependent of the supply frequency and one findsp.r.f.s from about 100 to 250.

Power OutputIn older radars peak power outputs of about 50 kWor even higher were common, while maximum rangewas modest. In modern radars, peak power is aboutl0 kW with increased range compared with previoussystems. This apparent spectacular improvement isput in perspective by considering the range equation(see A9.7) where we see that maximum range isproportional to the fourth root of the peak power.Thus reducing power by four-fifths reduces maximumrange by about one-third. This shortfall of one-thirdhas been more than made up by improvements inrerial design, receiver design and sophisticated signalprocessing.

Beam WidthAlthough the larger the beam width the greater thevolume of the target contributing to the echo thiseffect is more than cancelled out due to the inverserelationship between aerial gain and beamwidth(G o l l02). A narrow beam is always preferred,since the net effect is to increase range and improvebearing resolution. Simple geometric considerationsshow that with a 4" beamwidth two targets separatedby about 3! nautical miles, at a range ) 50 nauticalmiles, wil l appear as one on the p.p.i. Bearingresolution, unlike range resolution, is dependent onthe target range. An equally important consideration

is that ground returns will appear at closer ranges forwider beamwidths, thus masking the cloud retums.

Tilt and StabilizationThe reason for requiring stabilization is related to theprevious paragraph. A weather radar may scan up to300 nautical miles ahead of the aircraft.within azimuthscan angles of typically t 90". Unless the beam iscontrolled to move only in or above the horizontalplane part or all of the weather picture may bemasked by ground returns. Imagine the aircraft rollingwith port wing down. If the swept region is in thesame plane as the aircraft's lateral and longitudinalaxes, then when the scanner is to port the beam will

be pointing down towards the ground, while when tostarboard the beam will be pointing up, possiblyabove the weather. Figure 9.4 illustrates thisproblem.

In fact stabilization holds the beam not in thehorizontal plane but at a constant elevation withrespect to the horizontal. This constant elevation isdetermined by the ti l t control as set by the pilot.Details of stabil ization and ti l t are discussed later.

ContourThe pilot will be interested in those regions where theprecipitation is greatest. In order to make thesituation clearer, those signals which exceed a certainpredetermined level are inverted so as to show thecells of heavy precipitation as dark holes within thebright 'paint' caused by the cloud surrounding thecell. The width of the 'paint' around the cell is anindication of the rainfall gradient. The narrower thewidth the steeper the gradient, and hence the greaterthe probabil ity of encountering turbulence. Thistechnique is known as iso€cho (equal echo) contourpre sen tation.

Sensitivity Time Control, s.t.c.For correct contour operation the signal strength

No stabilitv

./

<_ _____ __Stability

7///////////////////////////////ruFig. 9.4 Scanner stabilization

B\\

No stabilitY

7//////27'�////////////////////////ru

1/13

strould depend only on the characteristics of the

target, but of course the range also affects the receivedpower. As a consequence ifthe contour inversion

level is set so as to indicate storm cells at a certain

range then innocent targets closer than that range

may cause inversion while storm cells beyond that

range may not. In order to solve the problem the

receiver gain is made to vary with range, beingminimum at zero range and increasing thereafter(i.e. with time), hence sensitivity time control or

swept gain, as it is sometimes called.

------+Time

Rxga in(S.t.c.)

Fig.9.5 Sensitivity time control

The aim is to make the receiver outputindependent'ud range. Unfortunately the receivedpower decreascs as the square of the range for targetswhich fill the beam, but as the fourth power of therange otherwise (see Appendix). To achieve the aimwould require a complex gain control waveformwhich would, even then, only be correct for a certainsized target. Many systems have been designedassuming a 3 nautical mile diameter cloud as standard;among such systems, s.t.c. has operated to a rangewhere such a target would fill the beam; beyond thatgain is constant with time. It has been observed that

it is uncommon for water droplet retums to comefrom a region which fills the beam vertically exceptat close ranges.

Following from the above, s.t.c. may be arrangedto compensate for the range squared law out toabout 30 nautical miles for a 6o beam. A change in

scanner size would lead to a change in maximum s.t.c.

range since the beamwidth would alter. Alternativeiy,a modified law may be compensated for out to, say,70 nautical miles, ignoring the possibility of beamfilling.

Automatic Gain Control, e.g.c.It is neither practical nor indeed desirable withcontour operation to have a'g.c. determined by signal

level as in receivers for other systems. The normal

1 U

arrangement is to have a.g.c' noise'derived. During

the time immediately before transmission the output

from the receiver is noise only, since the p.r.f. will

have been chosen to avoid second-trace echoes, i.e.

the time corresponding to maximum range expireswell before the next pulse. The a.g.c. circuit is gated

so that the receiver output is connected to it only for

a short t ime before transmission.The result of having such a.g.c. is to keep the

receiver noise output constant, which under normalconditions means the gain is constant. If, for anyreason there is excessive noise generated or received,the gain will fall, so keeping the background noisedisplayed at a constant level, although signals willfade and contour will not highlight all storm cells.

An alternative arrangement is to keep the receivergain constant regardless of how the receiver outputmay change. This has the virtue of keeping theconditions for inversion in the contour circuitunchanging. Such preset gain is found in moderndigital systems, whereas noise-derived a.g.c. is foundin older analogue systems.

DisplayA problem in display design for cockpit use is thelarge range of ambient iighting conditions. Somestorage of the information received is inevitable if i t isto be viewed, owing to the relatively low refresh rateof the fleeting basic information.

In older and simpler radars the c.r.t. screen iscoated with a long-persistence phosphor whichcontinues to glow some time after the electron beamhas passed on its way thus storing the informationover the scan interval. Unfortunately. such phosphorsare not very efficient and very high shielding isrequired for adequate viewing in bright conditions.

The direct view storage tube (d.v.s.t.) is a solutionto the problems of a conventional c.r.t. A mesh ismounted immediately behind the phosphor-coatedscreen. Electrons arrive at the mesh from twosources: a locused beam, velocity-modulated by thesignal, comes from a conventional electron gun whilea flood gun provides, continuously, a wide beam ofelectrons over the whole mesh. Since the modulatedbeam is deflected across the mesh in accordance with

scanner position and time since transmission, a chargepattern is written on the mesh. The pattern determineswhere and what fraction of incident electronspenetrate the mesh and strike the phosphor. Sinceeach glowing element of phosphor is excitedcontinuously, very bright displays are possible. A

Tx

i

slow discharge.path for the mesh must be provided toprevent saturation; the possibility of changing thedischarge rate exists and a pilot-adjustable controlmay be provided. On changing range the mesh will be'instantly'

discharged to prevent confusion between1ew and old screen positions of the targets. If thedischarge path is broken and updating inhibitrd *"have a frozen picture.

.. The modern approach to information storage is todigitize the signal which is then stored in anintermediate memory at a location depending on thescanner azimuth angle and the time of arrivalmeasured with respect to the time of transmission.The p.p.i. time-base scan format can be eitherrho-theta or X-Y as per a conventional television.In either case memory can be read at a much higherrate than that at which information is received. Withmany more pictures per second painted we have abright, flicker-free display without the need forlong-persistence phosphors of low efficiency or thee4pensive d.v.s.t.

In the case of a rho-theta display thc scan format isthe same for receipt and display information but, asexplained, the repetition rate is different,so onlyscan conversion in time is required. With a television_type display both the scan format and repetition rateare different, so complete scan conversion is requiredfrom input (received) format to readout (displayed)format. Use of the television-type display makesmultiple use of the weather radar indicator relativelysimple so we find such indicators able to display datafrom other sensors (e.g. Area nav.) or alphanumericdata such as checklists.

Digitizing the signal involves the recognition ofonly discrete values of signal intensity. ihe standardpractice is to have three levels of non-zero intensity,the highest corresponding to the contour inversionlevel. Although these three levels correspond todifferent degrees of brightness of the paint, with theuse of a colour tube they can be made to correspondto three colours. While there is no standardization ofthe colour code as yet (1979) red is the obViouschoice for contourable tarsets.

Perhaps the mosl signifilant virtue of a digitalweather radar is the absence of noise on the display.The output of the receiver, the video signal, isdigit ized and then averaged in both time and position,i.e. the video output occurring p gs after atransmission is averaged with that occurring p ps afterthe next transmission and the video data which wouldappear at adjacent positions on the screen are subjectto a weighted aver-agingprocess. With a suitable choiceof min i rnum s igra l level for a 'paint 'uncorre latednoise is virtually elirninated from the display with the

exception of a few noisc spots per memory update.

ScannerThere are two types of scanner employed to obtainthe required narrow beam, namely a directly fedparabolic reflector or a flat plate planar array. Of thetwo, for a given diameter and wavelength, the flatplate has the higher gain/narrower beam/leastside lobe power, but is most expensive. Since the flatplate is almost twice as efficient as the parabolicreflector it is invariably used with a modern systemexcept where cost is an overriding factor or the spaceavailable in the nose of the aircraft allows a larse-

. parabolic reflector to be used.The flat plate antenna consists of strips of

waveguide vertically mounted.side by side with thebroad wall facing forward. Staggered off-centrevertical slots are cut in each waveguide so as tointercept the wall currents and hence radiate. Severalwavelengths from the antenna surface, the energyfrom each of the slots wil l be summed in space,cancellation or reinforcement taking place dependingon the relative phases. In this application the phaseof the feed to each slot, and the spacing betweenslots, is arranged so as to give a resultant radiatedpattern which is a narrow beam normal to the planeof the plate. The greater the nurnber of slots the .better the performance; since the spacing betweenthe slots is critical we can only increase the numberof slots by increasing the size of the flat plate.

The parabolic reflector works on a similar principleto a car headlamp reflector. Energy striking thereflector from a point source situated at the focus willoroduce a plane wave of uniform phase travelling in adirection parallel to the axis of the parabola. Thefeed in a weather radar parabolic antenna is usually adipole with a parasitic element which, of course, isnot a point source. The consequence of a dipole feedis that the beam departq from the ideal and there isconsiderable spill-over, $iving rise to ground targetreturns from virtually helow the aircraft, the so-calledheight ring.

In both types ofaptenna, scanning is achieved byrotat ing the complete antenna a5sembly, thus arotat ing waveguide jo int is requi red. Anelectronically steered beant is possible with the flatplate but the considerable complications of arrangingthe correct phasing of the feed to all slots have madethis impractical for airborne weather radar systems.Some of the simpler. cheaper weather radarsemploying a parabolic reflector rotate the reflectoronly. leaving the feed fixed and so eliminating theneed for an azimuth rorating joint. A disadvantage ofthis latter system is that the feed is not at the focus

145

except when the reflector axis is dead ahead; theresulting deterioration of the beam shape means theangle through which the beam is scanned must berestricted.

lnsta l la t ion

Figure 9.6 i l lustrates a typical installation in blockform as represented by the Bendix RDR 1200. adigital weather radar designed with the upper end ofthe general aviation market in mind. The installationwill be discussed in terms of the RDR I100 first. andthen variations and refinements wil l be described.

WaveguideVRG

Trarrece

---i---l scannerve r J

2 8 V d . c . t t s v ' c f fSupply

lndrcator

Fig. 9.6 Bendix RDR 1200 installation

The transmitter receiver (t.r.) contains all the r.f.circuitry and components as well as the rnodulator,duplexer, IF stages, analogue to digital (a/d) converterand power supply circuits. Unlike older systems, thebasic timing circuitry is in the indicator rather thanthe t.r. This timing controls the p.r.f., scanner

rotation, memory write/read and all display circuitry.Thus in a modern weather radar we see that the 'brain'

of the system is situated in the indicator while theheart'remains in the t.r. This view is reinforced bythe fact that the pilot/system interface is achievedcompletely through the indicator, both for displayand control.

The scanner is a flat plate array plus associatedcircuitry for scanner stabil ization and.azimuth drive.The t'lat plate is mounted on a gimballed surface u'hichallows rotation in response to pitch and roll signalsfrom the aircraft vertical reference gyro (VRG).Radiation is through a radome which ideally istransparent to the X-band energy but at the sametime provides protectittn for the scanner, preservesthe aerodynamic shape of the aircraft and hasadequate structural strength.

Most of the interconnections are made usingstandard approved cables with signal and control l inesscreened. The interconnect ion bet iveen t . r . andscanlrer is, of necessity. by way of waveguide. lossesin co-axial cable being unacceptable at X-band (oreven C-band).

While the above is a simplif ied description of theBendix RDR 1200 the units and tl.reir contents arevery much the same for all modern digital systems.A trend in l ightweight systems for general aviationaircraft is to combine the transmitter and scanner inone unit. The nrain advantage of a two unit system,t.r./scanner plus indicator, is that the waveguide run iseliminated, so reducing capital and installation costsand waveguide losses. The argument againstcombinins the t.r. and scanner is that the unit

Radome

N,l )l /

V

Fig.9.7 Primus 200 multifunction colour weather radar.Two-unit sensor on the left, multifunction accessories on ther ight (courtesy RCA Ltd)

1tt6

installed in the nose is costly in terms of maintenancestrould it require regular replacement; thus reliabilityassumes even greater importance in such systems.

Singlecngined aircraft have not been neglected, theproblem having been tackled in two different ways,both of which involve a combined t.r./scanner unit.Bendix have gone for an under-wing pod-mountedunit, while RCA have developed a wing leading edgemounted unit in which a section of a parabolicreflector is used, as well as a pod-mounted unit.

Corrosion due to moisture collection is a problemin the waveguide run, which may be eased bypressurization. The ideal is to have a reservoir of dryair feeding, via a pressure-reducing valve, a waveguiderun which has a slow controlled leak at the scanner;this method is not used on civil aircraft. Severalinstallatiorp ailow cabin air to pressurize thewaveguide run via a bleeder valve, desiccant and filter.If the cabin air is not dried and fi l tered more problemsmay be created than solved. Two interesting casesthat have been brought to the author's attention are:a nicotine deposit on the inner wall of the waveguidecausing excessive attenuation, and rapid corrosioncaused by fumes from the urine of animalstransported by air. lfno active pressurization isenrployed the pressure within the waveguide may stillbe higher than static pressure if all joints are tightlysealed. A primary aim of waveguide pressurization isto reduce high-altitude flash-over.

Fig. 9.8 Installation of Weather Scout I t.r./scanner unit(courtesy RCA Ltd)

The waveguide run should be kept as short aspossible. Inaccessibil i ty or inadequate cooling maynrean that the t.r. cannot be situated so as tominimize the length of the run. Straight rigidwaveguide should be used, avoiding bends, twists.

corners and flexible sections, except where necessary,so reducing costs and losses. A choke flange to plainflange waveguide joint is normal practice, the chokeflange having a recess to take a sealing ring.

The radome is usually a covered honeycombstructure made of a plastic material reinforced withfibreglass. The necessity for mechanical strength,small size and aerodynamicshape may compromisethe r.f. performance. Lightning conductors on theinside surface of the radome will obstruct the beam,but their effect is minimized if they are perpendicularto the electric f ield of the wave. Horizontalpolarization is normal since there is less sea clutter,although with moderate to rough seas the ldvantageis minimal.

On large aircraft a dual installation is used.Obviously the scanner cannot be duplicated but botht.r. and indicator can be. In some cases only theindicator is'duplicated, thus eliminating the need fora waveguide switch. Unless one indicator is purely aslave with no system controls other than, say,brilliance, a transfer switch will be necessary totransfer control from one indicator to another. Evenif two t.r.s are fitted a transfer switch is sti l l necessaryfor scanner stabilization on-off and tilt control.

Controls

The following list of controls is quite extensive; mostwil l be found on all radars but some are optional.The nomenclature varies but alternative names forsome of the controls are l isted where known.

Range Switclr Used to select displayed range. Willalso change the range mark spacing. Selection may beby pushbutton or rotary switch, the latter possiblyincorporating rOFF', 'STANDBY' and 'TEST'

positions.

OfflStandby Pushbutton or incorporated in the rangeswitch. With standby selected there wil l be notransmission while indicator extra high tension (e.h.t.)may or may not be on.'

Functibn Switch Selects mode t-rf,operation.NORMAL" 'CONTOUR"'CYCLICi , .MAPPING"'BEACON'. The last two of these modes aredescribed later. Normal operation allows the use ofa.g.c. (preset gain) or var iable gain: s . t .c . is usual lyact ive. Contour operat ion select ion causes b lankingof s t rongest s ignals, a.g.c. and s. t .c . automat ica l l . ,se lected. Cycl ic operat ion causes nornra l a i rd c . r i r tourpresentations to alternate. Pushbutton or rotar.y

147

control may be used. 'NORMAL'switch may beomitted, this mode of operation automatically beingselected when a range switch incorporating 'OFF' and'STANDBY' is selected to any range while'CONTOUR' or'CYCLIC' switches are off. (See also'gain control'.)

Gain Control Used t6 set gain of receiver manually.A continuously rotatable or click-stop control isnormal. The control may incorporate contouron-off; by rotating the knob past the maximum gainposition contour plus preset gain wil l be selected. Inthis latter case a separate spring return pushbuttonmay be used to turn contour off momentarily. Inother systenrs the gain control may simplyincorporate a preset gain on-off switch at itsmaximum position.

Test Switch A special pattern specified by themanufacturers replace5 weather (or mapping) picturewhen test is selected.

Scanner Stob Switch On-Off Switching.

Tilt Control Adjustment of scanner elevation angletypical ly t 15" .

Brilliance or Intensitlt Control Adjusts brightness ofdisplay to suit arnbient l ighting.

Free:e or Hold Su,itch Dara update of displaystopped. last updated picture displayed. Transmissionand scanner rotation continues. Warning lamp rnaybe provided. Only available on digital system orwhere d.v.s . t . is employed.

Erase or Trace Contol Spring return switch torapidly discharge mesh in d.v.s.t., so wiping pictureclean or continuously variable control which altersdischarge rate.

Range Mark Cttntrol Alters intensity of range marks.

Azimuth Marker Sv'itch Electronically geiterated.azimutli rnarks nray be turned on or ofl ' .

Target Alcrt Sx'itct On-off. When activated flashesan a ler t on the screen i f a contourable target isdetected in l 'window' ahead of the aircral't (windowsiz-e for RCA Pr inrus 200 is 7 '5 'e i ther s ide of headingi i t a runge o. l '60- l i0 naut ica l rn i les) . Wlrn ing g ivenregardless rl i selected range also rvhen in 'freeze' mode

Sec'tor Srurr Srl ' i lci Allows selection of one of two

148

(usually) sector scan angtes, e.g. Bendix RDR 1200has t 30o or + 60o options.

Contrast Control Adjusts video amplitier gain andhence allows some control of picture as opposed todisplay brightness even when i.f. amps are operatingunder a.g.c. Sonretimes called intensity.

Manual Tune Contol Associated with automaticliequency control (a.f.c.) on-off switch. When a.f.c.is selected to off, local oscil lator may be tunedmanually for best returns. Generally not used onmodern systems.

Operation

The actual operation ofa weather radar is quitestraightforward, but to get the best use of the systema considerable amount of experience and expertise isrequired on tl're part of the pilot. Beginners areadvised to avoid by a wide margin any contourabletarget within s.t.c. range and any target at all outsidethat range.

With experience the pilot is able to distinguishbetween sal-e and unsafe targets to the extent thathe may be able to penetrate, rather than just avoid,weather. Several words of warning are in order whenattempting penetration: one should always select oneof the longer ranges before attempting to flybetween storm cells since the way through may beblocked further ahead; weather conditions can changerapidlyl the l imitations of X-band radar in so thr assignal penetration is concerned should be remembered.

Finger

Scalloped Edge

Fig. 9.9 RDR 1100 displaysDivision)

U-Shap€d

(courtesy Bendix Avionics

It has already been mentioned that a narrow paintaround a storm cell is a good indication of severeturbulence but whatever the width the cell should beavoided; the only question is by what margin. Thestrape of the return on the screen is also a pointer tothe type of weather ahead. Targets with fingers,hooks, scalloped edges or that are U-shaped have beenobserved to be associated with hail. Long hooks orcrescent-shaped indentations may indicate a tornado,but there are no guarantees either way.

This chapter, and its appendix, have thus far beenconcerned mainly with rain, so a word is in order hereabout returns from other types of precipitation. Drymowfall will not be seen but wet snowfall may besen with difficulty. Fog and mist will not bedetected. Hail may give strong or weak returns: if i tb dry and small compared with the wavelength,rctums are weak: if i t is water-coated. returns arestrong; if it is dry and of a size comparable with thewavelength (an extrelne condition with 3 cm radflr)lhe echo is very strong. The increase in scattering forwater*oated ice particles, hail or snow, may give riseto a'bright band'at an altitude where the temperaturebjust above 0"C. Lightning creates an ionizedgaseous region which may, if oriented correctly,backscatter the radar energy.

Several things may affect the weather picture onthe p.p.i. Icing on the radome will cause attenuationofthe transmitted and received signal, so targetswhich would have been displayed may, under thesecircumstances, remain undetected unti l very close;' innocent' precipitation wil l also attenuate the signal.Ground or sea retums may mask a storm cell; the tiltcontrol should be used to separate weather.and groundtargets, a diff icult task in mountainous regions.Interference, which takes the form ofbroken, curvedor straight l ines on the p.p.i. display, is caused byother radar systems. The exact effect varies with thetype of video signal processing and scan conversion(if any) and with the p.r.f. of the interfering radar.The older type of weather radar with no scanconversion and no averaging of the video output isparticularly susceptible to interference. High p.r.f.interfering signals, such as GCA, give many finebroken radial l ines on the screen. If the interferingp.r.f. is close to a harmonic of the p.r.f. of the radarbeing intert 'ered with, then curved broken lines,apparently moving into or away from the origin. willresult. Where motion is apparent, the interference iscommonly referred to as 'running rabbits', anothercommonll ' encountered term is 'rabbit tracks'.Selection of contour may alleviate interferenceproblems.

',dbnorrnal p.p.i. presentations wil l be observed if

scanner stabilization faults exist. With the scannertilted down and stabilization on a bright circular bandofterrain targets centred on the origin shouid bepresented. If the band is not circular but is severelydistorted, then most probably the gyro is 'toppled' or'spil led', i.e. the spin axis is not vertical. The gyrowill be in such a condition if i ts rotor is runningslow, as wil l be the case if i t has just been switched onor there is a supply or gyro motor defect. A gyro fasterect switch may be provided which, when depressed,increases the supply voltage so accelerating the run upto operating speed. The switch should not be held onfor more than half a minute or so, otherwise the rotorspeed may exceed that for which it was designed, socausing damage.

To complete an airborne check of stabilization, theaircraft should be banked and pitched, within thestabilization limits, satisfactory operation beingindicated by an unchanging circular terrain band.Note that with ti l t down selected. the amount of noseup pitch that the stabil ization circuits can deal with islimited.

Spoking is any p.p.i. presentation which resemblesthe spokes of a wheel. lt is almost certainly causedby a fault within the weather radar system, althoughit can be caused by unshielded electromagnetic devicesproducing strong changing magnetic fields. There aretwo main classes of fault which cause spoking:(a) video sigrral and noise spokes due to abnormalvideo output amplitude variations such as might becaused by the automatic frequency circuits (a.f.c.)sweeping through the local oscillator frequencies and(b) sweep spokes due to faulty display circuitry suchas damaged slip rings in the time-base resolveremployed in an older all-analogue radar. Todetermine which of (a) or (b) is the cause, the gainmay be tumed down and the antenna ti l ted tomaximum up; if the spoking persists, the fault is inthe display circuitry.

The above is only a brief discussion of some of thefactors one.must consider when operating weatherradar. Because of the degree of skil l involved, a pilotnew to weather radar should study the manufacturer'spilot's guide carefully; thqy are usually very good.Equally important, he should learn each time he usesthe system; for example, if a detour is made to avoidan unusually shaped return, a simple sketch and aphone call on landing to enquire about the weatherassociated with that target wil l add to his experience.Remember that to a large extent the body ofknowledge concerning weather returns is empirical.What better way to learn than to collect one's ownresults?

A final and most important point needs to be

149

/

made, and that is that weather radar presents aconsiderable hazard, when operated on the ground.Details are given later in this chapter.

Block Diagram Operation

We shall consider both analogue and digital systems:the former since it illustrates the principles of

weather radar directly and rystems of this type arestill very widely used; the latter since it is th; currenrapproach of all manufacturers. Two types of digitalsystem will be considered: rho-theta display and X-ydisplay.

All Analogue SystemThe p.r.f. generator provides timb synchronization forthe complete system; the output is often called the

P.R.F.gen.

Mod.

Tx

Timebase

BrightupRangemarksA.G.C./s.T.c.Video

tlI-l-,-

I+M

t l

, l

TI

Fig. 9. | 0 All analogue weather radar block diasram andwaveforms

lAz imuth II drive I- t i

FTI

1 Sl@

;l on/I off

Balencedmrxer .

150

pre-pulse, since the lagging edge is used to trigger themodulator. The transmitter is a magnetron keyed bythe modulator which determines the pulse width.The burst of r.f. energy (main bang) ii fed from thetransmitter to the scanner via a duplexer andwaveguide run. The duplexer allows common aerialworking in that it is an electronic switch whichautomatically connects the scanner to the transmitterfor the duration of the transmitted pulse, thusprotecting the receiver.

A sample of the transmitted frequency is fed to thea.f.c. (automatic frequency control) mixer along withan output from the l.o. (local oscil lator). If thedifference frequency is not equal to the required i.f.the a.f.c. circuit applies a control signal to ihe 1.o., s<jadjusting its frequency unti l we have equality. If thedifference frequency is outside the bandwidth of thea.f.c. circuit, the control signal is made to sweep unti lsuch time as the a.f.c. loop can operate normaliy.

The main receiver mixer is balanced to reduce l.o.noise. The i.f. amplif ier chain is broad band(bandwidth ) 2 X reciprocal of pulse wldtn) with gaincontrolled by the a.g.c./s.t.c. circuits or the manualpin control. The video envelope is detected and afterfurther amplif ication is used to intensity modurare(Z-modulat ion) the c. r . r .

With contour on, the video signal is sampled, and ifabove a preset inversion level the video fed to thec.r.t. is effectively removed.

The pre-pulse is fed to the a.g.c. gate which thusallows the video output through to the a.g.c. circuitonly for the durat ion of the pre-pulse (say l0ps) . Inthis.way the gain control l ine voltage level is made afunction of receiver noise. The lagging eclge of thepre-pulse triggers the s.t.c. circuit which reduces i.f.gain at zero range and returns it to normal after,typically, 30 nautical miles (about 370 ps).

The lagging edge of the pre-pulse also iniriates thestart of the time-base and gate waveforms, theduration ofwhich depend on the range selected. Thetime-base wavefornrl(r) is f-ed to a magslip (synchroresolver) in the scanner; since the rotor of the magslipis driven in synchronism with the scanner azimuthmovement the outputs are 1(r ) s in d and 1(r ) cos 0where 0.is the azintuth angle nreasurecl with referenceto the a i rcraf t heading. Using the cosine output forvertical deflection and the sine output for horizontaldeflection provides the necessary rotating time-base.

The start of the time-base run-down mustcorrespond to zero deflection of the c.r.t. spot. Sincethe magslip. being basically a transformer, removesany d,c. level a balancing half-cycle is requiredimrnediately after the time-base flyback to make theaverage (zero) value of the composite time-base

waveform coincide with the start of the run-down.If the balancing half-cycle is made larger thannecessary we have an open centre whereby zero rangeis represented by an arc, ofnon-zero radius, on thep.p.i. display.

The gate waveform is fed to the marker andbright-up circuits which provide the necessary feedsto the c.r.t. for the duration of the time-base. Rangemarks are produced at equally spaced intervals duringthe gate, and are used to intensity modulate the c.r.t.electron beam. The bright-up waveform provides abias which prevents the velocity of the beam berngsufficient to excite the phosphor coating on thescreen, except during the time-base rundown.

Pitch and roll stabilization is provided by aryro-controlled servomechanism.

Digital Weather Radar - Rho-Theta DisplayThe radio and intermediate frequency part of theblock diagram is much the same for analogue anddigital systems, so here only the video, timing andcontrol blocks will be considered. The following isbased on the RCA Primus 40.

Analogue video data from the receiver isdigitized in an analogue to digital (a/d) converter.The range selected is divided into 128 equal rangecells, for example, with 300 nautical miles selectedeach cell is 300/ 128 = 2.344 nautical miles, or, interms of t ime, 3607 ps is divided into 128 time-slotsof 28.96 ps. During each time-slot the video level isfirst integrated then encoded as a 2-bit word, thusgiving four discrete representations from zero tomaximum signal. In the Primus 40 a complementedGray code is used for the conversion but is thenchanged to standard binary.

The scanner is driven by a stepper motor such that1024 steps are taken for 120" ofscan. Thetransmitter f ires on every other step so providing 512azimuth dirt ictions from which echoes may bereceived. Thus on each of 512 azimuth angles data isacquired in 128 range increments.

The averaging/smoothing circuits reduce thenumber of l ines (azimuth directions) by a factor of4 to 128 (= 51214) and apply a correction to thegradient of the sigral in range and azimuth. The 4 toI l ine reduction is achieved by averaging the sum offour adjacent azimuth time cells as shbwn inFig. 9.13 and Table 9.1. Af ter averaging we have 128lines with 128 range cells per l ine grving128 X 128 = l6 384 data cel ls . The data is thencorrected first in range then in azimuth as follows:if in a series of three adjacent cells the outer two cellsare the same but the inner is different, then the inneris corrected so as all three are the same;for example,

1 5 1

icccivcr vidco

Range cel ls

Video levol (O thru 3)

7

z

0

0

256 LINES PER FRAJVIEFRAME REFRESH RATElS 60 .7 Hz (16 .475mS|

3

I

5

(NoisGl

128

0

127

0

1 5

2

1 4

3

13

1

10

1

1 1

,|

0

1

12

0

1

1

1

(,,

Complemented --{binary gray Icode

tt

Fig. 9.1I Analcgue to digital conversion in the Primus 40(courtesy RCA Ltd)

16.475mS -256= 64 355, rS L INE TIME

128 CELLS P€R LrNE X 256 LTNES=32.768 DlsPtAV CELTS

,"*\

Fig. 9.12 Rho-theta raster scan format in the Primus 40;

512 transmissions per scan, 128 l ines in memory, 256displayed lines (courtesy RCA Ltd)

"',*",j,ffik%-urr;fll.\

. . c4ce{

152

0-l2-56-9

r0-12

Table 9.1 Four to one line averaging

Sum of four azimuthadiacent time cells

Average

E'�l corrected

F8. 9.14 Rargc and azimuth smoothing and correction inthe RCA Primus 40

'nemory as it is received. Since the signal level withineach range cell is coded as a 2-bit word the memorycapacity must be 2X 128 X 128 = 32'168 bits. Thememory comprises sixteen 2 X 1024-bit shiftregisters, so application of clock pulses causes thecirculation of data provided the output is connectedto the input which is the case when new data are notbeing loaded. New data must be loaded into memoryat the correct t ime in the sequence of circulatingdata; this timing control is provided by the new dataline control circuit which synchronizes the loadingwith scanner position. Loading is inhibited when thefreeze button is pressed but circulation of datacontinues. The data is continuously read out as itcirculates at a rate of about 7'772\ines per secondcompared with loading every fourth main bang, a rateof 121'414 = 30.35 l ines per second. The differentload and read rates give the scan conversion in time,leading to a flicker free bright picture.

Although 128 lines of video are stored, 256 linesare displayed, so it is necessary to double the storedlines. The line-doubling circuit averages two adjacentstored video lines to generate a middle l ine, so givingthe required 256 lines each of 128 cells (i.e. a total of32768 displayed cells). In the averaging process therule is to average up if an integer average is notpossible. The following example, considering part of

0I23

313 would be corrected to 333 while 012 wouldremain the same (see Fig.9.l4). The above processes,together with the integration of the video signalwithin each range cell prior to digitization, reduce thedisplayed noice to negligible proportions.

Each of the 128 lines of 128 cells is placed in

After averaging-

Fig. 9.13 Fout to one line averaging in the RCA Primus 40

a--.,--=-la-----------l

Noise

153

Videofrom Rx

Txtr igg6r

Fig,9.15 Video processing and rho-theta display blockdiagram

Fig. 9.15 Simplified memory

three adjacent stored lines, illustrates the process:

s t o r e d l i n e . . . 3m i d d l e l i n e . . . 3s t o r e d l i n e . . . 3middle l ine . . .2s t o r e d l i n e . . . 0

Data out

mile is represented by 128125 = 5'12 cells, so the firstmark is located at cell 26 (= 5 X 5'12) with subsequentmarks a t ce l l s 5 l , 71 ,102 and 128 .

Azimuth marks are obtained by raising all 128cells of the appropriate lines by I in a similar way t^othat described above. The sweep is 120", so for l5-azimuth markers we require I + 120/ 15 = 9 l ines tobe enhanced in intensity. With 256 lines thosechosen are line 2 and line 256 Nl8 = 32ly' rvhereN= 1-8. Line 2 is used for the -60o marker sinceblanking is applied to the first 'trace l ine, t ltus theleftmosi azimuth marker is in fact at -59'0625".

The contour circuit converts a video 3 level to avideo 0 level. If we had a range sequence of cells0 2 3 0 0, say, then the contoured cell of level 3would not be bordered, the sequence being

3 2 2 1 . . .3 2 2 t . . .2 2 I 0 . . .I I I 0 . . .0 0 0 0 . . .

Range marks are obtained by raising the appropriaterange cell level for each of the 256 lines by l, i .e. 0

becomes 1, I becomes 2,2 becomes 3 while 3 remains

at 3. Thus range marks appear slightly brighter than

target returns except for level 3 targets' Identif icationof the appropriate range cell in each line is achievedby a counter. For example on the 25 nautical milerange there are five range marks 5 nautical miles apart.Since there are 128 range cells per line each nautical

154

Averagingsmoothingcircuits

CRTFrame retrace

tI

Freeze

32768 b i tshif t register

Clock

0 2 0 0 0. To avoid this, the range cell adjacent to acontoured level 3 cell is raised to level 2 if necessary,thus in ourexample O 2 3 O 0 would become0 2 O 2 0 after contouring. Bordering is guaranteedin azimuth as a result of the line-doubling processsince, for example, if we have adjacent azimuth cellswith video levels I 3 0 from'memory, thenline-doublingwill give | 2 3 2 0 and aftercontouring | 2 O 2 0 as required. This bordering

-featwe is.necossary where the video gradient is steep,such as when we receive returns from mountains ordistant weather targets.

vlotor-EvtLs 0 0 r I 2 3 2 t O

The rho-theta raster is generated by tlte deflectioncircuits which are triggered by the frame and X-Yretrace waveforms. A l inear ramp current rvaveformneeds to be generated for both_the )/ (vcrtical)deflection coils and the X (horiz.r,ntal) deflectioncoils which form the yoke. The duration ol'the rampis 53 '89 ps wi th a 10 '46 ps ret race ( f lyback)g ivrng atotall ine time (time-base period) of 64.35 gs. Theamplitudes of the ramp wavelbrms determine theamount of deflection in the X and X directions andthus the particular l ine which is traced on the screen;line I is at -60o, l ine 256 is at +60o. In.pracrice,since on the completion of one frame at l ine 256 onthe right we start the next frame on the left afterframe retrace, l ine I is blanked in order to allow thedeflection circuits to settle down. The franre rate is6O'7 Hz.

The X and )z ramp waveforms are initiated by thcX-Y relrace pulses. The amplitude of the I/ rampmust be a mininrum at the beginning and end of theframe and a maximum half-way through the frame;its polarity is constant throughout. The anrplitude ofthe X ramp must be a maximurn at the beginning andend of the frame and zero half-way through theframe, when the polarity reverses. To achieve theamplitude variations described the X and Y rampwavefolms are amplitude-modulated by appropriatelyshaped waveforms triggered by the frame retracepulse.

It should be evident that timing and synchronizationare all-important. We see from the simplified blockdiagram that the timing and control circuits areconnected to virtually all parts to ensure the

SINAiY CODI

* ( , o

, , 1 . o

ANALOG VIDEO

Fig,9.17 Digital to analogue conversion (uncontoured)(courtesy RCA Ltd)

The c.r.t. is intensity modulated by one of fourd.c. levels applied to its control grid. Since the outputfrom the contour circuit is digital we must employ adigital to analogue (D/A) conversion circuit.

Frameretrace

Mod

xRamp

-

I

I

ffi,,,

_ r l l l l l lY

X-Y retrace

Fig. 9.18 Ramp generation

'-- Ramp

t55

necessary synchronization. All timing signals arederived from d 4'972459 MHz crystal-controlledoscil lator (period 0.201 107 7 ps). Of particularsignificance is the scanner position when new data isloaded into the memory. A counter in the scannerdrive circuits counts every eighth step in the sweep,first clockwise then counterclockwise and so on.The counter thus gives the memory l ine number fromI to 128 (= 1024/8) which is used by the new dataload circuit. l t is possible that the scanner steppingmotor may miss a few beats, in which case the countreferred to above wil l not represent the scannerposition correctly. In order to prevent cumulativeerrors the count is Jammed' at 64 whenever thescanner crosses the dead ahead position goingclockwise. The information required for Jam centre'operation is derived from the X-axis stator output ofa resolver, the rotor of which is driven by the azimuthmotor. This output varies in amplitude, andphase-reverses when tl're scanner pases through thedead ahead positi<-rn..

The above is a much-simplif ied description of theessential features of the Prirnus 40; many details havebeen omitted. Other digital weather radars withrho-theta displays such as the Bendix RDR 1200 wil ldiffer in detail but will operate in a similar way.

Digital Weather Radar - Television (t.v.) DisplayIn the nrevious section we saw that a disital weather

radar with a rho-theta display required scanconversion in time only. This follows since the dataare collected in the same orderas they are presented;only the rates are different. With a t.v. display this isnot so, therefore we need scan conversion in bothposition and time. Since the basic differencesbetween the two types of digital radar are the rasters,scan conversion, and the organization of memory weshall concentrate on these topics. What follows rsbased on the RCA Primus 30.

The raster is similar to a standard t.v. displayexcept that the field and line directions ofdisplacement are reversed and it is quantitativelydifferent. The raster consists of 256 vertical l ines eachwith 256 cells, thus we have 256 X 256 = 65 536displayed data cells. Each frame of 256lines isdisplayed in two interlaced fields each of 128 lines.The field rate is approximately 107'5 per second sothe interlace gives approximately 53'75 frames persecond (faster than conventional t.v.). which gives aflicker-free picture.

Azimuth drive is similar to the Primus 40, the angleof scan being 120" achieved in 1024 steps. Anazimuth counter counts every fourth step so thatwhen the count has gone from 0 to 255 there is aphase reversal of the drive signal causing the scannerto reverse direction. When switching from standby toa transmit mode (normal, contour, cyclic or mapping)the scanner is driven counterclockwise to the

F lRsl t lttD RAsltR Lltrt. oNt tr 126 Lli l ts wRlTTiN AT RATI 0f 10, t Hz'r lR51 F l t l ,0 BTANK RITRACi t lN t oNt 0 f 128 GtNfRAl tD A l RATi 0 f l0 / tH2 'E tAt i rK f LYBACK L lN i rROn tND 0 t f lR5I r l tLo T0 BtCINN ING 0 f S tCo l ' lD f l i to A I RAl t 0 f 107.5 Hr

- - - - - s tcoNDFl iL0 RASTIR L IN€. 01 .1 t 0 f l?8L lNts ! !R l IT IN A l RAr t 0 f lo i 5Hz '- - s fc0ND t l tLD ELANK RITRACI L lNt . o t \ t 0 r I28 Ct f t tRATt0 AT RATI 0 f 107 t H ] '

F L Y B A C K L I N t f R 0 ! , ! t ! D 0 t S t C O t ! 0 F l t L D T 0 E t C l N N l N C 0 f r l R S I f l t L D A I R A T [ C [ 1 0 7 5 H .'l l t60 UMs PtR stc

Fig. 9.t9 Simplified raster for the Primus 30 (only elevenlines shown) (courtesy RCA Ltd)

)'

\r t

i !

\ i

\ l ' ,

\

\i\\

\

t \| lr l

t \

156

leftmost position, and 'chatters' there until theazimuth counter reaches 255 when clockwiserotation is initiated; this ensures synchronization ofcounter and position.

Digitized data is written into a random accessmemory (RAM) consisting of eight 4096-bit RAMchips giving a total of32K bit storage (lK bit = lO24bits). Thus with a cell containing a 2-bit word there isprovision for l6K cells (= 128 X 128). Conceptuallythe memory is arranged as a grid with orthogonalixes, so the address at which data is to be stored mustbe in X-Y format. SCan conversion is required toprovide the correct address given that the data is beingreceived in a rho-theta format.

Fig.9,20 Rho-theta to X/I scan conversion

Assume d word has been correctly stored ataddress (X,Y) the next word, assuming a unit rangestep, must be stored at (X + LX, Y + A)z) whereAX = sin 0 and AY = cos 0, 0being the scannerazimuth angle given by the azimuth counter (seeFig.9.20) . The rate ofgenerat ion ofnew addressesis determined by the rate of generation of new datacells, of which there are 128 for each azimuth count.The first data cell in each group of 128 correspondsto the address (0, 0). A read only memory (ROM) is

used as a lock-up table to give the values of cos 0 andsin 0.

The 128 X 128 = l6K RAM cel ls must beconverted to 256 X 256 = 64K display cells; this isachieved by a data smoothing circuit. Each RAM cellis converted into four display cells, the video level ineach display cell being determined by a weighted andbiased average of levels in the corresponding RAM celland some of its neighbours. With the cells designatedas shown in Fig. 9.21we have:

' Ia = inr. IQI6 + Is + It + l)lalIb = int. l(2ly +'t7 + IL + l)l4l/c = int. IQIy + Is + Ia + l)l4lId = inr. l(Zty + Ir + In + l)l4l

where int. [. . . .] means integer part of f . . . .1 .

An example oi the process for one RAM cell is givenin the figure.

Four concentric arcs, with centre at the middle ofthe bottom edge of the display, serve as range marks.The addresses of the display cells to be i l luminatedfrrr range mark purposes are stored in a ROM. The8K bit ROM is time-shared with the scan converterwhich uti l izes it as a sin/cos look-up table, as statedpreviously .

The rho-theta sector of target returns occupiesonly part of the X-Y display. The unused area of thescreen is used for alphanumerics identifying theoperating mode and the range marks. A ROM, usedonly lbr alphanumerics, contains the position codefor the bottom ofeach character on any given l ine ofthe raster.

Each of the circuits providing the functions ofraster generation, azimuth drive, digit ization, RAMand ROM addressing and transmission must besynchronized in time. All t iming signals are derivedfrom a 10.08 MHz crystal oscil lator. Suitablesub-multiples of the basic frequency are fedthroughout the system as triggers and clocks whichkeep everything in step.

Scanner Stabilazatioh

The need for scanner stabilization has already beenstated;here we shall review the implementation.There are basically two types of stabilization:platform and line-of-sight. With the former themoving part ofthe scanner can be considered as beingmounted cin a platform controlled, independently inpitch and roll, by a vertical reference gyro (VRG).With the latter, pitch and roll signals are combined,

lrlb td

l r lM l R

l a l c

l8

Fig. 9.21 A RAM cell to display cell conversion in thePr imus 30

L

( x + A X , Y + A Y )

I l[-+ (o'O)

AX--ri e

2

i-I

2 31 3 3

2 33

RCA

157

Timingandcontrol

Controlpanel(rangel

tsig.9.22 XIY display block diagram

taking into account the azimuth angle of the scanner,the composite signal being used to control the beamtilt angle. Since the platform system requiresrotating waveguide joints for azimuth, pitch and rollmovenrent plus pitch and roll motors, the l ine-of-sightsystem is preferred in most modern weather radars.Only the l ine-of-sight system will be explained below.' While the scanner is pointing dead ahead, aircraftmovement in roll will have no effect on the beamdirection since the axis about which the aircraft isrotating is in l ine with the bearn axis. With thescanner pointing 90" port or starboard pitchmovement wil l have a negligible eft 'ect since the axisabout which the aircraft is rotating is parallel and closeto the beam axis. Conversely with the scanner deadahead, aircraft pitch must be corrected in full whilethe scanner at t 90" aircraft roll rnust be corrected infull by pitching or t i l t ing the scanner.

158

The above paragraph is the basis for l ine-ot-sightstabilization. Pitch and roll signals fronr the VRG arecombined in an azirnuth resolver, the rotor of whichis driven by the azimuth motor. Tl.re stators of theresolver are connected to the pitch (P) and roll (R)outputs of the VRG, in such a way that the rotoroutput is P cos 0 + R sin d, where 0 is the azimuthangle.

The composite dernand sigrral is fed to a servoamplif ier which also has position and velocityfeedback inputs. If the sunr of the inputs is non-zero,an error signal from the servo amplit ier wil l drive themotor so as to reduce the error to zero. The positionfeedback from the ti l t synchro is modified by the ti l tcontrol so that the angle of the beanr above or belowthe horizontal may be set by the pilot. Velocityfeedback is provided by a tachogenerator to preventexcessive oversltoot. '

The components used in the stabil izatidn systemcan vary. The position feedback transducer and ti l tcontrol may be two- or three-wire synchros or indeedpotentiometers. Some equipments use a d.c. ratherthan a.c. motor although a.c. is normal for demandand feedback signals. On some systems no rollcorrection is employed if the scanner azimuth angle is

AntennaElevation Azimuth

rotary joint rotary

Azimuth motorcontrol logicfrom indicator

l e andreduc t ion

Fig.9.23 RDR 1200 scanner block diagram (courtesy BendixAvionics Division)

q) 6hIF

L - - -II

I Cable and I

I Oear reductionl

I(T)

Indicator

restricted to say 145o, as on various general aviationsystems.

Unless an azimuth stepper motor is used theazimuth angular velocity of the scanner is not constant.Reversal of direction at the extremities means thatthe scanner accelerates towards the dead aheadposition and slows down going away from dead ahead.

Pitch /rol lampli f iers

P c o s 0+

R. s i n 0

Summing -pre-ampli f ier

Pos i t ionfeedback

159

It follows that less time is available to makestabil ization corrections at the dead ahead position.If the servo loop response is fast enough to cope withthe nrost rapid rRovement in azimuth it wil l be toothst at the extremities. In order to vary response timethe velocity feedback may be modified so that it isgreatest in amplitude when the azimuth angle is amaxlmum.

With a flat plate aerial the beam is ti l ted bypitching the plate, thus a pitch-rotating waveguidejoint is needed. There is a choice when the systemuses a parabolic reflector, either the reflector andfeed move in pitch or the reflector only. In the lattercase no pitcli-rotating joint is used but the beam shapedeteriorates since the feed point is displaced tiom thefocus with ti l t applied.

Other Applications for Weather Radar

AJthough the primary function of a weather radar isto detect conditions l ikely to give rise to turbulence,various other uses for the system or part of the systemhave been; and continue to be, found. These wil l bebriefly described.

MappingVirtually all weather radars offer a mapping facility.At its most crude, selection of mapping merelyremoves s.t.c., whereupon the pilot can ti l t the beamdown to view a l imited region of the ground. At itsbest the beam is changed to a fan-shaped beam,whereby received echo energy is constant from allparts of the i l luminated ground region. In theAppendix it is shown that the received power isinversely proportional to the square of the range for abeam-fi l l ing target (A9.9). also if the beam isdepressed at an angle @ to the horizontal the rangeR = ft cosec @ wf "e /r is the aircraft height. So forequal returns from ground targets at differentdepression angles (hence range) the transmitted powerneeds to be distributed on a cosect d basis. since wewill then have (Pr) o(PtlR2i". lcosec2 Qlh2 cosec2 61=Qlh ' ) , i .e .P, is independent of range.

With a parabolic reflector an approximate cosec2beam can be obtained by use oi a polarization-sensitive grid ahead of the reflector surface. In theweather mode the grid is transparent to the beamsince the E field is perpendicular to the conductingvanes of the grid while in the mapping mode the gridreflects part of the beam energy downward since theE field is parallel to the vanes and therefore does notsatisfy the boundary conditions. To achieve remoteswitching between weather and mapping. either the

160

Grid vanes

Horizontal lypolarized feed

Fig.9.24 Weather-mapping facility using a parabolic

reflector

reflector is rotated through 90" (as in Fig. 9.24) orthe direction of polarization is rotated by using awaveguide rotating joint or a ferrite polarizationtwister.

The cosec2 beam is difficult to achieve with a flatplate array designecl specifically for a pencil beam.However, a fan-shaped beam can be obtained byreversing the phase of the r.f. energy fed to the slotsin the top half of the plate.

When selected to nrapping, rivers, lakes andcoastl ines are clearly identif ied, so allowingconfirmation of position. Built-up areas andmountains wil l give strong returns. An interestingphenomenon may be noticed over the plains of theUnited States: since fences, buildings and power l inestend to be laid out with a north-south or east-westorientation, returns from the cardinal points arestrongest, thus giving noticeable bright lines on theradar corresponding to north, south, east or west.

Drift IndicationWith downward tilt the retumed echo is subject to aDoppler shift due to the relative velocity of theaircraft along the beam. The spectrum of Dopplershift frequencies is narrowest when the beam isaligned with the aircraft track. The Doppler signalcan be displayed on a suitable indicator (A-typedisplay) where, due to the spectrum, it appears asnoise elevated onto the top of the return pulse. Withmanual control of the azimuth position of the scannerthe pilot can adjust until the Doppler 'noise'

(spectrum) is at a minimum, when the drift angle canbe read off the control. This option is rarely found.

aBeacon InterrogationThe transmitted pulse from the weather radar can beused to trigger a suitably tuned beacon (transponder)on the ground. The beacon replies on 93 l0 MHz,

Weather

I,Mapping

/---=--\z ' . - \ " '

so a weather radar with a local oscillator free.rency of9375 MHz and a transmit frequency of 934i MHz willproduce a difference frequency of 30 MHz for normalreturns and a difference frequency of 65 MHz for thebeacon reply. Two differently tuned i.f. amplifierscan be used to separate the signals. As an alternativetwo local oscillators may be used.

On some systetns the selection of beacon eliminatesthe normal returns from the display; on others it ispossible to show weather and the beacon response.

The ease with which this facility can be uied tofind offshore oil rigs makes radars offering beaconoperation an attractive proposition for helicopterssupplying the rigs. Such radars usually have a

short-range capability; for example, the Primus 50offers 2 nautical mile range using a 0.6 ps pulse, thusgiving good range resolution.

Multifunction DisplayThe weather radar indicator is increasingly used forpurposes other than the display ofweather ormapping information. X-I rasters in particular makethe display of alphanumeric data straightforward,hence all the major manufacturers now offer a'page-printer' option with one or more of the radarsin their range. Similarly display of navigation data isavailable as an option with the latest colour weatherradars.

nrotr

xon*rftoffi

Fig. 9.25 Primus 30 with page-printer option (courtesyRCA Ltd)

161

A page-printer option is normally used to displaychecklists. The alphanumeric data is arranged andstored in pages on EPROMs, either in the indicator orin an external auxiliary unit. Each Bage may becalled up in turn using a page-advance button. Pagescontaining the normal checklist index andemergency checklist index are particularly important,and usually have dedicated buttons used to call themup for display. Having displayed a page of one of theindexes a line-check button may be used to advance acursor (line highlighted by displaying, say, black alpha-numerics on green background rather than green onblack as for the other lines). With the cursor set, thechosen checklist can be displayed using a list button.

Apart from checklists other alphanumericinformation which may be listed includes waypoints,or indeed any pilot+ntered data if pages are allocatedto this facility. One method of allowing the pilot toenter data is to use a calculator keyboard;Hewlett-Packard and Texas Instruments makecalculators which can be modified to interface withthe page-printer system.

Display ofnavigation data from external sensorssuch as VORTAC, Omega, INS or Loran is achievedthrough an interface unit. Typically the pilot is ableto display waypoints joined by track lines, togetherwith the weather data. As an example the RCA DataNav. tr I system allows the display of current VORTACfrequency, range and bearing to current waypoint andup to three correctly positioned waypoint symbolsfrom twenty which can be stored. An additionalfeature of the Data Nav. I I is a designator symbolwhich may be set to any desired location on thescreen: this location can then be entered as a newwaypoint to replace the current one, so providing an

alternative route if storms are observed on originalintended course.

Progress in this area is rapid' ln 1979, with the

appropriate interface, the weather radar could displayprojections of aircraft position on straight or curvedpaths, ETA at waypoints and warnings of sensor data

failure in addition to the data referred to above.

Weather Radar Characteristics

ARINC Characteristic 564-7 allows the designer morefreedom than do most other such documents;however it is quite clear what performance andfacilities are to be made available. The following is a

summary of some of the more significant and/orinteresting items.

RangeAt least 180 nautical miles for subsonic aircraft and

162

300 nautical miles for s.s.t. is usually demanded by

customers.Range marks at 25 nautical mile intervals up to 100nautical miles are suggested, as are the available rangeselections of30/80/lS0 or 30/100/300 or 30/80/180/360. The advantages of a continuously variabledisplayed range from 30 nautical miles to maximumare stated.

Displayed Sectornt ieast + 90" but not more than t 120". Displayedrange at + 90" to be not less than 60 per cent ofmaximum range. The scan may be reciprocating or

circular. (Note in the case of the latter, since the

scanner rotates through 360' RAM (radar absorbentmaterial) screening is needed on the nose bulkhead toprevent excessively strong received sigrrals.)

Radio FrequencYC-Band 5400 MHz x2OMHz (nominal)

X-Band 9375MHzt 20 MHz (nominal), or9345 MHz L 2O MHz (nominal)

BandwidthMinimum bandwidth = l'216 where the pulse width 6

must be less than l0 Ps'

Dispby AccumcY ^Azimuth angle: t 2-.Range: the greater of t 5 per cent or I nautical mile.

Sensitivity Time ControlRange2 law from 3 nautical miles to the point where

a 3 nautical mile target ceases to be beam filling.

Two-wire logic from scanner to set s.t.c. maximum

range in accordance with antenna gain (and hence

bearnwidth).

Scanner Stab and TiltTwo-wire pitch and roll sigrrals each (E/2300) V+ 2 per cent per degree where E is nominal I 15 V400 Hz reference phase (50 mV per degree butexpressed in terms of supply). The phase of signals isspecified. Dummy load of 20 kQ where signal notused.Pilch and roll rate capability 20o per second.Line-of-sight (two-ais system): combined roll, pitchand tilt freedom t 35" with accuracy + lo, manualt i l t i 14" .Split axis (three-axis system): roll t 40', pitcht 20o, manual t i l t I 14", combined pitch and ti l t!25" ,accuacy t 0 '5" .Droop nose signal (tr/575) V per degree, same phase

as nose down. (Two-wire signal, 20 mV per degree

fryEnroute Navigation.

1) The intended t rack l ine or ig inat ing f rom lhe ai rcraf tsymbol d isplays lhe programmed route of f l ight . Waypointsand their numbers can be displayed on the t rack l ine. WhenDataNav is ut i t ized wi th RNAV, the associared VORTACsymbol is d isplayed along wi th i ts f requency as i l lustratedWhen used wi th a VLF/OMEGA or INS svslem. the DataNavwi l l d isplay s imi lar informatton. When weather is encoun_tered lhe current Waypoint may be ottsel by using RCA'Sexclusive Designator feature The Designator can be moveoto any location on the screen by means of the Waypoint

3) The aircraft has now completed the turn and interceptedthe new l rack l ine which wi l l safely c i rcumnavigale thedangerous weather. l f desired, you can re lurn to the or ig inalWaypoint and track line by pressing lhe cancel button onthe DataNav conlro l oanel .

FiE 9.26 En-route navigation with the (colour) Primus 200(courtesy RCA Ltd)

for SST aircraft where scanner mount is dropped withnose.)Scale required on scanner whereby ti l t angle may beread.

oftset contro l showing the Designator symbol at 35" r ight ,46 nm.

2) When the Designator is at the desired posi t ion, the newWaypoint can be entered into the Navigat ion System bymeans of the "ENTR" but ton on the DataNav contro l pane,.The current Waypoint wi l l be moved to the new locat ion anda new track l ine establ ished. The Range and Bear ing of theaircraf l to the VORTAC stat ion wi l l a lways be displayed inthe lower r ight hand corner of the display.

4) Waypoinl Listing Mode can be sebcted by means of-the"MODE" but ton on the DalaNav contro l panel . Informat ionfor 3 Waypoints can be displayed on the screen. The cur-renl Waypoinl wi l l be displayed in yel low wi th a l l other dalain green. Al l the Waypoints can be displayed in groups oflhree by moving the "Waypoint Offsel Control" left or right.

IlaveguideC-Band type ARA I 36 or WR I 37. X-Band typeRG-67U. Ridged waveguide rejected; v.s.w.r.max imum l . l : l .

163

Magnetron Magnctic FieldNo more than lo compass deviation with sensor l5 ftfrom the t.r. The t.r. should be mounted at least 2 ftfrom the indicators, other t.r. units and other devicessensitive to magnetic fields.

Maintenance and Testing

Safety PrecautionsThere are two hazards when operating weather radar,namely damage to human tissue and ignition ofcombustible material.

The greater the average power density the greater

the health hazard. A figure of l0 mW cm' is agene rally accepted maximum permissible exposurelevel (m.p.e.l.). Among the most vulnerable parts ofthe body are the eyes and testes.

The greater the peak power the greater the firehazard. Any conducting material close to the scannermay acf as a receiving aerial and have r.f. currentsinduced. There is obviously a risk, particularly whenaircraft are being refuelled or defuelled.

An additional hazard. which does not affect safetybut will affect the serviceability of the radar, is thepossibility of very strong returns if the radar isoperated close to reflecting objects. The result of

these'returrls is to burn out the receiver crystalswhich are of the point contact typ€'

The following rules should be observed whenoperating the weather radar on the ground:

l. ensure that no personnel are closer to atransmitting radar scannpr than the m'p.e.l.boundary, as laid down bY the sYstemmanufacturer;

2. never transmit from a stationary scanner;3. do not operate the radar when the aircraft is

being refuelled or defuelled, or when anotheraircraft within the sector scanned is beingrefuelled or defuelled;'4. do not transmit when containers of inflammableor explosive material are close to the aircraftwithin the sector scanned;

5. do not operate with an open waveguide unless

r.f. power is off; never look down an openwaveguide; fit a dummy load if part of thewaveguide run is disconnected;

6. do not operate close to large reflecting objects

or in a hangar unless r.f. energy absorbingmaterial is placed over the radome (RAM cap)'

The safe distances for radars vary widely,depending on average power transmitted and beam

164

width. It is normal to calculate the m.p.e.l. assuminga stationary scanner and a point source' in which casean average power of 6 X F X Pt is spread over an areaof n X D2 X 1Jin2 (e l2)i thus the distance D in rnetresfor an exposure level of l0 mW/cm2 is given by:

^ I | - 6FPt ' l o ' t -

D=m6 hffil *L ' J

where: 6 is pulse width in seconds;F is pulse repetition frequency in pulses

per second;P1 is peak Power in milliWatts;0 is beam width.

Thus consider an older airline standard weather radar(Bendix RDR IE/ED) using manufacturers' nominalfigures for the longest range option, i.e.6 = 5 ps;

F=2OO;Pt = 75 kW; 0 = 3" ;we have D = l8 '66 m(ev 60 ft), while for a modern general aviation radar(RCA Primus 20) where 6 = 2'25 ps, F = 107'5,

& = 8 kw ,0 = 86 we have D = l ' 12 m (e , : * f t ) '

To ensure safety precautions are observed consult

manufacturers' data for safe distances then, if

operating the radar for maintenance purposes, place

radiation hazard warning notices the appropriatedistance from the nose. When working by the scanner

with the radar on standby a notice should be placed

by the controls stating 'do not touch', or better still

the transmitter should be disabled or the waveguide

run broken and a dummy load fitted. If the radar is

switched on prior to taxiing, the transmitter should

not be switched on until clear of the apron'X-ray emission is a possible hazard when operating

the transmitter with the case removed, such as might

be done in a workshop. The likelihood of danger is

small but the manufacturers' data should be

consulted.

Check for Condition and AssemblYObviously the weather radar system as a whole is

subject to the same requirements as all other airborne

equlptn.nt in respect of security of attachment and

condition; however, certain points need to be

highlighted.-Thi waveguide run should be the subject of fairly

frequent inspections and should be suspected ifpoorperformance is reported. When inspecting the

waveguide, corrosion and physical damage such as

cracks and dents are obvious things to look for'

Flexible waveguide coverings are subject to perishing,

cracking and a detached mechanical bond at the

flanges. lnternal damage in flexible waveguide can be

founa Uy gently flexing while listening and feeling for

clicks. There should be no more than minor bends in

the H plane of flexible waveguide and the radius of l. Check warm-up time for magnetron.bends in the E plane shoukl be greater than about Zlrn.2. Check fan motor (a l ight piece of paper should

If it is thought necessary to dismantle the 'stick' to the fi l ter).

waveguide ,un1o curry oui an internal inspection, 3. Check internal power supply voltages and currents

or if i piece of waveguide needs to be replaced, care with built- in meter, if f i tted. or test meter connected

must be taken when re-install ing. Choke joint f langes to test socket, if supplied.

must mate to plain flanges. Seaiing or O-iings musi NB'. when disconnecting a test lrleter from the

be fitted to achieve u pr.rrut. seal. The E planes of test socket it is vital that the shorting plug

adjoining waveguide pieces should be paraliel. Undue should be replaced, otherwise crystal earth

force should not be used in aligning waveguide, either returns, broken for current measurement,

within the run or at the ends of the run. All wil l not be made'

waveguide supports should be secure and undamaged. 4. Check test facil i ty: pattern should be as specitied

If an internal inspection using a probe light reveals by manufacturer. In particular check the pattern

dirt or moisture it may be possible to clean by pull ing is centred and neither overfi l l ing or underli l l ingthrough a clean soft cloth andi or blowing out with an screen, and that the range marks are equally spaced

air pressure l ine. Care must be taken not to scratch (l inear deflection, rho-tl ieta). symmetrical arcsthe inside surface of the waveguide since thiswould (l inear deflection,X-Y)and correct in number

render it scrap, aswould signs of corrosion or deposits (t ime-base duration, rho-theta)' A rho-theta raster

which cannot be removed is suggested above. may show a small open centre (about { in'; but no

Any drain trap in the waveguide run should bechecked for blockage, and accumulated moistureshould be removed. Filters and desiccators in thepressurization feed, if f i tted, should be inspected fbrcleanliness (fi l ter) and colour (if desiccator is pink itis unserviceable).

The scanner should be checked for freedom ofmovement as well as general condition and security ofattachment. In carrying ()ut scanner inspcctions thedish or plate should not be turned directly by handbut through the gearing. Backlash in the gears can bechecked by gentiy applying forward and backwardmovement to the edge of the dish or plate in bothazimuth and pitch directions: in a 30 in. diameterimtenna, movement of ; T at the edge in<licates atotal backlash of nearly 1". No chafing of cablesshould occur due to scanner movernent.

When replacing the scanner, ensure shirns orwashers are used for the replacentent, in the samepositions as they were used lbr the item rentoved,this ensures the azimuth scan axis is nrutuallyperpendicular to the roll and pitch axis o1'the aircraft.Some scanners may be mounted with input waveguideflange up or down, but polarized connecliorts wil l becorrect for only one orielttaticln: tailure to check thispoint may result in an incorrect sense ol rotation inazimuth and pitch.

The t.r. is l ikely to have an internal cooling tan, inwhich case the fi l ter should be secure and l 'ree t 'romobstruct ion. I t may be possib le to c lear a b lockedl i i tcr by a i r b last in t l te opposi te d i rect ion to nornta la i r i lorv. whic l r should be in to the uni t .

Functional Ramp Check - RadarOhsr' rr.. S:rl ety l)reclu t ions

ta i l .5. Observe ground returns on all ranges. Operate ail

controls and ensure the desired effcct is achieved.

NotesThe above is only a brief outl ine of the checks to becarried out. One shtlLrld always use the manltfacturers'recornmended procedures when tcsting. Whencarrying out itenr (5) above. experience is necessary ifone is to s tate, wi th any cer ta inty , the condi t i t ln ofthe systenr. The picture obtained wil l depend on theheading of the aircrali; as an extreme exantple anaircraf't pointing out to sea wil l give very dif lerentground returns ot t i ts radar wl ten compared wi th anaircraft pointing in the direction of a range of hil ls.The technicirn should be aware of the f orm of thctarget /noise p ic ture expected at a par t icu lar a i r l le ldon a par t icu lar heading.

The test facil i ty varies from system to system. Atits sirnplest a ramp or tri{ngular voltage is generaledwhich varies from siSnal level zero to a maxitnunt ofsignal level three. If suclr a waveform is applied to thet l ig i l izer then a l l t l r rec levels of i l luminat ion lndcontour opcrat ion carr b6 checked. Adt l l t ional testswhich l ravc been providdd nrc lude a gated noise sourct 'for checking s.t.c. and contparing with nornral

' rcceiver noise. ref lectet f power t t t t r l t i tor , a . l .c .rnoni tor and a.g.c. tnoni tor .

Radar Systems TesterBecause of the possible ntisinterpretation of thesystenr cclndition when.iudged olt returns I 'rt l tnground targcts a systctns tester is stlnletimes used togive nrore control ttvefr t lre fcalures ofthe target.

A p ickup horn is at tached to the radorne at dc: ld

165

NAHOOTI EACKGROTINDrSISE PIAKS

(uGr{T t{olsE }

coHrounTE'T BAND

Ftg.9.27 RDR IE/F test pat tern (courtesy Bendix Aviontcs

Division)

Fig 9.2E Simplified radar systems tester

centre position. A co-axial cable connects the horn tothe tester which is inside the cockpit. The testerserves as a beacon and returns a signal which shouldappear at zero degrees on the p.p.i. display at a rangedetermined by the tester delay. An input attenuatorcan be adjusted so if the tester just fails to betriggered, the attenuator wil l give a measure of theradiated power. An output attenuator can beadjusted so that the 'echo' is just visible (radar onpreset gain with standardized bril l iance setting), thusthe attenuator wil l give a measure of systemsensitivity. The output attenuator may also be usedto check the contour inversion level setting.

Note the tester wil l check the wavesuide run.

166

r/tFf sTrsT sAt{ffi

TESr ilfffE B*lw{ ilff{*s $iltctr }

-lT{o EFFscr{ *fffiIcrD sAlH }

scanner and radome as well as the t.r. If bothattenuators read less than normal then the fault mustbe in the common r.f. components; if only the inputattenuator reads low then transmitter power output isdown; while a low output attenuator setting indicatespoor receiver sensit ivity.

Functional Test - Scanner StabilizationObserve Safety Precautions

1. Remove radome. Switch to standby withstabil ization on and ti l t zero. Check, using a spiritlevel, that the flat plate or the plane across therim of the reflector dish is vertical with thescanner dead ahead and at the extremities of thesector scan.

2. Remove VRG from its mounting and fit on a tilttable adjust.ed so that its surface is horizontalalong both axes.

.3. Inch scanner to the dead-ahead posi t ion. Wi th. - \stabil ization on, adjust t i l t control unti l scanner )is vertical.

4. Simulate a suitable pitch angle @ nose up. Usinga protractor spirit level ensure scanner elevationchanges by @ degrees down. Repeat for nosedown.

5. Sinrulate aircrali roll, ensure no change in .scanner e levat ion.

6. Inch scanner to por t ext rent i ty , az- imuth angle 0.

il

Simulate a suitable roll angle a port wing downand ensure scanner elevation changes by o sin 0up. Repeat for starboard wing down.

7- Simulate aircraft pitch angle @ and ensure scannerelevation change is @ cos 0 in opposite sense.

8. Repeat (6) and (7) but with scanner at starboardextremity noting sense of roll correction isreversed.

9. Inch scanner to dead ahead. Simulate suitablepitch angle. Switch stabilization from on to offand back to on, and ensure scanner moves inelevation without excessive overshooting.

10. Switch stabilization offreplace gyro in aircraftmounting.

I l. Carry out f inal check of t i l t control beforeswitching off and replacing radome.

Notes:The above applies to a typical line-of-sight system.For any particular equipment the manufacturers'procedure should be followed, observing statedtolerances.

A platform system checkout varies from the abovein that changes in elevation are independent ofazimuth angle.

In items (6), (i) and (8) an azimuth angle of 90"strould be chosen if possible, since this will makecos 0 = I and sin 0 = 0, so simplifying checking.

Pitch and roll correction potentiometers are oftenaccessible in which case pitch correction should becarried out with the scanner dead ahead and rol-correction carried out to balance any errors at theextremities.

A gyro simulator may be used for a checkout ofthe complete system less the VRG. The VRG isdisconnected and the simulator connected in itsplace. The procedure is then similar to that above,with the appropriate pitch and roll angles beingselected on the simulator. Since the VRG is nottested this is not a full functional test but is useful forfault-finding. The signals from the simulator shouldcorrespond to the VRG output signal standards(e.g. ARINC).

Check of v.s.w.r.If the waveguide run or scanner is suspect, v.s.w.r.checks can be carried out to find the faulty itcnr. Tocarry out such a check a directional coupler must befitted in the run, and power in the forward andreverse directions measured using, for example, athernristor bridge power meter. The check should becarried out at both ends of the wavesuide run so that

a measure of waveguide losses can be obtained fromthe differences in forward power at either end.

Practical difficulties occur in fitting the directionalcoupler in a rigid waveguide run. In someinstallations a coupler may be permanently fitted atone end, usually at or close to the t.r.

In order to measure the v.s.w.r. of the waveguidethe scanner should be disconnected and a dummyload fitted in its place. ARINC 564-7 specifies amaximum v.s.w.r. of I . l : I for a new installation.

When measuring the v.s.w.r. of the scanner theradome should be removed and the scanner tilted upto maximum to avoid returns. Figures obtainedshould be checked against manufacturers' data. Onfitting the radome the v.s.w.r. at the antenna willdeteriorate, measurement at various azimuth and tiltangles will give an indication of the radomeperformance.

The most likely cause of loss of radome efficiencyas a transparent material to r.f. is ingress of moisturethrough small pinholes and cracks which may appearon the outside surface of the radorne. Such pinholesand cracks may be seen by shining a light on theoutside surface and viewing from the inside. Amoisture detector can be used to measure theresistance between two adjacent points on theradome. The detector has two probes which arepressed firmly against the inside surface of theradome. Where the resistance measured is lower thannormal it may be due to an ingress of moisture.

Bench TestingThere is not space here to consider bench testing'indepth, but mention should be made of a special

Fig.9.29 RD-300 weather radar test set (courtesy IFR.Electronics Inc)

weather radar test set, the IFR RD-300 which,together with an oscilloscope, can be used to performall the common radar tests without the proliferationof signal generators and other instruments normallyfound on a radar test bench.

.c.frd

167

Ryan Stormscope

Being a relatively new development the Stormscopeis, as yet, not to be found in service in anything l ikeas many aircraft as is weather radar' Since space is ata premium the coverage of the Stormscope in thisbook has been limited to allow a more detailedcoverage of radar. The number of pages allocated herereflects the importance of each of the systems tomaintenance staff and aircrew today. The situationmay well be reversed in the future or, more l ikely,equalized.

As stated in the introduction to this chapter, the

Ryan Stormscope depends for its success on detectingelectrical activity which is associated with turbulence.Since the radiating source is natural only a receiver isrequired; an immediate advantage over weather radar.To obtain directional information, use is made of anADF loop and sense antenna, both either dedicated tothe Stormscope installation or t ime'shared with ADF.The rest of the installation comprises a display unitand a computer/processor unit.

Care must be taken in installation planning sinceStormscope is prone to interference lrom generators,

motors, strobe lights, etc. Interference fromcommunication transceivers is avoided by inhibit ingthe Stormscope whenever a transceiver is keyed'

measurement possible and reasonably accurate' Forrange measurement the received signals are comparedwith a 'standard' to give so-called pseudo range. Themethod uscd to deterntirte ratlge lrteans that particularlystrong signals appear to be closer than they actuallyare. which is not really a disadvantage since the sourceof such signals is a region of severe electrical activityand hence turbulence.

Each discharge appears as a bright green dot on thecirculal display screen at a position representative ofthe source position relative to thc aircraf't. Anaircraft symbol is located at the centre ol-t lre displaywi th radia l l ines marked at 30" i t l tcrvals at td twt tequally spaced range rings. The range ol't lre outerring is as selected on the panel-mounted receiver'40, 100 or 200 nautical miles. Since tlre outer ring isnot at the periphery of the display the rnaximuttlrange available is of the order of 260 nautical rniles.

Each discharge is only a momentary event sostorage in memory is required. The trlemory canstore the position of 128 dots, these being displayedto form a mapJike picture. When the 129th dischargeoccurs the oldest dot in memory is replaced; in thisway the image is continuously updated. lf theaircraft heading changes or a new range is selectedthe dot positions are incorrect unti l all I 28 have beenupdated, a process which can take up to 25 s on a

Fig. 9.30 Ryan Stormscope display unit and computer/processor (courtesy Ryan Stromscope)

OperationSignals from the two orthogonal loops and the senseantenna are utilized to give range and bearing of thesource of electrical activity received. The propertie:.of the loop and sense antenna make bearing

lSe

stormy day (only 5-10 s with tomado activity withinrange). On quiet days a dot may remain a long timebut after 5 min it is automatically erased. A'clear'button allows the pilot to erase the display startingwith the oldest data and progressing to the newest in

a total erase time of I s. Another button allows thedisplay of that activity which is forward of theaircraft with, of course, the full memory dedicated tothese signals.

The 128 dots appear in clusters on the display,indicating where bad weather can be expected. As theweather becomes more severe there is a spreadinginward towards the centre due to the pseudo rangereducing as a result of the stronger signals. Witllincreasing storm intensity the display becomes veryanimated.

A test facility is built in whereby when theappropriate button is pressed a dot cluster appearsnear to a position of 45o; 100 nautical miles. Inaddition, a test set is available which can simulatesigrrals at various ranges and azimuth angles forsystem checkout and calibration.

Comparison of Stormscope and RadarIt would appear that, purely from a functional pointof view, i.e. ability to avoid turbulence, there is littleto choose between the two systems. Some advantagesofStormscope over radar are:

l. no moving parts and no transmitter, hencemean time between failures (m.t.b.f.) should behigher;

2. simpler antenna installation which keeps downinstallation costs;

Appendix

Factors Affecting Weather Radar Performance

The Radar EquationIf power P1 is radiated from an omnidirectionalantenna then the power density (power per unit area)decreases with range. At a range R a sphere ofsurface area4nR2 is illuminated by the e.m. wave,thus:

Power density from omni antenna =# (A9.1)

Since a directional antenna is used with gain G (overan isotropic antenna, i.e. perfectly omnidirectional)we have:

Power density from = PrG-directionalantenna 4rRz

(Ae.2)

The target will intercept part of the radar beam, thesize of the part depending on the radar cross-sectionofthe target o. Since the reflected power is subjectto the same spreading out in space as the incidentpower we have:

Power density of _echo at aircraft

P,GO(4rR2)z

(Ae.3)

3 .4.

only the cheapest radars compare in capital cost;fully operational on the ground with 360" fieldof view.

The radar antenna intercepts part of the echo signal,the size ofthe part depending on the effective captureareaA, so received echo powerPr is given by

Some disadvantages of Stormscope compared withradar are:

l. sensitive to interference;2. limited rate and accuracy of data acquisition

although it would appear from operationalevidence that this does not prevent theStormscopefrom being used for efficient andsafe weather avoidance ;

3. lacking in any other applications such asmapping, navigation data display or pageprinter option.

Two things should be pointed out here; firstly theextras with weather radar have to be paid for;secondly the Stormscope is a relatively newdevelopment by a company small in comparison tothe giants ofweather radar. It should not be beyondthe ingenuity of the designers to develop the systemso as to eliminate some, if not all, of thedisadvantages.

P r = (Ae.4)

The relationship between antenna gain and effectivecapture area can be shown to be

4 n AG = f (Ae.s)

where ), is the wavelength of the e.m. wave. Soequation (A9.4) becomes:

- P,G2).3 o& =1ffi . (Ae.6)

Replacing P, by the minimum detectable sigtal pow.errn and rearranging, we have the well-known radarrange equation:

R*"*t = ffi (Ae.7)

The Radar Equation for Meteorological TargetsWith meteorological targets we have a large number of

169

independent scatterers of radar cross-section o; so;providing the target fills the beam, we may representthe total radar cross-section bv:

o = V42oi (Ae.8)

where: Xoi is the average total backscattercross-section of the particles per unitvolume;

V^ is the volume occupied by the radiatedpulse which can be approximated by(n l4)R2 02 c6l2;

0 is the beamwidth (equal in horizontaland vertical planes for a pencil beam);

c is the velocity of propagation:6 is the pulse duration.

Substituting for o in (A9.6) we have:

D _ PtG2^ ,2 02 c62o i'r - -SIZFF-

Equation (A9.9) is applicable where the targetisbeam-filling, for example a spherical cloud of 3nautical miles diameter will fill a 4" beam up to about43 nautical miles. For a ta-rget outside the beam-fillingrange the proportion oi'beam filled can be shown tobe (Dl0R)2 where D is the target diameter. Soequation (A9.9) would become, for such a target:

P,G2)y2 c6D22o;r, = *-:;.IFFF- (Ae. r 0)

an equation in which the fourth power of R occurs asin the basic radar equation (A9.6) and contrastingwith equation (A9.9) where we have the square of R.Again we can produce a range equation assumingnon-beam-filling targets:

R*a*o = PrG2) t2 c6D22o i5 l 2 r 2 m

If the wavelength is long compared with thediameter d of a scattering particle then it can beshown that:

ns lklz>d;6zoj = --Ia= (Ae.t2)

where lkl2 is related to the dielectric constant andhas a value of about 0'9 for water and 0'2 for ice.

It is helpful to replace 2d16 by an expressioninvolving rainfall rate, such an expression is providedby 2di6 - 200rr'6 where r is the rainfall rate inmm/h. lt should be noted that this is an empiricalrelationship, the constants being subject to variabilityfrom one experimental observation to another.Replacing 2 o; in equation (A9.9) and using therelationship between ,dir we have

170

(Ae. l3)

as the echo power received from a beam-filling raincloud. (Note that 180 x2OO lkl2).

Minimum Detectable SignalEquat ion (A9.1 l ) g ives the maximum range of aweather radar in terms of the minimum detectablesignal from a non-beam-filling target. A thresholdlevel must be chosen which is greater than the r.m.s.value of the noise occupying the same part of thefrequency spectrum as the signal. If the signalexceeds the threshold it is detected: if not it is missed.Too low a threshold will give rise to false alarms.

In choosing the threshold level the interpretationof the operator is significant, particularly inconventional (analogue) weather radars. In digitalweather.radars the choice of threshold is taken out ofthe hands, or eyes, of the operators. Noise spikeswhich do not occur in the same time-slot afterseveral successive transmissions are not displayed;they are said to be averaged out. As a result in adigital weather radar a lower threshold, or minimumsignal to noise ratio (s.n.r.), can.be tolerated withconsequent improvement in maximum range. I

In introducing the factors relating to noise into theradar equation it is convenient to use the noise figureF grven by

p = *l* (Ae.l4)' S"IN"where: S;/N; is the input s.n.r.;

Sofly'o is the output s.n.r.

The input noise can be taken as kTBwhere: k is Boltzrrrann's constant, = l '38 X 10-23

joules/degree:?nis temperature in degrees Kelvin;.B is the noise bandwidth (different from3 dB bandwidth but often approximatedbY i t ) .

So rearranging equation (A9.14) and substituting forly'; we have:

Si = kTBF SslNe (A9.15)

Substituting for m in equation (A9. I I ) gives:

o n 2 ) . 3 c 6 2 o ;R."*' = n#ftngl;i;; (Ae.r6)

Equation (A9.16) results from considering a singlepulse;ho.vever many pulses are normally receivedfrom a target during one sweep of the aerial. Thenumber of pulses returned from a point target is

lS0l,t'6 P,G02 c6 n3P, = - '7ff;n-

(Ae.e)

( A 9 . 1 l )

0 X p.r.f.n = + (n

(Ae.r7)

where c^.r is the scanning rate in degrees per second;p.r.f. is the pulse repetition frequency.

We can utilize some or all of the n pulses to improvedetection in a process known as integration. Use of ac.r.t., together with the properties of the eye andbrain, constitutes an integration process. Digitaltechniques, whereby the signal occuning in successivecorresponding time-slots is averaged, is also a form ofintegration in this sense. We can define the integrationefficiency as follows:

- (s/.^/),L" = reET,.where (S0/)r = s.n.r. of a single pulse tbr a $ven

probability of de tection ;(S/ffh = s.n.r. per pulse for the same

r i probability of detection when npulses are. integrated.

The integration improvement factor nEn canbeincluded in the range equation.

The average power P of the radar is related to thepeak power P; by:

P = Pt X6 Xp. r . f . (Ae. l8)

Substituting for Ps in equation (A9.1 6) andincorporating the integration improvement factor wehave:

PG2l,2 o3 c EnD o; (Ae.le)R*"*o5t2r2 VTBF Q (S/l/),

ln an effort to simplify the range equationEUROCAE and RTCA have derived a Performancelndex, PI, from the basic radar equation. Details ofthe calculations involved are given in ARINC

Characteristic 564 together with an empirical fornrularelating PI and maximum range. The primary purposeof the PI is to enable a comparison to be madebetween different radars rather than effect theaccurate calculation of maximum range.

Atmospheric EffectsThere are three effects. namely attenuation, refractionand lobing, which can degrade or even enhance theperformance of a radar operating in the earth'satmosphere.

Attenuation due to absorption by gases, primarilyoxyBen and water vapour, wil l reduce the maximumrange attainable. Precipitation particles also absorbthe e.rn. energy and caus.' scattering. The scatteringis csscntial l irr the opelution of weather radar butabsorption wil l decrelse the range and reduce theabi l i ty o l ' t l re radar to penetrate c louds in order to'see' what is bc.r"ond. Ernpirical results are availablebut we nray sirnply state that degradation increaseswith frequency. I.rencc the descriptions of C-bandequipment as weather-penetration radar and X-bandas weather-avoidance radar.

Since the density of the atmosphere is notuniform, refraction or bending of the radar wavesmay take place. Water vapour is the main contributorto this effect. The radar.waves will normally be bentaround the earth since the atmospheric densityusually increases with decreasing altitude, thus leadingto an increase in radar range.

Lobing is the arrival at the target of two radarwaves, one via the direct path and one by way ofreflection from the earth's surface. Depend'ing on therelative phases range may be enhanced or degraded.In an airborne weather radar with small side lobes thisdoes not create the same problem as withground-based radars.

l t l

10 Doppler navigation

Introduction

A Doppler navigator is a self contained dead.-reckoningsystem giving continuous readout of aircraft positionusually related to waypoints. Military aircraft havemade use of such equipment since the mid- 1950swhile bivil use for transoceanic navigation commencedin th6 early 1960s. In recent years ihr ut. of Dopplernavigators in long-range commercial aircraft haslargely been superseded by inertial navigators, triplesystems being fitted to airliners such as the 747 andConcorde. Military developments include compositeDoppler and inertial systems and we may expect tosee such systems'go civilian' in the future.

There are still many civil aircraft carrying Dopplernavigators but these are older, long-range airliners andas such they are fitted with equipment which,although not in any way primitive, does not employthe very latest techniques. A class of civil aircraft forwhich there is a continuing need for Dopplernavigators is survey aircraft which, by the very natureof their work, operate in areas not covered byground-based navigation aids (except Omega) andrequire accurate positional information. Use ofDoppler navigators in helicopters is not unusual; thereis equipment specifically designed for such aircraft.

The advantage of Doppler navigation lies in thefact that it is a self-contained system which does notrely on ground-based aids and can operate in any partof the world. This advantage is shared by inertialnavigation which also shares the disadvantage ofdegradation of positional information as distanceflown is increased. The degradation of informationarises from the fact that starting from a knownposition subsequent positions are computed bysensing the aircraft velocity and integrating withrespect to time. Errors, once they are introduced,can only be eliminated by a position fix.

A simplistic example will illustrate the build-up oferror. An aircraft takes off from A to fly to B, 3000nautical miles away in a direction due west from A.Using heading information from a directional sensorsuch as a gyromagrletic compass the Dopplernavigator senses that the iircraft is flying due west at

172

300 knots. Thus after I rnin the system gives areadout of position as being 2995 nautical miles fromB on trackt after I h 2700 nautical miles from B; afterl0 h the indicated position is B.

3OOO n.m. J-' l "

l o ,l

Fig. l0.l Effect ofheading error

With a + | per cent error in computed speed, thetolerance on the position readout after 1 min, I h andl0 h wil l be I 0'05. + 3 and t 30 nautical milesrespectively. With a + 1" error in the headinginformation we have the situation given in Fig. 10. r .We can see that d = 2 X 300/ X sin 0'5, thus afterI min, I h and l0 h the aircraft may be up to 0'087,5.24 and 52'4 nautical miles away from the indicatedposition. In both cases the absolute error increaseswith distance flown. In practice it is the headinginformation which usually limits the accuracy of thewslem.

Fig. 10.2 Doppler effect - transmission

Doppler Effect

ln 1842 the Austrian scientist Christian Dopplerpredicted the Doppler effect in connection withsound waves. It was subsequently found that theeffect is also applicable to e.m. waves. The Dppplereffect can be described as the change in observedfrequency when the source (transmitter) and observer(receiver) are in motion relative to one another. Thenoise of moving trains and road traffic is ademonstration of the effect commonly observed.The applicability to e.m. waves is illustrated by theuse of police radar speed traps, to the cost ofoffenders.

In an airbome Doppler radar we have a transmitterwhich, by means of a directional antenna, radiatesenergy towards the ground. A receiver receives theecho of the transmitted energy. Thus we have thesituation where both transmitter and receiver aremoving relative to the ground; consequently theoriginal frequency transmitted is changed twice. Thedifference between transmitted and receivedfrequencies is known as the Doppler shift and is verynearly proportional to the relative motion betweenthe aircraft and the ground along the direction of theradar beam.

Consider the transmission of e.m. energy towardsthe ground. Let the relative velocity of the aircraft inthe direction of the beam be r, the frequency of theradiation / and the speed of the electromagneticwaves c (= 3 X tO8 ms-l ). Referring to Fig. 10.2 wesee that in t seconds the wave will have moved adistance ct to b while the aircraft will have moved adistance vt to a. The waves emitted in time r will bebunched up in the distance between a and b which isct - vt. The number of waves emitted will be /rcycles. Thus the wavelength observed at the target,).', is given by:

t r ' - (cr -v} l f t=(c-v) l f ( 1 0 . 1 )

We can see that if the transmitter is stationary withrespect to the ground then y = 0 and equation (10.1)reduces to the familiar relationship s =/tr. If thetransmitter is moving away from the ground target,that is the beam is directed towards the rear, thenr in equation (10.1) becomes -y and we have\ ' = ( c + v ) l f .

Now consider the received sigtal. In a time / theaircraft would receive all the waves occupying spacecf in Fig. 10.3. However in this time the aircraftmoves a distance r/ and hence will receive the numberof waves occupying ct + vt in f seconds or (c + r)/trwaves in I second. Thus the received frequency, fr,is given by:

f r= (c+v) l \ . (10 .2)

Again we see y = 0 leads to c = )\,f and the aircraftmoving away from the ground target givesf r = ( c - y ) / I .

We must now consider both effects simultaneously.The wavelength tr in equation (10.2) is thewavelength of the echo which must be ).' in equation(10.1). .Thus substituting ). ' for X we have:

^ k + v \ ^Ir = (-fi I.

We are interested in the Doppler shift, /2r, which isthe difference between transmitted and receivedsigrals thus:

- _ ( ( c + v ) . ) 2 vf o = f , - f = f { - _ l } = = . f .- ' ( ( c - v l

t c - v "

Now c = 186 000 miles per second so I is obviouslyvery small compared with c, so with negligible errorwe may write:

fo = 2vf lc. ( 10 .3)

This equation is the basis of a Doppler radar.Observing, as above, the convention that y is positivefor movement towards, and negative for movementaway, the ground target gives an increased receivedfrequency on a forward beam and a decreasedreceived frequency on a rearward or aft beam.

From Figs 10.4 and 10.5 we see that the relativevelocity of the aircraft in the direction of the beamcentroid is v = V cos I cos c where Iz is the masnitude.of the aircraft velocity with respect to the grou-nd.

'

So equation (10.3) becomes:'

fp = (2Vf cos 0 cos c)/C. (10.4)

It is at this stage that the student is often convincedthat a Doppler radar could not possibly work due tothe smooth earth paradox and the mountaln paradox,which are hopefully dispensed with below. i

It is falsely argued that if an aircraft is movingparallel to flat ground then there is no change in rangebetween the aircraft and the ground and therefore noDoppler shift. That this is false can be seen by

l. ct ,ll l r lr r l l

t lAircraft Ground

Fig. 10.3 Doppler cffect - reception

173

Fig. 10.4 Airborne Doppler single-beam geometry in thcvertical Dlane

Fig. 10,5 Airborne Doppler single-beam geometry in thehorizontal plane

considering the actualtargets which producebackscattering of the energy. These targets areirregularly shaped scattering objects such as pebblesand there is. of course. relative motion between theaircraft and individual targets - hence a Dopplershift. If the illuminated area were perfectly smoothno reflected energy would be received at the aircraft.

The othdr false argument concerns sloping terrain.If the aircraft is flying horizontally above a slope thenits range to the ground along the beam is changingand therefore the Doppler shift will be affected.Again this falsehood is exposed by the fact that theactual targets are individual objects whose 'slope' withrespect to the aircraft is random and hence is notrelated to the slope of the ground.

Antenna Mechanization

The aircraft velocity has three orthogonal@mponents:

Vfi the heading velocity component;V) the lateral velocity component; and

171

Vi the vertical velocity component

as shown in Fig. 10.6. These velocities are in antennaco-ordinates which, with an antenna fixed rigidly tothe aircraft, are airframe co-ordinates. As the aircraftpitches and rolls the antenna moves with it and henceVil, Vn' and Vf will not be the velocities in earthco-ordinates required for navigation.

Fig. 10.6 The resolved velocity vectot

Fig. 10.7 Drift angle and ground speed

One conceptually simple solution is to stabilizethe antenna in pitch and roll, in which case the earthco-ordinate-related velocities Vg, V1 and VV areequal to Vi, Va' and Vpi respectively. If the lateralvelocity V4 is non-zero it means that the movementof the aircraft is not in the direction of the headingand a non-zero drift angle, 6, exists as in Fig. 10.7.The resultant of Vg and Va is the velocity vectorwith magnitude equal to ground speed s, anddirection that of the aircraft's track. It is convenientfor navigation purposes to present the pilot withground speed and drift angle inforrnation rather thanVs urd V1.

With moving antenna systems, the antenna isstabilized in pitch and roll and also aligned inazimuth with the track of the aircraft, that is to saytrack-stabilized. The drift angle is given by the anglebetween the antenna and aircraft longitudinal axesmeasured in the horizontal plane. Some Dopplers usepitch but not roll stabilization since error due to rollis small for small drift angles and furthermore tendsto average out over the flight.

vir

vi

Fixed antenna systems must compute thevelocities Vry,Vn and Vv each being a function ofyi, V;, Vf , R andp, where R and F are roll andpitch angles appearing in the expressions astrigonometric functions. The relationships are derivedin the Appendix.

Doppler Spectrum

The beams are of finite width, hence energy willstrike the ground along directions of different relativevelocities. As a consequence a spectrum of Dopplershift frequencies is received as shown in Fis. l0.gwhere the effects of side lobes are isnored.

%power-3 dbs

fd Doppler shift

Fig. 10.8 The Doppler spectrum

Such a phenomenon is undesirable but providingthe spectrum is reasonably'peaky'the mean Dopplershift is easily measured. If Z is the angle between thebeam centroid and the aircraft velocity vector thenequat ion ( 10.3) becomes:

Wilh A? typically 0.07 radians (4") and I typically70- we have Afp lfp rypically 0.2.

Since backscattering from the illuminated targetarea is not constant over the whole area there is arandom fluctuation of the instantaneous meanfrequencyfp. To determine the aircraft 's velocrtyaccurately the time constant of thevelocity-measuring circuits must be sufficientlv loneto smooth this fluctuation, but not so long asio be-unable to follow the normal accelerations-of theaircraft.

Beam Geometry

Since there are three unknowns Vi, Va' and Vf , aminimum of three beams are required to measurethem. ln practice three or four beams are used in aconfiguration involving fore and aft beams; as such itis known as a Janus configuration after the Romangod who could see both behind and in front.

The beams radiated can be either pencil. as inFig. 10.10 or narrow in elevation (nO) Uut wide inazimuth (Ac) as in Fig. 10.9. The hyperbolic l inesfo, h , f*, f-u are lines of constant Doppler shiftscalled isodops and are drawn assuming a flat earth.When wide azimuth beams are used a fixed antennasystem would lead to the beam crossing a widerange of isodops under conditions of drift, resultinginan excessively wide Doppler spectrum. Consequentlysuch beams virtually dictate a track-stabil ized anrenna.A wide azimuth beam has advantages in that smallerantenna areas are required and roll performance isimproved in the case where no roll stabilization isemployed.

Figure 10.9 shows that for a fully stabilized systemthe Doppler shift on all four beams is the same.Without roll stabilization small errors are introducedwhich tend to average out. Stabil ization can beachieved by servo loops which drive the antenna so asto equalize the Doppler shifts. Alternatively pitchinformation (and possibly roll) can be fed to theDoppler from a vertical reference such as a gyro oreven a mercury switch leavlng azimuth stabilization tobe achieved by equalization of Doppler shifts.Typically in such radars ground speed and drift angleare the only outputs where ground speed is given byequation (10.4) and drift angle by the amount ofazimuth rotation of the antenna. Headineinformation is usually added to the drift ingle to giveaircraft track.

Figure 10.10 shows that for a fixed aerial systemthe Doppler shifts on all four beams are, in general,not equal. It is shown in the Appendix to this

- 2Vfl o = ; " o t r .

Differentiation with respect to 7 gives a firstapproximation to the half-power bandwidth of thespectrum:

Lfo = 2A gy. r inT (r0.6)

where A7 is the half-power beam width. The ratio ofLfp to fp is thus given by dividing equation ( 10.6)by ( l0 .s) .

( 10.s)

Lfp-f; = a 7 . t a n 7 . (10 .7)

175

Fig. t0.9 Moving aerial system - typical beam geometry

chapter that the velocities in airframe coordinates,Vri, VA' and Vf , depend on (f2 - f l),(f2 -/3) and(.f | + f3) respectively, where /l ,f2 and f3 are theDoppler shifts on beams I ,2 and 3. Relationships forthese velocities can also be derived using/4 and twoother Doppler shifts. With this redundancy we havethe possibility of self-checking or continued operationafter one beam failure.

Computation of ground speed and drift angle in afixed antenna system can be divided into three parts:firstly computation of Vi , V; and Vl, using theDoppler shifts from three of the four beams; secondlycomputation of Vg, V4 and Zy using pitch and rollinformation and the previously computed airframeco-ordinate velocities; and lastly, drift angle =

uctan (ValVn) and ground speed = (Vn'+ Vnzlo's(see Figure 10.7).

176

Transmitter Frequency

The choice of r.f. is, as ever, a compromise. Theadvantage of using a high frequency is that thesensitivity of the radar in Hertz per knot is high, ascan be seen from equation (10.3); furthermore, for agiven antenna size, the higher the frequency thenarrower the Doppler spectrum. If, however, theradiated frequency is too high atmospheric andprecipitation absorption and scattering become moreof a problem. Another consideration is theavailability of components for the various frequencybands which might be considered. Most Dopplerradars operate in a band centred on 8'8 GHz or13'325 GHz, the former, to date, being perhaps themost common for civil aircraft use.

Heading

Fig. 10.10 Fixed aerial system - typical beam geometry

Modulation

At first sight it would appear that no modulation isnecessary, indeed c.w. Doppler radars have been builtand operated, a great attraction being simplicity.Difficulties, however, arise in transmitter receiverisolation and discrimination against reflections fromnearby objects in particular the dielectric panel(radome) covering the airframe opening for theantenna. At other than low altitudes unwantedreflections are comparable in amplitude to groundreturns. Noise like variations in vibrating radomeechoes will more than likely be in the same frequencyband as the gxpected Doppler shifts, and thusindistinguishable except where the s.n.r. is sufficientlyhigh at low altitudes.

To overcome the above problems both pulsed and

frequency modulated (f.m.c.w.) radars have beenused. The earliest Dopplers were pulsed so thatechoes from nearby objects were received during therecovery time of the diplexer and hence were notprocessed. In the so-called incoherent pulse systems aDoppler signal is obtained by mixing received signalsfrom fore and aft beams;this has two undesirableconsequences. Firstly the returns on the fore and aftbeams must overlap in time if mixing is to take place,this means stabilization and/or wide beams must beused. Secondly, the Doppler shift on the individualbeams is not available, hence the sense of direction ofthe velocity vector (forward or backward) and thevertical velocity cannot be computed.

With modern radars f.m.c.w. is the most commontype of transmission. The spectrum of the transmittedsigrd consists of a large number of sidebands as well

177

as the carrier. Theoretical analysis of f.m.c.w. revealsan infinite number of sidebands spaced by themodulation frequency f,n amplitude of individualsidebands being determined by Bessel functions of thefirst kind of order n and argument rn where n is thesideband concerned (first, second, third, etc.) and rnis the modulation index, i.e. ratio of deviation tof.. By using the Doppler shift of a particularsideband, and choosing an appropriate value of m togive sufficient amplitude of the sideband concerned,suppression of noise due to returns from the radomeand other nearby objects is achieved.

A problem common to both pulsed and f.m.c.w.Doppler radars is that of altitude holes. In a pulsedsystem ifthe echo arrives back at the receiver whena subsequent pulse is being transmitted then it isgated out by the diplexer and no Doppler shift canbe detected. Similarly with f.m.c.w., if theround-trip travel time is nearly equal to themodulation period a dead beat wil l occur whenmixing transmitted and received signals, and againno Doppler shift wil l be detected.

If a low modulating frequency is use'd the firstaltitude hole may appear above the operatingceiling. However low p.r.f. in pulse system leads tolow efficiency and the possibility of interference ifthe p.r.f. is in the range of Doppler frequenciesexpected (eudio). For f.m.c.w. given a choice ofsideband used, typically third or fourth, andmodulation index, typically 2l or 3, such as toavoid radome noise, the modulating frequency mustbe fairly high to allow a reasonable deviation. Afairly high modulating frequency is usually variedeither continuously (wobble) or in discrete steps toavoid altitude holes at fixed heishts.

Over-Water Errors

Doppler navigators measure the velocity of the

Fig lO.l I Frequency modulated continuous wavetransmitted signal and received ground echo spectrum

174

aircraft relative to the surface below them. Whenflying over water that surface itself may be movingdue to sea currents or wind-blown water particles.Random sea currents are of speeds usually a goodbit less than half a knot, and this small el'fectaverages out since the currents are in randomdirections. Major sea currents do not exceed, say,3 knots and since direction and approximate speedare known they can be compensated for.Wind-blown droplets would give an error less thanthe wind speed, about 3 knots error for l0 knotswind with the error varying as the third root of thewind. On long flights such an error will be reducedby averaging.

.

Frequency

Fig. 10.12 Over-water calibtatiory shift

When flying over land the beams il luminate anarea containing many scattering particles. Generallythe backscattering coefficients over the wholeilluminated area will be of the same order giving riseto the Doppler spectrum shown in Fig. 10.8. Oversmooth sea the situation is different; a largerfraction of the incident enerS/ will be returned onthe steepest part of the beam since the surfacebackscattering coefficient will depend on the angleof incidence. The net result is to shift the Dopplerspectrum as shown in Fig. 10.12, so that the meanDoppler shift is less than it should be for theaircraft velocity.

The error introduced, which could be up to 5 percent, is known as over-water calibration shift error.The narrower the beam widih the less significant theerror, so some Dopplers are designed to producebeams narrow enough to keep the error withinacceptable limits. Other Dopplers have a manualland-sea or sea bias switch which, when in the sea oron position respectively, causes a calibration shift inthe opposite sense by weighting the response .of theDoppler shift frequency processing in favour of thehigher frequencies. For a carefully chosencompensation shift the error can be reduced by af-actor of about ten.

oio-

k___ Overwater: ca l to ra lonI shr f t e r ro r

Antenna axis N(ml

Ground track

Lobcswitchedbeams

It '

Frequency

o]o

.L

Fig. 10.13 Lobe switching

Lobe switching is a more successful method ofreducing calibration shift error. The beam angle ofincidence to the ground is switched by a smallamount periodically. The illuminated areas for thetwo switched angles overlap, as do the Dopplerspectra. The Doppler shift frequency used forvelocity measurement is where the spectra cross.This crossover point corresponds to the retum fromthe same group of scatterers at the same angle ofincidence and is thus not affected by over-waterflight. Figure 10.13 illusrrates the technique, whichporks far better when track-stabilized antennas sincethen the lobe switching is at right angles to theisodops.

Navigation CalculationS :

The ground speed and drift angle information isnormally presented to the pilot but in addition isused, together with heading information, to give theaircraft position relative to a destination orforthcoming waypoint. To achieve this the pilotmust set desired track and distance to fly beforetake off. In Fig. l0. | 4 the pilot wishes to fly fromA to B, a distance of 50 nautical nri les, with desiredtrack 090. Thc aircraft has flown for 6 min at aspeed of 500 knots on a heading of 100 with a drifto[ 27" starboard, thus the total distance is 50nautical nri les and the aircraft is at point C. Thetrack error is 37-, t lre along distance to go isX = l0 nautical miles: the across distance isY = 30 nautical nri les.

In order to see how the Doppler navigator arrivesat the along and across distances indicated to the pilotwe must consider the information available:' Ground speed (s) and

dril't angle (6) - Doppler radarHeading (I/) - gyromagnetic compassDesired track (Til)andDistance (D) - pilot

Fig. 10.14 Navigation calculations

To arrive at along distance to go (X) and acrossdistance (y) the true track (?') and track error angle(E) are needed. We have, assuming drift to starboardas positive:

T = H + 6

E = T - T d

x = D -J jScos f , 'd r

Y = t s s i n E d t

(r0.8)(10.e)

(10 .10)

/ ( 1 0 . 1 l )

where I is the time of flight, and the sense of theacross distance is positive to the right.

Block Diagram Operation

Moving Antenna SystemFigure 10.15 illustrates a block diagram based on theMarconi AD 560, a system introduced in themid-I960s and used on a'variety of civil aircraft. It isstill to be found in service.

The sensor is an f.m.c.w. type employingwobbulation of the modulating frequency f^ toavoid altitude holes and using the Nth sideband(tr/ = 3 in the AD 560) to avoid unwanted interferencedue to radome vibrations. For the choice.of the thirdsideband a suitable modulation index is 2.5. obtainedby using a deviation of t I MHz on the 8800 MHzcarrier and a modulating frequency of 400 kHz.

Two mixer stages give the Doppler shift frequency

179

I

Transmitter/receivcr Aerial unit

p-- Heading

F----+ Drifi

Set tiack

Autopilot

Steeringi n d .

Groundspeed

SetzeroSetdistance

Trackcr + =T-

i Computer I Display unitMemory To diStance ! :

Fig. 10.15 Moving aerial Doppler block diagram based onthe Marconi AD 550

fo. The first mixes the received signal with a sampleof the transmitted signal, the required sideband(1200 kHz in the AD 560) being selected by theintermediate frequency amplifiers. The second mixes.fy' times f^ with the selected sideband to extract fi.;by means of a low pass filter with a cut-off frequency

' ofabout 20kHz.There are two transmit and two receive linear

slotted arrays. Anti-phase and in-phase arrays areused for transmit and receive, an arrangement whichcan be shown to compensate for changes inwavelength. The arrays are connected to appropriateinlets/outlets by an r.f. switch (varacter diodes in theAD 560). Fore and aft beams are obtained byproviding for connection to either end of each array,while port and starboard deflection is achieved by useof side reflectors. The beam-switching sequence is

180

important where pitch and azimuth drive of the aerialis concerned; the AD 560 sequence is port forward,starboard aft, starboard forward, port aft, a completecycle taking I second.

The Doppler spectrum is fed to two loops: onecoarse and one fine. The search loop provides forcoarse adjustmenl of a ground-speed measuring shaftwhich determines the frequency of a voltage controlledoscillator (v.c.o.) in the tracking loop through afeedback potentiometer. With the search loop nulledthe v.c.o. frequency is approximately equal to themean Doppler shift frequency, a discriminator withinthe tracking loop is then able to apply an error signalto the speed drive so as to position the ground-speedshaft accurately. The Doppler can then be said to belocked on, and any change in ground speed will betracked by the fine tracking loop.

1 second

Fig 10.16 Change in Doppler shift with aerial misalignment

If the antenna axis is not aligred with the track ofthe aircraft, either in pitch or in azimuth, the Dopplerstrift will change with beam-switching. Withmisalignment in azimuth the change in Doppler occursat a rate of I Hz, while if the misalignment is in pitchthe rate is 2Hz. This follows from the beam-switchingsequence and rate (see Fig. 10.16). Referencewaveforms of 2 and I Hz are fed to the pitch andazimuth drive circuits respectively; any misalignmentof the antenna will be detected by the drive circuits,resulting in the antenna rotating so as to align itselfwith the aircraft track. At this stage the Doppler shiftis the same on all four beams and accuratelyrepresents ground speed, also the angle between the

aircraft and antenna longitudinal axes is equal to the

drift angle.The s.n.r. is measured in both the search and

tracking loops. If the search loop check is notsatisfactory for three out of the four beams the three

servodrive circuits are disabled. The sigrral to noisecheck in the tracking loop causes the system to.go to

memory if it is not satisfactory. In memory theantenna and ground-speed shaft are fixed and a

memory flag appears in a ground'speed and drift

angle indicator. The sensor continues to give the

last-measured ground'speed and drift angle for aslong as the poor s.n.r. continues.

Ground-speed output from the sensor is in twoforms. - A synchro rotor is mechanically coupled to

the ground-speed shaft giving a three'wire feed' The

tracking v.c.o. frequency, which is proportional toground speed, is fed to a sine'cosine resolver in the

computer and also to a frequency divider which scales

and shapes the sigral so that one pulse per nauticalmile is fed to a distance flown indicator (integrating

counter).A synchro transmitter in the antenna unit gives a

drift angle output since the body is bolted to thefixed part ofthe antenna and the rotor is driven bythe azimuth motor. A differential synchrotransmitter is used to add heading (from compass) to

drift angle (azimuth drive) and so give track toanother differential synchro in the display unit. Therotor of the second differential synchro is set by theoperator at the desired track angle, hence the outputis the difference between track and desired track,i.e. track angle error 6.

The resolver in the computer unit resolves groundspeed S into its along and across speed componentsS cos.E and S sin ̂ E respectively. In the AD 560 a ballresolver is used, thus giving mechanical analoguecomputing. The ball is driven by a trackingoscillator-fed stepper motor, hence the rate ofrotation is proportional to ground speed. The axis ofrotation depends on the angle of the drive wheelwhich is set by a servo position control system to beequal to the track error angle. Two pick-offwheelsmounted with their axes at right angles rotate at arate depending on the along and across speeds. Theserotations are repeated in the display unit by means ofservo drives and cause the counlers to rotate. Thealong distance counter is arranged to count down fromthe initial distance to the waypoint until it reacheszero when the aircraft will be on a line perpendicularto the desired track and passing through the waypoint.If both along and across distances read zerosimultaneously the aircraft is over the waypoint.

Fixed Antenna SystemFigure 10.17 may be used to explain the principles ofa fixed antenna system to block diagram level. Theantenna consists of planar arrays of slotted waveguideor printed circuit, separate arrays being used fortransmission and reception. Beam'switching isachieved using varactor diode or ferrite switches tocouple the transmitter and receiver to theappropriate port.

The received signal is mixed with a sample of the

f.m.c.w. transmitted signal and the wanted sidebandfiltered out and amplified. If only ground speed and

drift angle are required further mixing may take place

to extract the Doppler frequencies as in the movingantenna case; however the sense of the shift (positive

or negative) is lost. If the three velocity vectors in the

direction of the aircraft cq'ordinates are required, an

intermediate frequency,fe is retained which will be

reduced or increased by an amount depending on

which beam is being radiated.The time-multiplexed Doppler shifted ,fq siSnals

are separated by a demultiplexer driven by the

beam-switching control and feeding four tracking

loops. Voltage-controlled oscillators become locked

to the incoming frequencies fO ! fo by sweepingthrough their range until they lock on. The four

tracker outputs are then summed and differenced, as

SAPf

Sterboarddrift

Nose up

Timo

181

Fig 10.17 Fixed aerial Doppler block diagram

appropriate, in a combining network to providesignals /r, fy and /2 proportional to aircraftco-ordinate velocities (see Appendix, Al 0.3).

Within the computer/display unit (CDU) theaircraft referenced velocity components aretransformed into track-orientated earth-referencedcomponents using attitude signals (pitch and roll)from a vertical reference gyro (see Appendix, A10.4).With a heading input and waypoints, in terms ofdesired track and distance, set in by the pilot thecomputer can integrate the along and acrossvelocities to give distance to go and across-trackerror (distance) respectively.

The CDU may offer latitude and longitudercadout of position by transforming the aircraftvelocities into north-orientated horizontal components.True north, as opposed to magnetic north, may beused as the reference if the pilot is able to enter thevariation. With attitude, heading and true air speedinputs the output from an airspeed transformationcircuit or routine may be compared with the velocitytransformation to give wind speed and direction.

Installation

The Doppler navigator system illustrated inFig. 10. I 5 requires five units as indicated; i.e. antenna,transmitter-receiver, tracker, computer and displayunit. The Marconi AD 560 comprises the five unitsmentioned plus a junction box, ground-speed anddrift-angle indicator, distance-flown indicator and acontrol unit (or simply a panel-mounted switch).The weight of the AD 560 is about 30 kg whichshould be compared with Marconi's latest Dopplerthe AD 660 which weighs 5 kg (sensor only).

The AD 660 is a single-unit Doppler sensor giving

'182

Heading Attitude

Air speed Variat ioninit ial posit ionand waypoints

ground-speed and drift-angle outputs and, with acompass input, wil l provide navigation data to a CDUgiving a two-unit Doppler navigation system. Digitaloutputs are provided in accordance with ARINC 429(DITS) and ARINC 582, there is also an optionalsynchro output for drift angle. This unit, introducedn 1979, may herald a comeback for Doppler inairline service since it has been ordered by Boeing forinstallation in their 727s and,737s. to be used inconjunction with Lear Siegler's performance andnavigation computer system (PNCS).

oxo

=E

x

]z

-E

Fig. 10.18 AD 550 (courtesy Marconi Avionics Ltd)

: 1 I

it .{:,-:"*l:: .r i l , : . :tri li ,i f

Fig. 10.19 Doppler 7l antenna/electronics unit (courtesythe Decca Navigator Co. Ltd)

Figure 10.18 shows the AD 660 with cardsremoved. Note the use of large-sca1e integratedcircuits common in all modern systems as weapproach the 1980s. The antenna is a printed circuitmicrostrip producing four beams transmittedsequentially. The transmitter is a f.m. Gunn diodeoscillator generating over 200 mW at a carrierfrequency of 13.325 GHz. Computation and controlis achieved through a microprocessor.

The Decca Doppler 7l and 72 are desisned forv./s.t.o.l. (vertical/short take-off and landing) aircraftoperating below 300 knots and fixed-wins aircraftoperatigg up to 1000 knots respectively. theseEfstems have had civilian sales limited to aircraft withspecial needs such as certain helicopter operations andsurveying. Figures 10.19 -10 .22 show units of atypical Decca Doppler 7l installation. Interconnectionsare simple; all three indicators being driven directlyfrom the antenna/electronics unit. A heading input isrequired for the PBDI which, together with theantenna/electronics unit, forms a basic two-unitsystem, the two meters being optional. Anotheroptional unit is an automatic chart display driven bythe PBDI or a more sophisticated replacement, aTANS computer.

lq._19.?0 Doppter 7l position, bearing and drift indicaror(PBDI) (courtesy the Decca Navigator Co. I_tay

183

Fig. 10.21 Doppler 7l ground-speed and drift meter(courtesy the Decca Navigator Co. Ltd)

Fig. 10.22 Doppler 7l hovermeter (courtesy the DeccaNavigator Co. Ltd)

The Doppler 70 series systems are c.w., three-beam(not switched) K-band radars. Adequate decouplingbetween transmitter and receiver is inherent in thedesigr, so allowing the use of c.w. A local oscil latorsignal is used for mixing, so providing an intermediatefrequency t the Doppler shift. Further details of thesystem are included below.

184

Across-track error and track-angle error are usuallyavailable as outputs from a CDU fbr use by anautopilot. Obviously warning signals must also beprovided to indicate the integrity of the steeringsignals to the user equipment.

Controls and Operation

We shall consider the Doppler 71 as an example,although obviously considerable variations exist.

P.B.D. I .Indicator, controller and general purpose processorwi th programme capaci ty of 1500 16-bi t words.Battery-protected memory.

Switchesl. DOP TEST: ground checking of sensor;

ST BY: inputs inhibited, display tlashes;LAND/SEA: allows correction for overwater

calibration shift error to be switclted in.2. LMP TEST: check of display and lamps;

HDG/VAR: display of heading input and insertionof m agnetic variations;

FIX: position displayed is f ixed; Dopplerincremental distances are stored; warning lampflashes: slew switches operable:

POS: aircraft latitude or longitude displayed;GS/DFT: ground speed and drift angle displayed:BRG DIST: bearing and distance to next waypoint

d isp layed;WP: selected waypoint latitude or longitude.

3. WAYPOINT I to l0 : a l lowsforwaypointselection. Waypoints can be inserted or changedat any time.

4. LAT LONG: three-posttion latitude, longitude orboth (alternately) displayed.

5. SLEW: two switches used for inserting variation.present position, waypoint co-ordinates andresetting the numeric displays as required.

\Displays

l. Numeric: twb groups of three seven-segmentfilaments show data selected by seven-positionswitch.

2. Sector display: indicates latitude no*fh (N) orsouth (S), longitude east (E) or west (W).

3. Track error: analogue display showing track error,in degrees, to the selected waypoint.

4. Warning indicators: incorporated in analoguedisplay to give warning of Doppler failure(or memory), computer failure or test modeselected.

Ground-Speed and Drift MeterDisplay ofground speed up to 300 knots and driftangle + 30o (expanded scale).Power failure and memory warning flags.Manual setting of drift and ground speed provided for.

Hovcr MeterDisplays:

along-heading velocity - range -10 to +20 knots:across-heading velocity - range t l5 knots;vertical velocity - range t 500 ft min-r.

Characteristics

ARINC Characteristic 540, airborne Doppler radar,was issued in 1958 and last printed in January 1960;it is no longer maintained current. For this reasondetails of a currently available system, the DeccaDoppler 7l are l isted. Since this is primarily ahelicopter system a'very brief data summary for theMarconi AD 660 system aimed at the airliner markethas also been included.

Decca Doppler 7lPower: 100 mWFrequency: 13'325 and 13.314 GHz.Intermediate frequency: 10.7 MHz.Beam width: 5o in depression plane, I lo in broadsideplane.Depression angle'. 67".Modulation: none, c.w.Number of beams: three continuous.Along-heading velocity range to computer: -50 to+300 knots.Across-heading velocity range to computer: ! 100knots.Supply: I l5 V, 400 Hz, single-phase.Altitude range: 0-20 000 ft over land or over waterwhen surface wind ) 5 knots.Accuracy of sensor - less than O'3Vo or 0.25 knots(whichever is the greater (overland)).Acquisition time: within 20 s.Indicated accuracy, ground speed and drift meter:3'5 at 100 knots; 5 at 300 knots, drift t 0'5o.Indicated accuracy, hover meter: along and acrossvelocities I I knot, vertical velocity t 40 ft min-I.

Marconi AD 660Power: 200 mW.Frequency: 13'325 GHz.Modulation: f.m.c.w.Number of beams: four, sequential.Velocity range: l0-800 knots.

Drift angle range: i 39.9o.Altitude: 45 000 ft above sround level.Supp l y : 28 V d . c . , 2 A .

Testing

Modern Dopplers have a considerable amount ofbuilt- in test equipment with which to carry outchecks. Synthetic signals may be generated byswitching antennas at a much higher rate than normal,thus leading to the memory flag clearing and a givenreading possibly appearing on the ground-speed anddrift-angle indicator. This might be the effect ofpressing the test switch on the ground under memoryconditions. If airborne and in memory a similarcheck could be carried out, but if in the signalcondition, i.e. Doppler shift present and s.n.r.satisfactory, then a good and easy check is to operatethe slewing switches to offset ground-speed anddrift-angle readings; on release the readings shouldreturn to their original positions. This check willcause a small error in the computed position but this

Simulatedfl ight path

151

Sta$ 2

065

WPT 2

Stage 1

Start

Fig, 10.23 Test conditions for simulated flight to checkcomputer (see text)

End

185

can be eliminated by sleiving in the opposite directionby the same amount to produce a cancelling error.

To check the computer/display part of a Dopplernavigator a course must be simulated by settingcompass heading (d.g., directional gyro, selected),drift anglei ground speed and waypoint course anddistance using appropriate slewing controls. Havingset everything up the computer is switched on for atimed run, at the end of which the displayed readingsd-rould be as independently calculated. Usually awritten procedure will give the necessary figures forsuch a check but in any case they are reasonably easyto work out. As an example, starting with thefollowing:

Waypoint I track 335" distance 15 nauticdmiles

Waypoint 2 track 065" distance 15 nauticalmilqs

Heading OlO"Drift l0o starboardGround speed 600 knots

at the end of the simulatcd two-hg flight the acrossdistance should be zeto, the distance fTown 42nautical miles and the time taken 4 min l4'5 s, all towithin the tolerance laid down for the system.

Ramp test sets have been produced for Dopplersystems, usually purpose-built by the manufacturer ofthe radar and not general-purpose as are, for example,VOR, DME, ILS, etc. test sets. Sometimes one wil lfind meters with associated switches which can beused to monitor various internal voltages andiorcurrents but this is more l ikely on older multi 'unitequipment.

It is important for accuracy to ensure the antennais aligned with the aircraft's longitudinal axis. TheDoppler will interpret any slight misalignment as a

drift-angle error. lnit ial alignment of all antennas is

important but with a fixed antenna system once the

hole is cut in the airframe, correctly aligned, the onlycause for concern afterwards is that the antenna isfitted the ccrrect way round. With moving-anteniasystems an alignment procedure for the antenna

mounting is carried out initially by using sighting rods

on the mounting and the aircraft ' Viewing the rods

from a distance to ensure they are in line, and then

tiglrtening the securing bolts through the slottedholes in the mounting plate, wil l ensure that the

antenna can be subsequently changed without a need.

for an alignment check.- although one should be

carried out on rnajor inspections.

Appendix

Relationships Betrrveen Aircraft and EarthCo-Ordinates

As in F ig. Al0. l le t i ' , i ' ,k 'be or thogonal uni tvectors defining a right-handed co-ordinate systemwith the positive direction of the axis spanned by i'being forward along the aircraft's longitudinal axis

and ihe positive direction of the axis spanned by i'being starboard along the aircraft's lateral axis.

FE. At0.l Aircraft co-ordinates

As in Fig. A 10.2 let i, i , k be orthogonal unitvectors defining a right-handed co-ordinate systemwith the positive direction of the axis spanned by fbeing forward along the aircraft 's longitudinal axisprojected on to a plane parallel to the ground and thepositive direction of the axis spanned by 7 beingstarboard along the aircraft's lateral axis projected onto a plane parallel to the ground.

k

Fig. A10.2 Earth co-ordinates

Further le! positive pitch be nose'up and positive

roll be starboard wing-down, then from Figs. A10.3and A10.4 we have:

i ' = i c o s P - k s i n Pk ' = i s i n P + k c o s P

i ' = T c o s R + / r s i n Rk ' = - i s i n R + k c o s R

r86

k '

F8. A10.4 Aircraft roll

Thus the matrix of transition from l, i, k to i', i',/c' is given by:

w=Mxv,,\

f cos f sin P sin R sin P cos Rl= 1 0 c o s R - s i n R l = U

l-sin P cos P sin R cos P cos RJ

If the aircraft velocity vector l/ has co-ordinates l!1,Vt, V v with respect to i, i, k and Vfi , Va', V f withrespect lo i ' , i ' ,k 'we have:

since Z is the vector sum of Vd,VA' and l/y'. Thus:

v n = h v i + a v { + v v lwhere ft, a and v are the magnitudes of the projeitionofz on to each axis of the co'ordinate system

i . e . u = h i + a i + v k

From Figs A10.5, A10.6 and A10.7 we see that for:

b e a m I h = - H a = Ab e a m 2 h = H a = Ab e a m 3 h = H a = - A

b e a m 4 h = - H a = - A

where I/ = cos 0 cos a;.4 = cos 0 sin a; Z = sin d.

Fig. A10.5 Velocities in longitudinal/lateral plane

V(/

Fig. A10.6 Velocities in plane normal to longitudinal/lateralplane

Fig. A10.7 Aircraft velocity vector

Now the Doppler shifts are given by/2 =2Vnf lc'therefore:

fi = 2f (-H Vl +,af r = 2 f ( H V i l + Af s = 2 f ( H V r i - Afc = 2f(-H vi 'l

v = Vv = Vv = Vv = V

F:: :-il '[i lrr hn]vi{

(A l0 . l )

i.e. Vy = Vrt cos P + Vj sin Psin R + Vi sin PcosRV.e = Va' cos R - Vl sin RVv = -Vi sin P + Va' cosP sin R + Izf cos P cos R

The Doppler Shifts for a Four-Beam JanusConfiguration

The magnitude of the relative velocity vector 24, inthe direction of any beam is the inner product of the.aircraft velocity vector V and the unit vector alongthe beam centroid, a. Thus:

V R = V ' u= ( V i + V 7 1 ' + V v ) . u

Va' + Y Vl)lcVx' + V Vf)lcvA' + v vi)lcv; + v vi)lc

(A r 0.2)

187

The Aircraft Velocity in Earth Co-OrdinatesExpressed in Terms of Doppler Shifts

- From equations (A10.2) we have:

v1i = c(fz;fr\ =+*-fr

v; = c(20p. =

vl = c( f i+f t ) -

W=MxWir;:itilwhere Kp =

47cof .", "

(Ar0.4)

c(h - fe)-rrrc(fz + fq)__4TT

(A10.3)ci,4 =

AEos;1il-;a

! __:_" Y 4f sin 0 are known constants.

rl4 is obtained from pitch and roll signals and f1,f2,fgare the Doppler shifts measured on beams l, 2 and 3.Note beam 4 is redundant but could be used forchecking purposes.

Substituting for Vfi, Vt' , Yi from equations (A10.3)into equations (Al0.l) we have:

188

11 Radio al t imeter

Introduction

The meaning of the terms aircraft altitude or height iscomplicated by the various references used fromwhich the height can be measured. A barometricaltimeter senses the static pressure at aircraft level andgives a reading dependent on the difference between{his pressure and the pressure at some reference level.For aircraft flying above about 3000 ft, the referenceof paramount importance is that level correspondingto a pressure of 1013.25 mbar (29-92 in.Hg), theso-called mean sea level. The other barometricreferences used are local sea level and airfield level.The pilot is able to set the reference level pressure atl0l3'25 mbar, QNH (local sea level - regional) or QFE(airfield level), the Q codes being used incommunication with air traffic control (ATC).

Conversely the radio altimeter measures the heightof the aircraft above the ground. lf an aircraft is inlevel flight the barometric altimeter reading will besteady while the radio alt imeter reading wil l bevarying unless the aircraft is ffying over sea or plain.It follows that radio altimeters are most useful whenclose to the ground, say below 2000 ft, andparticularly so when landing providing the finalapproach is over a flat surface. As a consequence,radio altimeters designed for use in civil aircraft arelow-level systems, typical maximum ranges availablebeing 5000, 2500 or even 500 ft in the case of use inautomatic landing systems. Military aircraft canutilize high-level radio altimeters.

Basic Principles

necessary in order to 'mark' t l ie t ime of transmission.both f.m. and pulsed transmissions are used. Themethod of t ime measurement depends on the type ofmodulation used and the complexity of the airborneequipment which is acceptable. Three basic types ofalt imeter are marketed: pulse, conventional f.m.c.w.(frequency modulated continuous wave) and constantdiflerence frequency f.m.c.w.

The bgsic principle of a pulsed system is simple,since the transmitted and received pulses clearlyrepresent events between which the time can bemeasured. With f.m.c.w. there is no single eventduring one cycle of the modulating frequency; howeverspecific t imes during one-half cycle can be identif iedby the instantaneous frequency being transmitted.Since the transmitter frequency is continuouslychanging, the received signal, which has been subjectto delay due to the round-trip travel t ime, wil l bedifferent in frequency to the transmitted signal atany instant in time. The difference frequency, fi,can be shown to be proportional to the height asfollows.

Assume a triangular modulating waveform offrequency fm and amplitude such that the carrier,'/",is modulated over a range A/. This situation isil lustrated in Fig. I I . l . Th€ two-way travel t ime is2Hlc where 11 is the height and c the speed of l ight.The magnitude of the rate of change of transmittedfrequency is 2 . A/. f^ (= 0.5 Lf lO10.25 fi l . Theproduct of the elapsed time and the rate of change offrequency wil l give the diflerence in frequencybetween transmitted and received frequency thus:

f n = 2 . L f . f ^ . 7 ,= 4 . A f . f ^ . H l c ( r l . r )

Radio height is measured using the basic idea of radio Tltus the measurement of the beat l iequencvranging, i.e. measuring the elapsed time between determines the height since 4 .Lf . f^jc is a knowntransmission of an e.m. wave and its reception after constant frrr any particular system.reflection from the ground. The height is given by The beat frequency is constant, for triangularhAlf the product of the elapsed time and the speed of modulation, exccpt at thc turn-around region twicel i gh t : &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&=492t ( rbe ing thee lapsedt ime in percyc le . S ince turn-aroundtakesp lace in? .gswe

microseconds). have that the average beat frequency for 2Zps isEnergy is radiated at a frequency h the band fnl2 l 'or the rest of the moclulating cycle;the beat

42OO'44O0 MHz. Modulation of the carrier is frequency is constant at fi . If the average beat

189

Transmittcd

Fig. I l.l A FMCW radio altimeter - frequency/timerelationship

frequency over one modulating cycle is detected wewill have a measured frequencyfi given by:

fn = (((tlf; -2r)fn + rfnl2)fm'= fm (r - 3Tfmlz) (l1.2)

With the aircraft at about 2000 ft and f^ = 2OOHzwe have:

fn = fn ( l _ 0.0012)

i.e. an error of about 2.4 ft at 2000 ft. On the groundthere wil l sti l l be an elapsed time, due to built- indelay, ofabout 0.12 ps corresponding to a residualalt itude (see later) of 57 ft, say. When we useIll l l l= 0'12 ps and f^ = 2OO Hzin equation (l 1.2) wefind that the error on the ground due to the averagingof the non-constant beat frequency is about 0.002 ftwhich is insignificant.

In practice a perfect triangular modulatingfrequency is diff icult to achieve, some rounding atthe turn-around taking place. In fact with anyreasonably shape-modulation waveform, it can beshown that the average beat frequency when used inequation ( I I . l ) wil l yield a height reading which iscorrect wi th in acceptable l imi ts . Appendix ILIproves this for sinusoidal frequency modulation,whichis easier to obtain since the rate of change of carrierfrequency is bounded in magnitude by unity.

lf there is relative motion between the aircraft andthe ground imrnediately below the received frequencywill experience a Doppler shift,/a (see Chapter l0).

190

Thfi will result in'a vertical dhplacetnent of thc graphof the received sigral in Fig. I l.l. The effect for adescending aircraft (positive shift) is shown inFig. I1.2. As can be seen the beat frequency is

fn - fa and fi, + f6 for equal periods. If we take theaverage over half a ntodulation period we get

Un + fa + fn - fil2 = fh as required. This assumesfn ) fa which will be the case with a radio altimeter.

Fig. I 1.2 The effect of Doppler shift on beat frequency

tlsing equation (l l . l) we see that for a system withA.f = 100 MHz (42504350 MHz) and f ̂ = 200 Hzthe beat frequencyfi = (4 X 100 X 106 X 2OOH)/(984 X 106) = 8011so the range ol fp lor a0-2500 ft alt imeter with a residual alt itude of 57 ftwould be 4'56-?.04'56 kHz. This wide bandrvidth canbe reduced by' maintaining a constant differencefrequency thus improving sensitivity. Since fidepends on both Lf and f * we can vary either ininverse proportion to the heig.ht so keeping ficonstant.

The necessary servo loop nrust monitor thediff 'erence frequency and control the modulator so asto change Ll'or frt accortl ingly. The control signalrepresents the height of the aircraft, Since thecontrolled variable is made inversely proportional toheight it becomes diff icult to employ the techniqueat lower heights. particularly when A/ is varied. Thislatter problenr can be overcorne b1' the contpromise ofoperat ing convent ional ly at low al t i tudes and rv i th

o

oCotcto

Eo.9!oG

C'

o

o

oQ'

6

Received

constant j[ otherwise, or by modifying feedback inthe loop such that fp varies with any change inaltitude but not over such a great range as it wouldwith no feedback. The French lnanufacturer TRTproduce altimeters with constant f i at all heights,control being achievedby varytngf^; the tolerance+ I ft or I per cent is met.

Factors Affecting Performance

Ihe accuracy of a radio alt imeter dependsfundamentally on the precision with which the timeof transmission is marked. For precise timing a widetransmitted spectrum is required since this would leadto a steep leading edge in a pulsed system or, in thecase of f.m.c.w., a large frequency deviation whicheffectively gives an expanded scale. Ofcourse a finitespectrum is transmitted and in fact is l imited to a totalspread of between 4200 and 4400 MHz byinternational legislation (the band 1600-1660 MHzhas also been allocated for use by radio alt imeters butis no longer used).

In an f.m.c.w. system a counter may be used whichmeasures the number of cycles or half-cycles in oneperiod of the modulation. Since the counter is unableto measure fractions of a cycle, other than possibly ahalf, the count is a discrete number. The processresults in a quantization error called step or f ixederror. We may rewrite equation ( I I . l ) as:

phase of the modulation frequency will also have thedesired effect providing the wobbulation rate, sayl0 Hz, can be fi l tered out.

lf a frequency discriminator is used to measurefrequency the step error is not present sincemeasurement is continuous rather than discrete. Withconventional f.m.c,w. alt imeters. however. the ranseof frequencies to be measured is large anddiscrirninator circuits sufficiently stable and linear arediff icult to achieve. With a constant differencefrequency f.m.c.w. alt imeter a discriminator caneasily be used to detect the small changes whichoccur in l7r.

The received signal strength varies with the heightofthe aircraft. From the radar range equation thisvariation is as the fourth power of the range; howeverwith an altimeter the greater the range (height) thegreater the area of the target (ground) is i l luminated.For radio alt imeters therefore. the variation is as thesquare of the range. The gain frequency characteristicof the difference frequency amplif ier (f.m.c.w.)should be such that the higher frequencies areamplif ied more than the lower. Such a characteristichelps by reducing the dynamic range of thefrequency-measuring circuit and also reduces lowfrequency noise.

Part of the two-way travel t ime is accounted for bythe aircraft installation delay (AID). Since, ideally,the radio altimeter should read zero feet clntouchdown, the residual alt itude which accounts forthe AID is made up of cable length, multiplied by afactor (typically l '5) to allow for propagation speed,and the sum of the antenna heights. It is importantthat the system is calibrated so as to allow for AlD.One method which has been r.rsed for an altimeteruti l ized for blind landing is to measure the differencefrequency on touchdown on a number of f l ight trialsof a particular type ofaircraft. The frequency arrivedat empirically can be injected into the differencefrequency amplif ier on the bench and the set can thenbe adjusted to give a zero feet output. A molecommon method is to calibrate by cutting theantenna feeders to an appropriate length; this wil l bedescribed under the heading ' lnstallation'.

Transmitter-receiver leakage can be viewed as noisewhich l imits the receiver sensitivity and also maycause an erroneous reading. Use of separate transmitand receive antennas wil l give space attenuation, afigure in the regicn of about 75 dB should be aimedfor in an insta l la t ion where, the a l t imeter output isused in crit lcal systems such as automatic Ianding andground proximi ty warning (g.p.w.s. ) .

Signals occurring at near zero range are caused byreflections frorn landing gear and other appendages,

( l r . 3 )

where ,rV = fnlfn is the number of beat frequencycycles (to nearest integer below) in the modulationcycle. Since ly' is an integer the height measured willbe subject to a quantization error called step errorequal to cl4Al'which with A/= 100 MHz is2.46 ft.

In practice in any one rnodulation period theactual count wil l depend on the nurnber ofpositive-going zero crossings of the beat frequency,this could be N or N + I depending on phasing. So ifthe phasing varies thc count wil l jurnp between y'y' andIy' + | and back so avertging out the step errorprovid ing the t i r r rc const ln t < l f the indicat ing c i rcu i t islarge cornpure ' t l wi th t l t0 t i r r re between countf luctual ions. l t c ln be showtr lhat the count wi l lchange lirr a charrgc- in lrcight ol 'u quarter of awavelcngth o l t l rc r . l ' . At t l re l ' rcquency used forradio a l l i r r rc tcrs i . t ( lu i t r tcr o l 'a wavelcngt l r is less thanI in . lnd l rcncc r r r r lcss l ly i r rg ove ' r vcry sntoothsur laces thc l lLrct t r l t i r rg r ld io l rc ight wi l l causeaveraging orr l o l ' thc error . l )e . l iberate ly wobbl ing the

H = & , = #

191

(clTx(al

r.caraf \ Multipatht

\\

Fig, I1.3 Some factors affecting performance

as well as the leakage signal referred to above. Inboth constant-difference frequency and pulsealtimeters a tracking loop is employed to follow thechanges in altitude. Initially, the altitude will beunknown so a search mode is entered which, whileseeking the correct altitude, will vary receiver gain.As previously discussed, at low altitudes the gain wil lbe low, thus while searcfuing in the frequency(f.m.c.w.) or t ime (pulsed) region of the unwantedsignals the large gain reduction ensures they are weak.The ground return is relatively strong ensuring lock-onto the correct signal and hence indication of theactual radio alt itude.

Multipath signals arise since the first-time-aroundecho will be reflected from the airframe back downto the ground and return as a second-time-aroundecho. While this multipath signal wil l be considerablyweaker than the required signal the height-controlledgain wil l in part null ify this favourable situation. Intracking altimeters the init ial or subsequent searchcan be in the direction of increasing altitude solocking on to the correct signal f irst, a similarapproach to outbound search in DME.

Aircraft pitch and roll will mean that the beamcentre is no longer vertical; however if the beam isfairly broad, at least part of the transmitted energywill take the shortest route to the ground. Providedreceiver sensitivity is adequate there wil l be sufl lcientenergy received from the nearest point for accuratemeasurement.

A consequence of broad beams is that in flyingover rough terrain, reflections wil l be received fromangles other than the vertical. Since the non-verticalpaths have a longer two-way travel t ime the spectrumof the difference frequency wil l be spread (grounddiffusion). The spectrum shape wil i be steep at thelow frequency end corresponding to the correcialtitude much the same as the pulse shape in a pulsedsystem will have a steep leading edge,(see Fig. I l.3band c). This spectrum widening is increased byaircraft roll and pitch. Leading edge tracking in

192

Time (pulsed).Freq. (f .m.c.w.)

pulsed systems and lowest frequency tracking(spectrum fi ltering) in f.m.c.w. systems is used toretain accuracy.

The altimeter can be designed with a response timein the order of a few mill iseconds; however one doesnot normally wish to follow the snrallest variations ofthe ground below. The output is usually f i l tered witha time constant of say a few tenths of a second.

Block Diagram Operation

As already mentioned there are three main approachesto radio alt imeters. Most alt imeters are of the f.m.c.w,type, the majority of these being conventional for thesake of simplicity. Although tracking f.m.c.w. andpulsed systems are more complex they do haveadvantages over conventional f.m.c.w. as wil l beappreciated from the previous paragraphs. Simplif iedblock diagrams for the three types wil l be considered.

Conventional f.m.c.w. AltimetersThe transmitter in a modern equipment comprises asolid-state oscillator frequency modulated at typically100-150 Hz rate. While most of the power(0'5-l W) is radiated from a broadly directionalantenna a small portion is fed to the mixer to beatwith the received signal.

The echo is mixed with the transmitter sample in astrip-l ine balanced mixer to produce the beatfrequency. Use o{ a balanced mixer helps in reductionof transmitter noise in the receiver. The gain of thewide band beat frequency amplif ier increases withfrequency to compensate for the low signal level ofthe high frequencies (high altitude). Signal l imitingremoves unwanted amplitude variations and gives asuitable signal form for the counter.

A cycle-counting frequency-measuring circuitprovides a d.c. signal to the indicator. Basicallysuitable switching circuits control the charging ol'acapacitor so that a fixed amount of charge is

Fig. I1.4 Conventional FMCW altimeter block diagram

Ft. | 1.5 Constant difference frequency FMCW altimeterblock diagram

pnerated for each cycle (or half-cycle) of thermknown beat frequenry. With the simplest type ofindicator the total charge per second (current) ishdicated on a milliammeter calibrated in feet.

Condant Differencc Frequency f.m.c.w. AltimetenThis approach is similar to a conventional f.m.c.w.dtimeter at the r.f. end. The beat frequencyanplifier is a narrow band with gain controlled by&e loop so that it increases with altitude. A trackingdiscriminator compares the beat frequency,/6, withainternal reference,fr;if the two are not the same,tr crror signal is fed to the loop control.

The outputs of the loop control circuit are used to-t the modulator frequency,f^ (or amplitude if&viation, Al is controlled), to set the gain of theanplifier and to drive the indicator. The change inf- (or A/) is such as to make/6 = /' . Obviously anydage in height will lead to a change in/6 and

consequent loop action to bring/6 back to therequired rate;in doing so the indicator feed willchange. lf f6 and f, are far removed, search action isinstigated whereby the modulator frequency (or A/)is made to sweep through its range from low to highuntil lock on is achieved.

hrlsed AltimetersA simplified block diagrarfi of a typical pulsedaltimeter is shown in Fig. I 1.6. Such altimeters aremanufactured by, among others, Honeywell; thefigures mentioned in the description of operationwhich follow are for the Honeywell 7500Be series.

A p.r.f. generator operating at 8 kHz keys thetransmitter which feeds the antenna with pulse of r.f.of 60 ns duration and frequency 4300 MHz. Theradiated peak power is about 100 W. A time referencesignal, /6, is fed from the transmitter to initiate aprecision ramp generator.

19:t

Tx

to

-JI||l--

Ramp

r l \1r6ck gate I Ipulse | |

I NVideo rcturn

Fig. I 1.6 Pulse altimeter block diagram

The ramp voltage is compared with the rangevoltage, /4, which is proportional to the indicatedheight. When the ramp voltage reaches Vp atrackgate pulse is generated and fed to gate B and anelongated gate pulse is fed to gate A. The detectedvideo pulse is also fed to gates A and B. A furthergate pulse is fed to the a.g.c. circuits.

Unless a reliable signal is detected within theelongated gate pulse the trackisearch circuit wil lsigral the commencement of a search cycle and breakthe track loop by removing its reference current feed..During search the search generator drive to the rangecircuit ensures that I/p, starting from a voltagerepresenting zero feet, runs out to a voltagerepresenting 2500 ft. The search cycle repeats unti l a

194

reliable signal (tive or six pulses) is received when thetrack loop becomes operational.

During track the overlap ofthe track gate pulseand video pulse determines the magnitude of a currentwhich is compared with a reference (offset) current.Where the two currents are equal, the output of therate circuit (integrator) is zero, otherwise a positive ornegative voltage is fed to the range circuit. The rangecircuit (integrator) adjusts its output voltage, Vp, ifits input is non-zero. Since I/p determines the timingof the track gate pulse any change in /p will causethe previously mentioned overlap to alter until suchtime as the loop is nulled, i.e. overlap current =reference current. Any change in height willtherefore result in a change in Zp to bring the loopback to the null condition.

Automatic gain control (..g.c.) and sensitivity t imecontrol (s.t.c.) are fed to the receiver where theycontrol the gain of the i.f. arnps. During search thea.g.c. circuits monitor the noise output of the receiverand adjust its gain so as to keep noise output constant.The s.t.c. reduces the gain of the receiver for a shorttime, equivalent to say 50 ft after transmission, andthen its controldecreases l inearly unti l a timeequivalent to say 200 ft. This action preventsircquisit ion of unwanted signals, such as leakage,during the search mode.

During track the a.g.c. maintains the video signal inthe a.g.c. gate at a constant level. This is important toensure precise tracking of the received signal since anyvariation in amplitude would cause the area of overlapto track gate and video signals to change. At lowheights on track the a.g.c. reduces the receiver gain,so helping to avoid the effects of leakage. When the

height increases the leakage signal is, of course, gatedout. giving time discrimination.

Monitoring and Self-Test

The integrity of the radio altimeter output is vital,particularly in automatic landing applications. Circuitredundancy and comparison is an effective way ofdealing with the problem. For example two separatedtitude-measuring circuits may accept a feed fromthe mixer and independently arrive at the aircraft'sheight. Should the two heights be different by morethan an acceptable amount, a warning signal is sent tothe indicator and any other systems to which theheight inlormation is fed. A disadvantage of usingredundancy and comparison only is that no attempt iimade to eliminate the cause of failure, so informationis lost.

Self-calibration is an approach which is able tocompensate for small errors. If a part of thetransmitter output is passed through a precision delayline the resulting signal can be used to provide a'check height'. For example in a conventional f.m.c.w.system the delayed transmitter sample may be mixedwith an undelayed transmitter sample to give a beatfrequency which may be compared with a suitablereference frequency. lf the two frequencies aredifferent the modulating signal is adjusted to bringthem in line; for example, if the beat frequency is toolow, equation ( I I . I ) tells us that increasing A/ byincreasing the modulating signal amplitude will, inturn, increase the beat frequency as required. Similarideas may be applied to constant-difference frequencyf.m.c.w. and pulsed altimeters where suitableparameters are adjusted as necessary.

A self-calibration loop such as described abovemay operate continuously using what is essentiallyredundant circuitry. However in equipment wherethe altitude is measured by means of a loop, such as aservoed slope (controlled f^) f.m.c.w. system. wemay have sequential self-calibration where themeasuring loop is switched, perhaps three times persecond. into self-calibrate mode.

Checking received signal quality is a feature ofmost altimeters. In a pulsed system the presence ofdetected received pulses in a gate pulse is feasible; in aconstant-difference frequency altimeter the presenceof a spectrum centred on the required beat is checked.With a conventional f.m.s.w. alt imeter one cannotcheck a particular part of the time or frequencydomain but signal plus noise to noise ratio may bemonitored.

When the aircraft is flying above the maximum

height reading of the radio altimeter it is likely thatthere will be a signal failure detected. Since this isdue to attenuation because ofexcessive range, andnot a failure or degradation of the altimeter, it isdesirable that no warning of failure is given and thatthe pointer on the indicator is parked out of view.A cruise-monitoring circuit may be incorporatedwhich, using a delayed and attenuated feed betweentransmitter and receiver, checks continuingsatisfactory operation in the absence of a detectablereceived signal. Any warning to the autopilot mustnot be affected by cruise monitoring; it should beactive whenever there is a loss of r.f. or when anyother failure is detected. Contrary to the above,many radio alt imeters react to a loss of signal byparking the pointer and displaying the flag.

The antenna and feeder, if not properly matchedto the transmitter, will give rise to a reflection whichmay cause problems since it wil l be delayed withrespect to the transmitted signal. A directionalcoupling circuit may be used to monitor the reflectedsignal and so give a warning of excessive v.s.w.r.

The monitoring and self-calibration circuits varygreatly in detail and in how comprehensive a check iscarried out. All, however, on detecting a failure wil lprovide a warning signal to operate a flag in theindicator and a similar, but usually separate, warningsignal to other systems, dependent on radio alt itudeinformation. In particular if a tie-in with autopilothas been established, the warning signal wil l controlan interlock circuit within the autopilot system whichprevents erroneous information dictating the fl ightpath. Some manufacturers provide latched indicatorlights on the front panel of the transmitter-receiverwhich give an indicatioq of the area of failure.

A self-test facil i ty is usually provided whereby adelay l ine, ideally between antennas, is switched in,thus giving a predetermined reading on the indicator.A disadvantage of such a facil i ty is that it introduceselectromechanical devices such as co-axial cable relayswhich are, of course, something else to go wrong. Itmay be argued that checking that the reading on theground before take-off is some specified Iigure nearzero is adequate, but nevertheless, some form ofin-fl ight test facil i ty is usually required.

On pushing the self-test button, providing theequipment is operating correctly. the failure warningoutput should be active so causing the warning t ' lag toappear on the indicator and, moie important.preventing the autopilot uti l izing radio alt imeterinformation. As an extra safeguard. an interlockshould be provided so as to prevent self-test once theautopilot or any other system has begun to make useof the radio alt imeter height output.

195

Indicator

As mentioned previously, a mill iammeter may be usedto indicate height but an altemative is a servo-drivenpointer. A decision height (DFI) facil i ty is alsoprovided. The pilot sets tl le DH bug to the requiredheight reading and in doing so determines the voltageV1 , fed to a comparator. The other comparator inputis a d.c. analogue altitude signal which if less than Iz4wi l l cause the DH lamp to l ight , so warning the p i lo tthat the aircrali is f lying below the DH setting. Ablock d iagram of an indicator is shown in F ig. I 1 .7.where isolation amplif iers have been omitted forsimplicitv.

used particularly laterally to avoid roll error in whichcase leadingedge tracking (pulsed) or spectium-fi ltering(servoed slope f.m.c.w.) must perform adequately.The antennas must be mounted sufficiently far apartto avoid excessive leakage but not so far apart as toproduce a large parallax error at touchdown, a spacingbetween 20 in. and 8 ft may be required.

lf the spacing between antennas, mountedlongitudinally, is 8 ft and the midpoint of the l inejoining the antennas is, say, 7 ft above the ground ontouchdown, then half the shortest distance betweenthe antennas via the ground wil l be (42 + 72)o's x g 11giving a parallax error of I ft on landing. This may betaken into account when calculatine the residual

t :l E

a-L-$oH set\ E

DH bugor index

Comparator

Dif ferent ia l amp

Fig. I 1.7 Simplified servodriven indicator

Installation

Figure I 1.8 i l lustrates a single radio alt imeterinstallation showing interface and selection links.Co-axial feeders pass r.f. energy to and from theseparate transmit and receive antennas by way ofswitches. When self-test is activated the transmittedenergy is fed to the delay unit where it is attenuatedand delayed before being fed back to the receiver.For a particular installation feeder length and delay isknown so the correct reading on self-test may becalculated and entered in the pilot check l ist andfunctional test procedure.

The antennas are broadly directional, f lush-mountedhorns often being employed giving a beam widthbetween about 20o and 40o. Broader beams may be Fig. I1.8

196

Supp ly

f a t oLrnKs l MOD

To othersystems:autopi lot,GPWS,fl ightdirector

Indicator

--T-Sel f lF la (test I

Transmitter receiver

Radio al t imeter insta l lat ion

Fig. I1.9 ALA-5 lA (courtesy Bendix Aviohics Division)

lrt Lamp\

Mast( _

DH Select-_''Buq

DH ind .

Alt displaytape

Flag

AlC ret.symbol

DH adjustand selftest button

DH ref.symbol

DH tao

./oH select/'Test Knob

FiE I l.l0 KI 250 indicator for use with KRA l0 radioaltimeter (courtesy King Radio Corp.)

alt itude. Good performance in respect of leadingedge tracking or spectrum filtering will make parallaxerror worse. A further consideration in antennapositioning is avoiding excessive reflections fromprotuberances such as the undercarriage; in thisrespect side lobe energy must be kept low (say 40 dBdown).

The transmitter receiver must be mounted within

Fig. ll.ll Typical moving vertical scale indicator

reach of the antennar since the feeder length is

crit ical. Since the transmiiter is relatively low power,

cooling is not a severe problem but forced-air cooling

able to cope with up to 50 W of dissipated energy

may be required. The power supply wil l be I l5 V a'c'

4OO Hz if the equipment is to ARINC specificationsbut 28 V d.c. equipments wil l be found.

Aircraft Installation DelaY-

Aircraft installation delay (AID) is the elapsed time

197

between transmit and receive when the aircraft is attouchdown, and is due to the delay in the feeders andthe height of the antennas above the ground. In orderthat the indicator will read zero feet on landins theinstallation must be calibrated. Various metho-ds havebien used, by far the most common being that laiddown in ARINC 552,{; AID is defined by the formula:

A I D = P + K ( C t + C r )

P------->

Fig. ll.l2 An AID calibration chart (& = 1.5)

198

interchangeabil ity of transmitter-receivers betweendifferent installations.

In practice on a new installation, having determinedsuitable positions for the antennas, a minimum cablelength for feasible t.r. location will be found.Equation ( I I .4) can now be used to decide the AIDand to calculate the cable lengths. ARINC 552A and,usually, manufacturers' installation manuals provide agraph from which cable length can be read oif. Ar un

.example considerP= l0 f t , min imum tota l cablel e n g t h = l 0 f t a n d K = 1 . 5 . W e h a v eP + K (Ct + Cr) = 25 ft so the 20 ft AID cannot beused. If we choose 40 then total cable leneth is(40 - l0)/l .5 = 20 ft, whereas with 57 *.luu.3l'3 ft. One should be careful, when using a graph,to ensure that it corresponds to the type of cablebeing r"rsed (RG - 9/U in ARINC 552A) and furthercheck the axes which may be total cable length oreach cable and total path or antenna height (eachcable ys. antenna height in ARINC 552A).

Interface

Were it not for the use which is made of radio alt itudeinformation by other systems, it is doubtful whethermany civil aircraft would carry radio alt imeters. Theoutputs available are heigirt, rate of change of height,trip signals and validity (l1ag or warning) signal. Someof these wil l be fed to the autoland/autopilot system,the g.p.w.s. and a fl ight director.

Most frequently used are a d.c. analogue of aircraftheight wherever a system needs to continuouslymonitor radio height and, essential, a switched,lail-safe validity signal (invalid low). The rate signal,i.e. rate of change of height, may be derived in thesystem uti l izing the height signal, but if provided wil ltake the form of a phase-reversing a.c. analogue sigral(ARINC 552A). The trips are switchable d.c. voltages,switching taking place when the aircraft transitsthrough a pre-set height, the DH bug is sometimescalled a pilot set trlp. Again trip signals may begenerated in those systems using the height analoguesigrral.

Autoland or blindlanding systems must have radioheight information which wil l be used to progressivelyreduce the gain of the glideslope signal amplifier (notradio) in the pitch channel after the aircraft passesover the outer marker, and wil l also be used togenerate trip signals within tlte autoflare computer.The following is a brief summary of events with radioheights:

140 ft (a) radio alt imeter interlock switched in;

( r 1 .4)where:P is total minimum path length between transmit

and receive antennas via the ground when theaircraft is in the touchdown position (minimumpath length is specified to avoid parallax error);

K is the ratio of the speed of light to the speed ofpropagation of the co-axial cable (typically 1.5);

C1 is the transmitter feeder length;C, is the receiver feeder length.

(AID is not in fact aircraft installation delaysince AID is an elapsed time whereas theright-hand side of ( I I .4) is in feet. A moreaccurate term would be residual alt itude.)

Calibration is achreved by cutting the cables to alength which gives an AID of 20, 40 or 57 ft (figuresof 40,57 and 80 fr are quoted in draft proposais fromARINC in 1978). The transmitter-receiver is benchcalibrated for the 57 ft AID standard. Grounding oneof three pins on the t.r. plug by means of a jumperexternal to the unit selects the appropriate zero-biasadjustment to give the 20,40 or 57 ft AID asrequired. The result of choosing this method is

(J

+o

(b) changes in response to glidepath signals;120 ft (a) preparatory functions;90 ft (a) check 140 ft operation:50 ft (a) glidepath signal disconnected;

(b) throttle closure init iated;(c) pitch demand maintains correct descent

rate;20 ft (a) rudder servo disconnected;

(b) ailerons centred.

This sequence is applicable, for example, to aBAC I -l I series 500 aircraft using an Elliott seriesll00 auto touchdown system. The 140 and 90 fttrips are fed from the radio altimeter; the others aregenerated in the autoflare computer.

The g.p.w.s. needs radio height trips on all modessince the profi le changes abruptly at different heights.Mode 2 operation ise xcessive terrain closure warningand so depends on the rate of change of radio height.The trip signals and the rate signal will normally begenerated within the g.p.w.s.using valid heightirrformation from the radio alt imeter.

Some multi-function fl ight director instrumentshave a rising runway symbol which moves up to meetan aircraft symbol as the aircraft descends totouchdown. Operation is typically over the last100 ft. The vertical movement of the rising runwaydepends on the height analogue signal from the radioaltimeter, while its lateral movement is controlled bythe ILS localizer output. Failure of either radioaltimeter or localizer causes the rising runway to beobscured by a'RUNWAY' flag.

Mul t ip le Insta l la t ions

All-weather landings will only be safe if informationfed to the autoland system is reliable. To achievereliabil ity of radio height a multiple installation is

used. Whether dual or triple installations are useddepends on the probabil ity ofan undetecteddegradation; consequently dual radio alt imeters wil lonly be used where monitoring, self-calibration andredundancy are deemed sufficiently comprehensiveand reliable.

As soon as'one fits more than one radio altimeterto an aircraft the possibil i ty ofinterference exists. If.the number I system were to receive a leakage signalfrom number 2 a false height reading may result. Alsosince the antennas are broadly directional and allfacing downwards the echo from number I will bereceived by number 2 (and 3) and vice-versa.

Various safeguards are employed. Minimumcoupling may be achieved by separating the pairs ofantennas sufficiently (at least 8 ft, say) and possiblyensuring that the E fields of adjacent pairs are at right '

angles (see Fig. I l. l3).As a iurther precaution multiple-installation

altimeters will employ different modulationfrequencies. As an example of how this helps,consider Fig. 11.14 where we have two altimeters, oneoperating with a modulation frequency of 100 Hz,

IOO MHz

1/105

Fig. I l . l4 Dual- insta l lat ion modulat ion l iequencies(100 and 105 Hz )

t l--rj t F-t t l

l - + :fb iA

oco {f ' cIo

r

1/100

tili

til-lEr-

mF> 2 0 i n

No. I

>20 in

No. 2

> 2 0 i n

No. 3

E*ltill.-

..]:GJ,:

Fig. I l. | 3 Triple-instaltation aerial arrangements

199

the other 105 Hz (e.9. Collins ALT 50). Assume thatat one instance in time 're' in one of the receivers wehave two signals both at fc F 4300 MHz) and bothincreasing in frequency; one at 100 Hz rate, the otherat 105 Hz rate. One-hundredth of a second later therewill be a non-zero beat frequency f6 glen by rate ofchange of frequency of the most rapidly changingsignal multiplied by the time lag of the slowesr. So:

f n = ( L f x 1 0 6 x 1 0 0 x 2 ) x ( / )= (100 x 106 x 100 x 2) x ( l /100 - l /105)= 9 '5 MHz .

Thus after one cycle of f^ the interfering beat is wellout of range of the difference frequency amplifierbandwidth. The beat will change at a 5 Hz rate,rcaching a nraximum of 100 MHz.

The different modulation frequencies are selectedin a sinri lar way to AID, i.e. by a jumper betweenappropriate pins, the jumper being part of the fixedinstallation. A similar technique for pulse altimeterscould be to employ sufficiently different p.r.f.s toensure that the 'height' change due to an,interferingpulse would be at a rate fast enough to preventlock-on by virtue of the altimeter time constant.

Characteristics

The following are selected and summarized fromARINC 5524 .

Input I Output r.f. Coupling50 f,) RG-9/U (or equivalent) co-axial cable.Cable + antenna s.w.r. less than l. l : I over frequencyrcnge 4210-4390 MHz.

Altitude RangeFrom 2500 ft to a'few feet'below touchdown.

Loop GainSufficient to ensure proper operation up to 2500 ftassuming a total feeder cable length of 30 ft ofRG-9/U, a ground reflection coefficient of 0'01 andwith an additional 9 dB loop gain for contingencies(e.g. longer or different type ofcable).

Outputs

Basic(a) d.c. alt itude analogue: V = 0.2h +0.4 below

480 ft; V -- l0 + l0 In ( (/, + 20)/500) above480 ft (ln being log to the base e).Accuracy: greater of ! 2 ft or 2 per cent up to500 ft, 5 per cent thereafter. Time constant 0.1 s.

200

(b) Two trips: single make contacts switching supplyfrom user equipment below pre-set height.Adjustable 0-2500 ft t 6 per cent and500-1500 ft I 6 per cent.

Optional (additional to above)(a) Synchro output representing height to be

employed for display purposes. Same accuracy as(a) above.

(b) Altitude rate 400 Hz phase reversing, 200 mV100 f t - r min- lAccuracy: greater than t 20 f.p.m. or f l0 percent up to 50 f t ; t 30 f .p .m. or t l0 per cent50-500 ft.

(c) Additional trips: three at 0-200 ft t 3 per cent t3 ft; two at 0-500 ft t 3 per cent t 3 ft; one at1000-2500 ft t 6 per cent. i

Ramp Testing and Maintenance

It is important to stress that due to the extensiveinterface it is essential that the radio altimeter outputsare compatible with those systems which it feeds.If we also consider the program pins used to selectmodulation frequency and AID and, further, criticalfeeder lengths, it is clear that replacement of units orparts of the fixed installation must only bd carriedout when complete compatibil i ty, both internal andexternal, has been established.

A functional test on the ramp is quite straightforward.The radio alt imeter should read nearly zero feet whenswitched on. If the antennas are mounted forward ofthe main wheels the reading will be less than zero;if aft of the main wheels greater than zero. The flagshould clear showing the r.f. path is not broken butnot proving the loop gain is sufficient; attenuationshould be introduced to check this but is unlikelv tobe called for.

When self-test is operated the correct readingshould be obtained and the flag should appear. Whilekeeping the self-test switch pressed the DH bug maybe adjusted from.a higher to a iower reading than theheight pointer, the DH lamp being first lit and thenextinguished as the bug passes the pointer.

Special-to-type test sets are available which allowvariation of simulated altitude: this is useful forchecking trip signals. On some altimeters operatingthe self-test causes the pointer to sweep again due to avariation in simulated altitude.

Appendix

Sinusoidal Frequency Modulation

If the carrier Vs sin 2nf.t is frequency modulated bya sinusoidal waveform, V* sin 2rf^t, then theoutput of the transmitter,vs, and the received signal,vr, are given by:

v t = V ts in (2 r f r t + rns in2n f * t ) (A l l . l )

rr = Vr sin (2nf, (t - f) + m sin 2nf* (t - T))( A l 1 . 2 )

where ?n is the two-way travel time and m is themodulation index (constant in this application).

If received and transmitted signals are fed to amultiplicative mixer we have, after somemanipulation, a difference frequency signal of:

v1 = kV1V, sin(2m sin(nf*T)Xcos(2nf*Q - f l2) '1 + Ztr f rT) (Al l .3)

where k is a constant of proportionality. Since T ismuch smaller than I I f^ we may write:

sin nf*T x f^T

Also zlz = Lf l2f^, where A/is total range'od 7 /frequency variation so: l

vl,, = kV1V, sin (nAlI cos (2nf^t * n f*T)+ 2nfrT) (A I I .4)

The beat frequency may be found by differentiatingthe argument (angle) in (Al 1.4) with respect to timeand dividing by 2z to give

fn = -((n LfT)(2nf^) stn (2nf*t - nf^T))l2r= nLfTf ln s in(2nf^ t -nf*T+n) (Al l .5)

Note the minus sign resulting from the differentiationof the cosine term has been replaced by a phase shiftof n radians. The average beat frequency over half amodulating cycle, l 12 fm, is:

7n = 2f* Ullrr^ 1a,1= r LffmT cos nf^T

Again since T 4 | I fm, cos n/rrI= l, so:

7n o 2Lff^r= 4Lff^Hlc (Al1.7)

This is the same as equation (l l , l) derived assuming alinear modulating waveform.

(Al l_6)

201

12 Area navigation

Development of Airspace Organization

Before radio aids were available, pilots navigated byvisual contact with the ground and were responsiblefor their separation from other aircraft. With theadvent of radio and improved instrumentation itbecame possible to fly in situations where the groundcould no longer be seen and separation could not beguaranteed. From such beginnings the need for air-and ground-based navigation aids and controlledregions became apparent:

In the 1930s the airspace s':rrounding certain busyairports began to be designated controlled zones withrestrictions, relating to weather and to qualif ications,placed on those who wanted to enter the zone. Withthe growth of air traffic has come the growth of aworldwide controlled airspace system.

The 'shape'of this controlled airspace wasinfluenced by the introduction of one of the earliestground-based navigation aids, radio range. Thisequipment, introduced in the 1930s, gave four beamswhich suitably equipped aircraft could follow. Beamflying was continued and intensified with the adoptionof VOR and so confirmed a system of controlledairways which radiate from ground stations. Theairways link control areas where a number of airwaysconverge over c€ntres ofhigh-density traffic. Neitherthe airways nor the control areas start at ground level,as do control zones which are centred on one or agroup of airports.

The airwavs. control zones and coritrol areasconstitute controlled airspace within which instrument Area navigation equipment is not new although the

flight rules (lFR) are in force. Only. instrument-rated acronym RNAV. is fairly recent. In fact RNAV could

pilcts flying aircraft fitted with a minimum have been implemented in the 1950s had the choice

iquipment complement can use controlled airspace for an international standard been the Decca

for navigational purposes although, for non-conforming Navigator. Other equipments providing navigation

flights, i special visual flight rules (VF'R) clearance facilities over a wide area and not tied to fixed points

can be obtained from air traffic control (ATC) to are Loran and Omega of the ground-based systems

enter or cross. Within controlled airspace separation and Doppler and Inertial Navigation Svstems of the

is the responsibil i ty of ATC, whereas in uncontrolled self-contained variety. Unfortunately all of these

airspace i1 is the responsibility of the pilot who, systems are more expensive than VOR/DME which is

however, can be given a separation service if in an in widespread use. The advent of airborne computers

advisory service area or on an advisory route. has now made possible sophisticated navigation

The above description of the structure of systems including an RNAV systern based on

m2

controlled airspace is lacking in detail but is sufficientto make clear the disadvantages. With heavy trafficwe have many aircraft occupying a relatively smallproportion of the airspace, in particular the scheduledair transport aircraft in the airways. To free aircraftfrom the airways one needs a navigation system which "can be safely used over a large area, hence areanavigation, and not irrevocably tied to fixed points,such as VORTAC beacons, on the ground. But onecan go too far; the thought of aircraft converging on , ',-an airport from all directions is frightening. What isneeded is new airways which can remove 'dog legs'and parallel existing ones, thus shortening routes andflight times.

The benefits of area navigation are not always easyor even possible to realize. For example in the UnitedKingdom the areas most used by air traffic of thescheduled transport type is largely covered withairways, control areas and zones already and anextension to area navigation which will benefiteconomically is difficult to achieve. However, ingeographically small but busy regions such as Britain,area navigation equipment is an extremely useful boolto the large number of general aviation aircraft whichpotter around beneath the airways and in uncontrolledairspace generally. It was possible before, but with

the correct equipment it is now much easier.

Generalized Area Navigation System

R/DME. The trick is to 'shift' the position of theco-located beacons to a phantom beacon or waypointlocation chosen by the pilot. The pilot uses hisVOR/DME instrumentation h the same way asbefore, except that steering commands are ielated toa waypoint remote from the beacon.

The computer power, of course, allows much morethan the generation of steering commands to phantombeacons, several navigation sensor and air dataoutputs may be mixed to provide a means of lateraland vertical navigation and a display of data whichcan take many forms. The all-purpose system isil lustrated in Fie. 12.1.

heading and drift angle ortrack angle and ground speed orwind direction and speed orcross-track distance and track error, etc.

Analogue presentation on HSI:heading and trackcourse display and settingdesired tracklateral steering command.

Analogue presentation on attitude director:pitch and roll steering commands.

Analogue map presentation:route, beacon and waypoint data.

Control anddisplay unitFl ight data

storage uni tConvent ionalinstrumentat ion

Navigationcomputer uni lAutomatic data

entry unttElectronic, movingor proJectedMap display

Possiblesensor inputs

tII

Aar data inputs

Fig. l2.l General area navigation system

The computer. using stored data and inputs from arzriety of sensors. calculates the aircraft positionabsolutely in terms of latitude and longitude and alsorelatively in terms of deviation tiom the desired flightpath. A variety of display formats may be used asfollows.

Digital read-out on display and control unit:present position, latitude/longitude or

No attempt has been made here to give a definitive listof displayed data since there is considerable variation.

The data required for the computer to perform itsfunction are of three types and can be input to thesystem in three different ways. For regularly f lownroutes 'hard' data such as location, elevation andfrequency of VORTAC beacons and airports,standard departure and arrival routes (SIDS andSTARS) etc. will be stored in a flight data storage

203

unit (FDSU), typically on magnetic tape. Waypointposition, 'soft ' data. may be entered or amended infl ight by means of a keyboard and 'scratchpad'

display on the control and display unit (CDU).Real-time data from navigation and air data sensorsare continuously available for input from a variety ofsources.

The data relating to waypoints are 'soft' in thesense that they can be amended but they may bestored as 'hard' data on a magnetic or punched cardand input via an automatic data entry unit (ADEU).This facility is useful since an operator could have thewaypoint data for all regularly flown routes recordedon cards, the correct one being chosen for a particularflight. {r

Since the information from the sensors is inanalogue form analogue to digital conversation(A/D) is necessary before the computer can handle it;A/D circuits may be in the area navigation systemitself or in the systems which feed it.

The form of area navigation systems is by nomeans finalized, and with the variety of inputs andoutputs possible it seents unlikely that functionalstandardization wil l be achieved to the same extent asit has with other systems such as lLS, VOR, ADF,eLc- Onecoukl write a bttok on those RNAV andVNAV (vertical navigation) eqttipttrents available now

(1979) but here space wil l only allow a briefdiscussion of VOR/DME-based RNAV. with

examples, and ARINC Characteristic 583'l ' Future

developments depend on use of microcomputers and

util ization of f lexible c.r.t.-based display systems (see

Chapter I 3).

VOR/DME-Based RNAV PrinciPle

The basic idea is simple; signals from existing VOR

and DME co-located beacons are used to give rangeand bearing, not to the station but to a waypoint

;pecified by its range and bearing from the station.

To achieve this the RNAV triangle (Fig. 12.2) has to

be continuously solved.We have:

pr: distance between beacon and aircraft;0 1: magnetic bearing from beacon to aircraft;p2: distance between beacon and waypoint,02: magnetic bearing from beacon to waypoint;p3: distance between aircraft and waypoint;03: maflnetic bearing from aircraft to waypoint.

The quantit ies p1 and 0I are known from normalVOR/DME operation. the quantit ies p2 and02 areentered by the pilot, hence two sides and an included

VOR / DMT

- \ / "

Waypotnl 1 , 3

t'z /

(

204

Fig. 12.2 RNAV. tr irnglc

41!s

angle of the RNAV triangle are known, so p3 and 0 3can be found.

h = (( iz-xr)2 + (y, - yr) ' )6tb, = i;:' rdi-i',l4,i:it Q2:)

where xL = p1- sin 0 py k = p k c o s o k k = 1 , 2

lf (y, - y r)l(x, - x,) ) 0, 0. is in either thenorth-east or south-west quadrant; ifOz - y t)l@z xr) ( 0, 03 is in either the north-westor south-east quadrant, if yz - y, = 0, 0g is either0 o r l 8 0 " , w h i l e i f x z _ � x r = 0 , 9 : i s e i t h e r g 0 o r270". The ambiguity can be resolved by observingthat 03 wil l only change by a small amount forsuccessive calculations. An example is given byFigure 12.5 where we have:

0 1 = 9 0 . 0 2 = 1 8 0 , p r = 4 0 , p 2 = 3 0 1 s o :x1 = 40 sin 90 = 4O,yr = 40 cos 90 = 0x 2 = 3 0 s i n l 8 0 = 0 , ! z = 3 0 c o s l 8 0 = - 3 0pr = (e4q' +(-30)2)o 's = 500r = tan-r (-30/-40) = 36.87 0r.

180 + 36 '87 = 216'87 .

If the previously calculated d3 was 216 then the new0 " is 216'87 .

Fig. 12.5 RNAV. triangle example

The program for the solution of the RNAVtriangle could be based on the above or some otherformulation of the problem. Since the program isfixed it will be stored in read-only memory (ROM).The trigonometric function values may also be storedin ROM to speed up calculations. Note that completesine and cosine tables need not be stored sincesin d = cos (0 -90), also

cos0 = -cos | 180 - 0 l i f90 10 1270 andcos 0 = cos(360 - 0) i f 270<0 < 360.

,,,LK

Fig. 12.3 Vector solution of RNAV. triangle

The solution of the triangle can be found byanalogue methods as in one of the earliest RNAVcomputers for the general aviation market, theKing KN74. The vectors P{0 1 un6 Pzf 02 ^t"represented by square waveslilfrose amplitirdes areproportional to p1 and gz and whose phases representto 01 and 02 respectively. From Fig. 12.3 we seethat:

P r l t = = p z l 0 z : p r l t l ( r 2 . l )where the minus sigr indicates vector subtraction.Thus if we reverse the phase of the square waverepresenting Orl0: anA add this to the square waverepresenting pz l!2 we will have a waveform thefundamental of which represents p3 and 03 inamplitude and phase respectively.

Fig. 12.4 Cartesian (X, Y) and polar (p, 0) co-ordinates

The solution may also be found by a digitalcomputer. Expressions for p3 and 03 can be foundby converting to cartesian co-ordinates (see Fig.12.4)then reverting to polar co-ordinates. Thus:

Y axis (N)

&-

Thus, for example, a cosine table for angles 0 to 90only is sufficient. A look'up table for the inversetangent function is more problematical and can beavoided by a reformulation of the equations (12.2) tobe solved, for example by using the cosine rulealthough here ambiguity is introduced in the solutionfor 0 3 which is slightly more complicated than thatarising from (12.2). DME/VOR a

RNAV. vector

WaypointCoursedeviationdistance

Inboundcourse selected

Fig. 12.6' Deviation triangle

Course deviation calculation involves the solutionof another triangle shown in Fig. 12.6. It is normalwith RNAV to give the deviation in terms of distancerather than angle, at least out as far as a specifiedrange. Solution of the deviation triangle is possible

since one side, p3,.and all angles are known. So usingthe sine rule:

#=m&'r ( r 2.3)

t . p ' - p 3 s i n ( 0 " - 0 t )

Ifp is negative from (12.3) then the aircraft is to the

left of the desired inbound course. For example inFig. 12.6 i f 03 = 270",0c = 306'87" ( to) andpr = 50 then p = 50 sin 36'87 = 30 nautical miles(a 3 ,4,5 triangle) with aircraft to right of course by

30 nautical miles, whereas if 03 = 306'87" znd0c.=2700then p = 50 sin (-36'87) = -:0 nautical miles, i.e.aircraft to the left of course by 30 nautical miles.

In the above for accurate navigation the RNAVtriangle;hould be in the horizontal plane;unfonunately distance to the DME beacon from theaircraft is given as slant range. To obtain ground

206

t l3

Mean sea level

Fig. 12,7 Slant range triangle

distance (pt) *e need to solve the slant range triangle

shown in Fig. 12.7. The beacon elevation must be fedinto the equipment from a FDSU, ADEU or bymeans of a keyboard. Aircraft alt itude is obtainedfrom an encoding altimeter. Using the notation ofFig. 12.6 we have:

P r = (@r ) , _ (A_E) \o3

Sim.ilar calculations are necessary if the system hasVNAV capabil ity, i.e. if steering commands in bothpitch and roll are obeyed the aircraft will achieve aspecified altitude at the active waypoint or at aspecified distance from the current waypoint.

Bendix Nav. Computer Programmer NP-2041A

IntroductionThe NP-2041A is a ten waypoint RNAV computer.The waypoint parameters may be entered from akeyboard on the front panel or from a portablemagnetic-card reader. Bearing and distance to theactive waypoint are found by solving first the slantrange triangle then the RNAV triangle. ln additionthe unit can be used for frequency management forboth v.h.f. communication and navigation.

The complete RNAV system comprises anNP-2041A, a CN-201 lA comm./nav. uni t , aDM-2030 DME, an IN-20 l44' electronic coursedeviation indicator and an bncoding altimeter.Presentation of HSI and RMI is achieved through an

IU-2016A interface unit. The above package can be

complemented with an ADF and a transponder to

make up a BX 2000 system. Other optipns are aweather radar interface and a magnetic-card reader(modified Texas SR52 or Hewlett-Packard HP-67

scientif ic calculator). Large business aircraft owners

would be possible customers for a full or nearly full

package, while small singlecngined aircraft may be

f i t ted wi th the basic VFR system: i .e. comm./nav. and

an indicator.

N(m)

(o8s)

Fig. 12.8 Bendix NP-2O4lA-based RNAV. system

Although here we are concerned mainly with theRNAV computer, a brief description of the otherunits wil l be given. The CN-201lA (Fig. 2.2) is apanel-mounted unit containing two v.h.f.communications transmitter-receivers, twoVOR/LOC receivers, an audio selection panel,glideslope receiver (optional), marker receiver(optional) and various system controls (a lesscomprehensive CN-2012A may be used with theNP-2041A). The IN-2014,{ indicator is discussed andil lustrated in Chapters 4 and 5 (Figs 4.7 and 5.3).The IU-20164 remote-mounted interface unitconverts VOR/LOC and glideslope outputs to levelsthat satisfy HSI and/or RMI requirements andperforms other functions not of interest in thiscontext. The DME, encoding altimeter, HSI andRMI require no comment here, having been dealtwith elsewhere. The calculators are simply modifiedto attach a plug-in connector.

Display and ControlFigure 12.9 illustrates the front panel of theNP-2041A. There are seven separate digital displayseach employing gas-discharge seven-segment indicators.The quantity displayed in each of the windowsdepends on the position of the Display Selectorwitch and, for some of the displays, the ModeSelector switch. With the Display Selector set to:

SBY: standby waypoint parameters shown in FREQ.,BRG/KTS., DST/TTS, EL Xl00 and CRSwindows.

ACT: active waypoint parameters shown as for SBY.BRG/DST: displays bearing (BRG/KTS window) and

distance (DST/TTS window) to active waypoint inRNAV or APR mode or to VOR/DME station inVOR/LOC mode.

KTS/TTS: displays ground speed (BRG/KTS window)and time to station in minutes (DST/TTS window)to waypoint in RNAV or APR modd or toVOR/DME station in VOR/LOC mode.

The SBY and ACT windows display the number(0-9) of the standby and active waypoint. The'IN' legend is illuminated if course shown (CRSwindow) is inbound while'OUT'legend (below'IN' legend) is illuminated if course shown isoutbound.

The Mode selector controls the mode of operationas follows:

OFF: self+xplanatory.VOR/LOC: conventional navigation, the waypoints

are the stations.RNAV: waypoints are remote from the associated

stations. Lrft/Right course deviation is l inearwithin 100 nautical miles, full-scale deflection(f.s.d.) being 5 nautical miles, from 100 nauticalmiles out deviation is angular.

APR: as RNAV but linear deviation up to 25 nauticalmiles, f.s.d. being l '25 nautical miles.

'TEST': specified display for satisfactory operation.

Data is entered by means of the keyboard or bymagnetic card reader. Of the l6 keys 1l are dualfunction e.e. FREQ./ I , NAV.2/. (decimal point), etc.

Data entry must always be in the correct sequenceas follows:

l . press SBY WPT, FREQ., COM.l , COM.2,BRG, DST, EL, CRS, NAV.I , NAV.2, ADFor XPR (transponder) key as required to selectappropriate address for data;

2. press number keys to enter data;3. check data in app.r-opriate window and if correct

207\__',,

Fig. 12.9 NP-2041A (courtesy Bendix Avionics Divrsion)

press 'ENTER' key.An annunciator light indicates when a key is beingpressed.

As an example the sequence for the entry ofNAV.I frequency is:

l. select KBD on COM.i NAV. unit;2. set mode selector to VOR/LOC;3. press NAV.I key, ensure dot in FREQ. window

flashes;4. press appropriate number keys, e.g. 109'80,

ensure readout in FREQ. window (scratchpad)is correct:

5. press 'ENTER' key. Frequency wil l betransferred from FREQ. window to NAV.Iwindow in COM./NAV. unit within which theNAV.I receiver wil l be tuned to that frequency.

A lurther example is given by the insertion of awaypoint parameter, say station elevation 200 ft forbeacon associated with waypoint 3:

l. mode selector to any position other than'OFF'o r 'TEST ' :

2. display selector to SBY;3. press SBY WPT key;4. press number key 3, ensure 3 appears in SBY

u,rndow:

20a

5. press EL key, ensure dot in EL X100 windowflashesl

6. press number keys 0 and 2 in that order, ensure02 appears in EL Xl00 window (scratchpad);

7 . p ress 'ENTER 'key .

The other waypoint parameters i.e. FREQ.,3RG,DST, CRS may be entered in a similar way. To enteroutbound course we press the CRS XFR key havingpreviously pressed CRS key and entered inboundcourse.

Two keys not previously mentioned are WPT XFR,which transfers SBY waypoint number to ACT, andLPP, which loads the present position of the aircraftinto waypoint zero.

Block Diagram dperation

Microcomputer The heart of the NP-2041A is amicroco-rnputer which comprises a central processorunit (CPU), system controller, ROM, RAM, systemclock and hput/output (l/O) ports. The CPU is an8080A 8-bit microprocessor, the associated chipsbeing drawn from the same 8000 series family e.g.8224 clock generator 8228 system control and busdriver, 8255 programmable peripheral interface etc.

The microcomputer accepts data in a suitable form

fRot{tPAr{tt-0 tSPLAYS

0tsPtAY0ArAcoitIR0t

CARD RTADTRINPUT M I CROCOM PUITR

v(n mflilPUl

|Il v^tNPUT

lq. t?.t9 Np-2041A block diagram (courtesy l lendirAvionics Division)

from the keyboard. select ion-switch,carr l reader, nav.receiver, DME receiver ano atti_ei.r.-ii"ln"o", a",,is subjected to arirhmeric ona tugj.oi'oo.r;tii3", _othen output in a suitable f*;;i"",h.. itoi,'Bra,comm./nav. and Interlace units. The operations ondata are performed by rr,r. cpti-*iri.r, ffi;;;;instructions and/or data from nOU', naii _ flOporrs. The list of instructions^(progia",i ' iri" i<OU,,tjr^e.

CPU storing the a<jdress of the current and nextrnstruction in internal registers. An inr;;;.;;;; ,uybe to carry out an arithsolution of one of ,h. JT:t::^:ptration

as part of tireor wnte data inro, ," ,/lfi:i::

or to read dara out of,The ROM provides a. t3t1l^19n-v^olatile (permanent)menrory of e6K bits (e6 x l0l4 = ,816; friri. rr,ostorage space is used for rhe prograr's tbr the fourbasic service loops VOR/LOC, ;fdsi,.'**OV

l"oAPR..The totat non-votatile [eM i, zriiir'lir^"td_t-lg. r,?.rage for the. gar1m9 te rs fo r I 0 way poin rs.Ine dara in rhe non_volatile RAM in onlyr"iuii,.O ,,

long as a memory hold voltage (external) ismaintained. In addition there is 2K bits of store inl:li,:1.

*o]r whie-h provid.,, ,".;;;;riJlug" ro,oara trom the CpU. disrance to *aypoini. L."rring towaypoint, qtc. Volati iwhen the equipment rl'r#;l:?js

lost (<lumPeri)

Inputs Waypoint pararneters.enter the microcomputervia the keyboard or card_reider;;.; i ;.;: ' ;?n a key

ild;:i:i'*:f?f int"iu" "t, i*n.,'i" ."microprocess". a*j"e pt,,iiiH:,?i|.,l,?i.:t:.r.A key can be pressed Zo rni urt., ,fr. p*ri.", ,"..The card-readir interface raises the signal level to thatsuitable for TTL operation ,"d s;;;;;;; rn' i" irrrrp,signal w-hich causei the microprocessor to break theservice loop in prosress while it ,.;;;; i"ii*onr.Data fronr tne VOR receiver is in rhe form ol twoconstant amplitude square. waves, reference (ref.)phase and variable (var.) phase. Th. ;L;;;;;;;*"*

PARAttttIUN INCDAIA

ll,AY POllil8 I A R I N GI U R I

t I N I A R I Z T DWAY POINTv A R I A 8 r t

zop

between the ref. phase and var. phase signalsrepresenting the bearing to the VOR station. Avoltage controlled oscillator (v.c.o.) is phase-locked tothe ref. phase using an exclusive -OR type phasecomparator. (If two square waves, of equal frequencyand phase, switchhg between '0' and ' l ' are appliedto an exclusive -OR gate the output will be zero since0@0 = 0 and I @l = 0; if not in phase the mean levelof the output will be non-zero). The ref. phase-lockedsignal is fed to the var. phase-lock loop where it isphase-compared with the var. phase, the differencecontrolling the repetition rate of pulses from a secondv.c.o. The VOR LSI (large-scale integrated circuit)counts the pulses from the variable rate v.c.o. toobtain the bearing which is fed to the l/O ports as afour-digit b.c.d. number two digits at a time (sincedata bus is only 8 bits wide).

The DME distance data input is in the form of apulse pair where the time interval between pulsesrepresents the distance (12'36 ps per nautical mile)'The DME ISI converts this time-interval into afour-digit b.c.d. number which, as in the case of theVOR LSI output, requires two readings fo transferthe data through I/O Ports.

An encoding altimeter feeds data via buffers'(interface) to set latches in the I/O ports. The datachanges in 500 ft increments since the C1, C2 and C4

lines from the encoding alti l l leter are not connected(see Chapter 8).

The flag signals of external equipment (e.g. nav.

and DN{E) are monitored by the flag interface. If an

invalid signal is detected the flag interface output istransferred through l/O ports to the computer.

Finally, the mode and display selector interfacetranslate the information relating to the position of

the appropriate switches into logic levels that are

applied to the I/O ports.

Outputs Data to be displayed on the front panel is

transferred through the I/O ports to the display datacontro l . The data consists of b.c .d. address, b.c .d.

data and decimal point. The display data control

decodes the data and provides the necessary cathode

and anode drives for the gas discharge displays'The appropriate data wil l not be displayed but wil l

be replaced by dashes if a flag signal is detected. For

example the BRG/DST flag in RNAV mode will showfor any of the following: nav. flag, DME search' DME

test, DME not frequency paired with nav. 1, loss of

nav. or DME input signals or ILS frequency selected.

The RNAV flag signal generated by the computer

must also be fed to appropriate external equipment.The selected inbound or outbound course of the

active waypoint and the computer distance to the

210

waypoint are serially shifted out to the ECDI lbrOspiay in the course and distance window.

Comm./nav. frequency management is achieved byparallel b.c.d. address and data outputs for tuningpurposes.

The bearing to the waypoint is fed to the ECDIand also the interface unit in the form of the RNAV30 Hz ref. phase and RNAV 30 llz var. phasg derivedfrom the waypoint bearing output LSI and waypointvariable d/a converter. The feed to the waypointbearing LSI from the I/O ports is in digital format.The ECDI processes the RNAV 3Q Hz ref . and 30 Hz

var. phases to produce left/right deviation signals todrive the 'bar'. The interface unit sirnilarly provides

left/right deviation signals for the HSI (and possibly

autopilot).The display intensity control sets the intensity

level of the seven segment indicators and front panelannunciators in accordance with the setting of the'DIM'contro l on the comm./nav ' uni t . A common'DIM' control is used for all units of the BX 2000system to ensure uniform intensity of lighting.

King KDE 566

IntroductionThe KDE 566 is an automatic data input/outputsystem (ADIOS) used with the KCU 5654 controluni t (F ig. 12.12) forming par t of the KNR 665digital RNAV system il lustrated in Fig. 12.11. The

complete systenr, which may have more units thanthose shown. wil l not be described since a system withsimilar capabil it ies has already been discussed'

We have not considered an ADEU in any detail soa brief description of the KDE 566 follows. In f 'actthe unit is called an ADIOS rather than an ADEUsince the magnetic cards may be recorded by theunit using data frotn the KCU 5654. memory as wellas providing the data entry or input function frompre-recorded cards.

The magnetic cards are about the size of a businesscard and can store the waypoint parameters (frequency,

course inbound, course outbound, and waypointdistance and bearing from the beacon) for up to tenwaypoints. A number of cards can be prepared for

frequently travelled routes. The route (from-to) canbe noted on each card and the top right cornerclipped off to fix the data so that they cannot bechanged.

Block Diagnm OperationThe KDE 566 operation is best explained in terms of

its modes of operation which are monitor, record,

DME Tune

DME d is t .

Fig. l2.l I King KNR 665 RNAV. system

Fig. 12,12 KCU 565A (courtesy King Radio Corp.)

enter and error. The mode control circuit nronitorsthe record and enter buttons and the belt positiondetector to determine which mocle shoul,i be actlveand so instruct the rest of the svstem.

MONITOR&ria l data is c locked in f iom the KCU 565A throughthe memory data gating and voltage translationcircuit. the clock pulses coming from the rnaster clockin the KCU 5654. lmmediately prior to a data cycle

Digital coursegl ideslope dev. and f lagRNAV./VOR/LOC dev. and ftag

there is a synchronizing sequence of 6 bits set to Ifollowed by 2 bits at 0. The data cycle consists ofl0 X 5 X l6 = S00 bits since there ire l0 waypornts,5 waypoint parameters and l6 bits for eachparameter. A further 96 bits following waypoint 9are designated 'test waypoint'. Thus we have a totalof 904 bits from the KCU 5654 which are stored infour shift register memories providing a more thanadequate storage space of 1024 bits. In this modethe KDE 566 memory is a mirror imase of theKCU 565,4 memory. Updat ing o. .urc every I 1 .3 ms.

RECORDThe record mode is activated whenever the recorclbutton is pushed. The mode wil l not be entered unti lthe end of a memory-refreshing cycle as signified bytl.re output of the memory synchronizer to the modecontrol circuit.

On inserting a card in the slot a microswitch isclosed by the corner of the card unless previousiyclipped off. The record button switch is in serieswi th the microswi tch, thus when the but ton is pressedthe ntemory is temporar i ly f rozen. the motor d i ivesystent is activated and the card begins to travel outof the slot provided rhe card is whJe and fullyinserted. The motor. drives a belt which has smallholes in it at appropriate points allowing l ight from a

Digital

distance

Card slot

KCU 565Amemory/control/display

KDr 571dist. spd/ttsindicator

KN 581 RMI

211

KDE 566 KCU 565

!KDE 566

Enter buttonI

Record button

PWR switch

Fig. 12.13 KDE 565 block diagram (courtesy King RadioCorp.)

lamp to shine through onto a photoresistor in the beltposi t ion detector input .

- When tl.re card has built up speed the mode controlis notif ied by the position detector that all is readyfor recording. The data is clocked out of the fourshift registers to four magretic heads which recordthe digital data at the appropriate points on the card

2 1 2

14v Itov I to al l l .C. 'ss v j

by magnetiz-ing thc ferromagnetic oxide in one of tlyodirections. depcrrdilrg on whether 0 or a I is toie--' 'recorded. A systt:ni counter is advanced one counteach tir lc thc nremory is clocked. Bctween writ ing 4bits (at a l irrre) the card advances and previouslyrecorded data is erased. After 256 counts the datahas all been recorded and the system returns to themoni tor ntode.

Frontpanel

Mastorclock in

Serraldata out

Serialdata in

Read/write

ENTERA card, on which a set of data are recorded, is pushedfully home in the slot closing a microswitch sopositioned that it will close even though the corner ofthe card is clipped. When the enter button switch, inseries with the microswitch, is pressed, the motordrive system is started and the card travels outward.When the card reaches a position slightly before wheredata recording began the belt positiop detectornotifies the mode control when then'enables all entercircuitry.

The magnetic heads read data from the cards sincethe changing magnetic field, as the card passes over amagnetized part, wil l cause a current to i low in thecoil wrapped around the head core. Because of theway in which the data were recorded this currentoccurs in pulses, positive or negative depending onwhether I or 0 was recorded. Each of the four headsfeeds an amplifier and thence the threshold detecrorswhich provide digital data outputs. The digital dataare fed to the data decoders which enter the data intothe correct memory channel sequentially. Thefour-channel count multiplexer gathers the countsfrom all four channels producing one count ourpurfor the counter and decoder.

When the card has travelled past the end of thedata tracks the belt position detector init iates theerror check phase via the mode control. The counterand decoder output is examined to determine if4 X 256 = 1024 bits have been counted. If the countis correct the mode control gates the master clock tomemory, actuates the read/write l ine to the KCU565A and enters the contents of memory into theKCU 565.4 memory. After data transfer is completethe KDE 566 returns to the monitor mode.

ERRORIf the count from the counter and decoder is not1O24 the mode control initiates a flashine red errorl ight and returns to the monitor mode. l io attempt ismade to enter data into the KCU 565A.

Standardization

The first meetings of the AEEC area navigationsub-committee were held in 1969 to discuss an ATAstatement previously prepared. Three possiblesystems for airl ine use were proposed: a simple Markl, a sophisticated Mark 2 and a Mark 3 which involvedan expansion of INS. The ARINC characteristics lorthe Mark I and 2 systems were published in i970:however, before publication of the Mark 3characteristic it was decided that the Mark I system,

which by this stage was no longer '$mple', and theMark 3 system should be combined,hence thepublication in 1974 of the ARINC Characteristic583-l Mark l3 area navigation system. The remainderof this section will be used to briefly describe tneMark 13 system.

The Mark 13 is a three-dimensional system desisnedfor use in all types of commercial transport aircrafi.The basic information required for lateral and verticalnavigation is derived from a mix of VOR/DME andINS data plus alt itude from an air dlta iomputer orsinri lar source. If INS data are not availableVOR/DME fi ing with air data/magnetic headingsmoothing is used in which case loss of VOR/DMEdata leads to an air data based dead reckonins mode.

The parameters of at least twenty waypoiits areprovided for using manual or automatic entry. Twosuccessive waypoints define a great circle leg withrespect to which navigation and steering commandor deviation signals are fed to conventional indicatorsto give lateral and vertical commands and in additionare produced for use by the AFCS. Both paralleltrack and vertical positioning at any point on thetrack are capabil it ies provided by the system; in thelatter case visual and aural altitude alert sisnals aregenerated.

The Mark l3 system comprises two units, anavigation computer and a control and display unit,with the possible addition of a flight data storageunit. Processing of the inputs and generating theoutputs should be performed by the NCU while theCDU provides the pilot/system interface includingcontrol of the INS when used as an RNAV source.

The sys.em inputs are as follows:

VOR omni-bearing: analogue or digital;DME slant range: pulse pair, variable spacing;INS: present position latitude and longitude;INS: true heading and velocity (NiS and E/W);altitude: analogue or digital;baronretric correction: only if analogue altitude

uncorrected;TAS: synchro, a.c. analogue or digital for data

smoothing and d/r;magnetic heading: synchro;VOR warning: discrete high or low level;DME warning: d iscrete;alt itude warning: discrete;TAS warning: discrete;magnetic heading warning: discrete;prograln control: pins wired to choose input

rlpi ions:a.c. reference phase: 26 V 400 Hz;go-around: discrete from AFCS,

213

altitude alert cancel: discrete;Mach number: synchro or digital;IAS: synchro;ILS localizer/glideslope deviation: d.c. analogue;localizer failure : discrete;glideslope failure : discrete;AFCS engaged: discrete;autothrottle engaged: discrete ;digital clock input: ARINC 585.

The system outputs are as follows:

Fig 12.t4 TIC T.34A RNAV. test set (courtcsyTel-l nstrument Electronics Corp.)

214

omni-bearing to waypoint: sin/cos from four-wireresolver;

relative bearing to waypoint: synchro;crosstrack deviation: high- or lowlevel d.c. analogue;vertical track deviation: as crosstrack;lateral track angle error: synchro or'digital (b.c.d.);drift angle: synchro or digital (b.c.d.);lateral track change alert:.28 V d.c.;vertical track change alert: 28 V d.c.;lateral steering (roll command): a.c. or d.c.

analogue;

\

(

vertical steering (pitch command): as lateral;to/from: high- or low-level d.c.;desired lateral track: synchro or digital (b.c.d.);track angle error plus drift angle: synchro;distance to waypoint: digital (b.c.d.) 0_399.9

nautical miles;present position (lat./ long.): digital (b.c.d.);ground speed: digital (b.c.d.) 0-2000 knoti:t ime to go: d ig i ta l (b.c .d. ) 0- . ]99.9 min:crosstrack distance: digital (b.c.d.), O-3gg.g

nautical miles;lateral track angle; digital (b.c.d.);desired altitude: digital (b.c.d.) 0-50 000 ft;second system data: two-wire data bus:RNAV system failure: high- or low-level discrete:alt itude alert failure: discrete;digital bus warning: discrere;autotune val id : d iscrete;alt itude alert: discrete aural and visual.:VOR frequen cy : 2l 5 selection;system status annunciation: 28 V d.c. once per

second;high deviation sensitivity: 28 V d.c. when selected;VOR frequency alert: 28 V d.c. when discrepancy;parallel offset track alert: 28 V d.c. when selected:speed error: d.c. analogue;speed error warning: discrete.

The preceding l ist of input and output signals israther lengthy but i l lustrates the computing po*.,available in modern equipment. Details of ihe sienalcharacteristics can be found in ARINC 5g3. The lasteight of the inputs and the last two of the outputs aredesignated as being growth inputs ar outputs. Themethod of achieving the necessary outputs, given theinputs, is left to the designer and will vary in bothhardware and software.

Testing RNAV

An RNAV system is simply a computer which acts ondata from external sensors. In order to check forstandard operation these sensors must give knowninputs to the RNAV system. For example we coulduse appropriate VOR and DME test sets to give abearing and distance from beacon of lg0'anA fOnautical miles respectively, then with a waypointposition of 090" and .10 nautical miles from thebeacon the RNAV bearing to the waypoint should be53.13" and the RNAV dislance 50 nautlcal milessince we have set up a 3 : 4 : 5 triangle. This simpleexample can be extended to incorporate the otherinputs demanded by more sophisticated systems.Flag operation must be thoroughly tested. A self-testwhich checks the display and display drive andperhaps other circuits is usually provided.

Tel Instrument Electronics produce an RNAV testset, the T-34A, which is in fact a combined DME/VORramp tester. The convenience of such anarrangement is obvious. The DME section is virtuallythe same as the TIC T-24A (Chapter 7) the VORsection is similaFffffie TIC T-278 (Chapter 4) butwith additional feitures, i.e. 108.05 MHi r.f. indbear ings of 45" , 135o, 225", 3 I 5o.

It should be remembered when trouble-shootingthat, if the system includes an ADEU or FDSU,mechanical problems may occur affecting input ofdata. Poor contact between reading head andmagnetic card or tape can cause a loss of data. If thecard or tape dnve slows down but the readinefrequency remains the same then the rate of

-magnetic

flux is slower and hence the head outpui is lowered.When recording a slow drive causes biis to be wnttenon top of each other (pulse crowding losses) while ifthe drive speeds up the bit is recorded over a longerlength of track leading to incorrect head output ifreplayed at normal speed.

215

13 Gurrent and futuredevelopments

lntroduction

Changes in aircraft radio systems occur more andmore frequently due to the improving state of theart. The first airborne radio equipnrents usedthermionic devices, cat's whisker detectors and largeparallel plate tuning capacitors; power, weight andsize were restrictions on the developrnent of suchequipments. In the 1950s transistorized equipmentbegan to appear although not completelytransistorized, the r.f. stages being reluctant tosuccumb to solid state. Even now the thermionicdevice is still with us in the shape of the magnetrortrand the c.r.t. but not, I think, for very long. Claimsconcerning an all solid-state weather radar were madeabout mid-1979, a commercially viable equipmentappeared in 1980 (e.g. Coll ins WXR700). The c.r.t.wil l remain with us for many years but wil l, I 'm sure,eventually be replaced by a matrix ofelectroluminescent elements.

Transistorized equipment is of course stillmarketed, but many of the transistors, diodes andresistors now appear on integrated circuits. Theemergence of first small scale integration (SSI) thenmedium scale (MSI) and now.large scale integration(LSI) of ever more components on one chip hasrevolutionized the design of air radio systems. Inparticular using LSI techniques to producemicroprocessors opens up a whole new world.

The rate of development in the last decade or someans that many aircraft fly with a range oftechnologies represented in their electronic systems.It is not inconceivable that an aircraft could be inservice with a valve weather radar, a transistorizedADF and an RNAV system employing amicroprocessor, or some other combination whichwould make it a flying electronics museum. That thishappens and wil l continue to do so is the companyaccountant's choice not the engineer's or the pilot's..The replacement of one system by anotherperforming essentially the same function must bejustified in terms of increased safety, increased payload, increased reliabil i ty or an improvement inperformance which allows flights to be made in

216

conditions where previously the aircraft would haveto be grounded.

The reluctance to replace an equipment which isperfbrming adequately reduces the size of the rnarketfor the radio system manufacturer. Of course there isno problem with new aircraft which will have thelatest proven equipment fitted. Paradoxically, thesituation we have is that the aircraft f itted withequipment employing the latest state of the art aremore l ikely to be in the general aviation category,since that market is very much bigger than that forconrmercial airl iners.

Completely new systems do not appear veryfrequently, although when they do it is oftenbecause the improvement in the state of the art hasmade the impossible possible. An airborne Omegareceiver was not a viable proposition unti l thecomputer power and memory capacity necessarycould be economically made available in a box ofreasonable size.

Syqtems such as VOR, DME, ILS, etc. requireenorrnous capital investment and so once adopted ona large scale tend to last an extremely long time.During and immediately after World War II manyairborne radio systems were developed but only a fewsurvived; new systems developed since the 1950s havenot been internationally agreed replacements forexisting systems but provided competit ion for them.The microwave landing system (MLS) which wil lsucceed ILS wil l be the first replacement system, asopposed to competing system, for decades.

It must be mentioned in the introduction to achapter such as this that the changes we are seeing,have seen and will be seeing, are to a large extent dueto vast expenditure on defence and space research.Having stated the obvious, I will now briefly give mythoughts, occasionally supported by facts, on what isto come.

The State of the Art

The microp*ocessor and other LSI circuits are used inthe current generation of radio systems (1979). lt

seems clear that such circuits will be used more andmore for succeeding generations not only forcomputing purposes, in the conventional sense, butalso for control of virtually everything. Often, inequipments, a microprocessor wil l be under-uti l izedbut nevertheless it will still be a cheaper approachthan using the minimal amounfof hard-wired logicand furthermore spare processing power is availablefor expansion. All one needs to do is add thenecessary software. I doubt if special-purpose LSIcircuits will appear in great variety since the volumeofproduction required for a reasonable unit cost isvery large. Having said that, we already see specialISI chips for VOR and DME signal processing.

The cqmplete all-purpose airborne computer isnot with us yet and may never be. By 'completely

all-purpose', I mean a computer system whichmonitors all sensors, drives all displays, performs allcontrol functions and is the only equipment withwhich the pilot can communicate directly. Such acomputer could be built today but there areproblems such as reliability, duplication ortriplication being necessary, the need for performingseveral tasks at the same time and the huge I/Ointerface. Certainly in the near future, and indeednow, airborne computers will work in specificfunction areas such as navigation, flight control, flightmanagement, etc. although some of these functionsmay be combined and performed by one computer.

Interwiring is beginning to change radically inaircraft installations due to our ability to handle largequantities of fast time multiplexed digital data. Theprospect of a main serial digital data highway withspurs out to sensors, for incoming data, and controland/or display units, for both incoming and outgoingdata, is a very real one. Again we have the problem ofreliability but the saving in interwiring will besignificant. This is happening now in certainfunctional areas such as frequency control andpassenger entertainment systems, and will be extended.In this context mention should be made of the use ofoptical fibres which can handle data at a very muchfaster rate than conventional cables (15 X 106 bits persecond; cf.2400 bits per second, say). In addition tothe increased capability the cheapness of the rawmaterials and the immunity to interference areadvantages which make it virtually certain that weshall see hbre optics used in the future and would seethem now were it not for the difficulties encounteredin connecting them together.

The increase in the use of digital sigrals and theability to process them has a most noticeable impactin the aircraft in that controllers, displays andfacilities are all changed, meanwhile progress in the

r.f. f ield continues with perhaps less obvious results.A logical outcome of the development of low-noise,reliable, small, solid-state r.f. amplif iers and sourcesmust be to mount t.r.s adjacent to antenna downleadsso that r.f. cables and waveguides longer than a coupleof inches are a thing of the past. Reliability is the keyin this latter development since such t.r.s would berelatively inaccessible.

The Flight Deck

The flight deck will be radically different in futurewith flexible electronic displays replacingconventional instrumentation and a keyboard withalphanumeric and dedicated keys replacing a mass ofrotary and toggle switches. The trend has alreadystarted, examples being discussed in Chapters 9 and12. This is not to suggest that conventionaldisplay/control wil l disappear completely ;electromechanical instrumentation wil l be needed asback-up for the electronic displays and toggleswitches will always be used for certain functions. Asexamples of the way things are going, work by Boeingand British Aerospace will be discussed.

Boeing 767There are a number of features of the 767 flight deckworthy of mention, such as spaciousness, visibilityand comfort; our main concern here, however, is thedisplay and control of radio systems. Rockwell'Collinsare to provide their multi-colour EFIS-700 electronicflight instrument system which includes two shadowmask c.r.t. indicators: an electronic attitude detectorindicator (EADI) and an electronic horizontalsituation indicator (EHSI). Computation andalphanumeric display willbe provided by the Sperryflight management computer system (FMCS). Therewill be an EADI, EHSI and FMCS for each pilot,while an additional centrally placed electronic displaywill be used as part of the caution and warningsystem.

The EADI presentation is similar to a conventionalattitude-director indicator- one of which is fitted as aback-up. Blue and black fields represent sky andearth respectively with a white line, separating thetwo fields, as the artificial horizon. There are scalesfor roll (upper), glideslope (left), speed deviation(right) and pitch (vertical centreline). In additionthere is a rising runway symbol and a radio altitudedigital readout. The information displayed fromradio aids is glideslope deviation against a scale,localizer deviation by lateral.,movement of a symbolicrunway and radio altitude by vertical movement of

217

q-"ffW:.

Fig. l3.l Boeing 767 flight deck mock-up (courtesy BoeingCommercial Aeroplane Co.)

the symbolic runway and by digital readout. Decisionheight selected and operating modes are alsodisplayed. The facility exists, with an EADI, forblanking of scales not in use.

The flexibility of a computer-driven c.r.t. display isfully utilized in the EHSI by providing for operationin three modes: map display, full compass display orVOR/ILS mode with a lull or partial compass rose.Weather radar data can also be displayed making adedicated weather radar display an optional extra.When presenting data in map form the display isorientated track up; a vertical track line with rangemarks joins a symbolic aircraft at bottom centre to aboxed digital readout ofthe track at top centre.Heading and preset cours€ are given by distinctive

2 1 8

pointen on a partial scale near the top of the display.Ground speed (from INS), a three-segment trendvector (projected path), wind direction and speed,planned flight between named waypoints, verticaldeviation, range.scale, operating mode and weatherradar data are all shown. A variety of colours areused to avoid confusion in the interpretation ofthelarge amount of data in a 4'7 X 5'7 in. display. Aconventional RMI provides back-up.

The FMCS will mix stored daia and data fromseveral radio and non-radio sensors to provideposition fixing, optimized flightpath and speedguidance, drive for the EADI and EHSI and alsohardware and software monitoring to assist introuble-shooting. A four-million-bit disk memory

system will be employed to provide storage of datasuch as location of airports and VOR stations,selected company routes, standard departure andarrival routes (SIDS and STARS) and aircraft/engineparameters. Communication with the system isprovided by a l4-line c.r.t. display with24 charactersper line, for display of navigation and performancedata, and also a full alphanumeric keyboard plusdedicated keys.

British Aerospace Advanced Flight DeckWork by the British Aircraft Corporation and HawkerSiddeley Aviation on the evolution of flight decks intoa form which would include integrated electronic dis-plays began back in the early 1970s. The results weresufficiently encouraging to commence, jointly, anadvanced flight deck program in January 1975. Elec-tronic displays and controls have been studied withregard to engineering feasibility and human factors.

The program has come up with an exciting viewof the flight deck of the future. The main displayconsists of seven 9 in. c.r.t.s split into two discretesub-systems. Three displays (Sl, 52 dnd 53) centrallylocated on the instrument panel present aircraftsystems and engine information while four displays(Fl, F2, F3 and F4) split two each side of the panel,present f l ight information. ln addition, two furtherc.r.t.s (Dl and D2) are provided, one each side of themain instrument panel, for documentation, e.g.checklist, performance data, etc.

The flight information displays are an EADI andm EHSI. Presentation of information is in aconventional format, the EADI being similar to anADI but with information in both analogue anddigital form surrounding the ADI earth/ground circle.The EHSI can present a conventional HSI typedisplay with full compass rose and lateral deviationbar or a map-like display complete with route andweather radar data. Like the EADI the EHSI hasflight information either side of the main display butin this case in digital form only. A facility to switchthe display from one c.r.t. to the other wil l helpshould a malfunction occur. In addition there areconventional back-up instruments.

Altemative InstrumentationThe Boeing and Brit ish Aerospace developmentsdiscussed above do not depart from the conventionaldisplay format for the main part of the EADI and theEHSI display. Obviously in using electronic displaysshowing computer-generated symbols a wide range ofpossibilities for display formats exist; however, anydeparture fronr convention would require pilotretraining and would have to be justif ied in terms of

easier and safer interpretation. Work has been doneby NASA using a Boeing 737 in which the EADIdisplay format is such as to give the pilot the nextbest thing to the VFR view when landing (Flight,I I September 1976).

As conventional ele ctromechanical instrumentsgive way to c.r.t.s so c.r.t.s wil l one day give way tosolid-state devices. Litton Systems of Toronto haverecently announced (1979) a 3 X 4 in. display madeup of nearly 50 000 LEDs. The matrix iscomputer-driven to provide the required display.The advantages over the c.r.t. are reduced size andlonger l ife (m.t.b.f.).

A display technique particularly suitable for ILSapproaches is that of a head-up display. Suchdisplays are commonplace on modern militaryaircraft. When landing using panel-mountedinstruments the pilot must look up to establish visualcontact. If such contact is not possible looking downagain to return to instruments could create problemsof fast assimilation of the data on instruments. Witha head-up display the approach guidance symbols areprojected by some arrangement of optical devices sothat they can be viewed through the windscreen.Civil airl iners wil l be fitted with such displays (e.g.Airbus 4300) although it should be noted that withtwo pilots one could be eyes-up and one eyes-downduring approach to avoid an abortive attempt toacquire visual contact.

Multi-System Packages

With the advent of micro-electronics and theconsequent small unit size, it is possible to bringvarious systems together in one package. In the daysof valves we had one system - many boxes, whereasnow it is possible to think in terms of one box - manyqystems. In fact we have already discussed examplesof multi-system packages in Chapter 12. As a further,and yet-to-be -implemented, example, consider aDoppler radar antenna. There is no reason why theelectronics for the Doppler radar, an inertial sensor,a radio-navigation sensor, srich as Loran or Omega orVOR/DME, and a navigation computer should not allbe mounted on top of the fixed Doppler antennaforming a single package. Control, display and radiosensor antennas would all need to be remote in such aninstallation.

Data Link

A two-way digitally encoded automatic information

219

m#G iG*-

::'$.

I

Fg. tf.Z Experimental advanced flight deck (courtbsyBritish Aerospace)

link has been the subject ofperiodic discussion byvarious working groups for over 30 years. If andwhen the system will be implemented and what r.f.dtannels will be used if any are still unknown. ARINC

?,20

project paper 586 provides detailed information on apossible system with an entertaining appendix on thehistory of automatic communications for aircraft bythe then Chairman of the AEEC. W.T. Carnes.

lFr

In fact an ATC automatic data l ink does exist inthe form of the secondary radar surveil lance radarsystem (Chapter 8) which wil l probably be extendedsome time in the future (see ADSEL/DABS below).Here we are only concerned with a two_wayautomatic data l ink uti l izing v.h.f., h.f. or Satcomfor universal use.

system will provide for a transfer of moreinformation than is possible with current SSR, enablentore accurate and reliable tracking by ATC and sohelp in the development of automatecl approachcontrol systems (CAAS _ cornplrter assisie.t approachsequencing) and further. form an indispensable partof a proposed beacon-based coll ision avoidancesystem (BCAS).

A memorandum of understanding was signed bythe FAA (for the USA) and the CAA (for tie UK)early in 1975 to allow for future deveiopment ofselectively addressed SSR systems on a co_operativebasis. We are sti l l some way off an ICAO standardsystem, but whenever it comes it wil l be compatiblewith SSR so that existing airborne equipm.ni maycontinue to be used.

ARINC Character is t ic 718 (November l97g _draft) lays down a specification for a transponderwhich lorms part of rhe standard SSR svstem(ATCRBS - ATC radar beacon systemjand DABS.In that characteristic pl , p2 and F: pulse parameters,frequencies and SLS provisions ur. ui fu, itandardSSR but the abil ity to respond to modes A and Cinterrogations only is required rather than A, B, Cand D.

- Two types of interrogation wil l be possible withthe new system, an ATCRBS/DABS ail_call or aDABS only. The all-call interrogation wil l consist ofthree pulses Pl, P3 and p4 together with a SLScontrol pulse P2. The pl, p2 and p3 pulse paramerersare as for the ICAO SSR while p4 is a 0.g irs pulse theleadiirg edge of which is 1.5 gs after the leading edge:f.P: A" ICAO transponder will ignore p4 *f.,it. aDABS transponder wil l recognize the interrogation asall-call and respond with the all-call replv.

A DABS interrogation consists of pt and p2preamble pulses and a data block. The data block is asingle r.f. pulse either 15.5 or 29.5 ps long employingdifferential phase shift keyed (d.p.s.k..y midutation.With this type of modulation a lg0" phase change ofthe carrier at a data bit phase reversal positionrepresents a ' l 'while no phase change represents a .0,.The first phase reversal occurs 0.5 pi after the leadingedge of the data block, this is the sync. phase reversal.Subsequent phase reversal positions occur at t ime0'25l/lrs (N>2) after the sync. phase reversal. Themaximum value of iy' is 57 or I l3 giving 56 or I l2data bits transmitted at a 4 M bitls rate. The trail ingedge of the data block has 0.5 ps of r.f. added toensure the demodulation of the last bit in the darablock is completed without interference.

An optional P5 may be radiated as an SLS controlpulse in the same way that p2 is radiated from anICAO interrogator. p5 wil l be transmitted 0.4 ps

Part icipating aircraft

\ \

\ \\ \

\ r

, / / t \ \, / / | \ \r ' / I \ \

Subscriber terminals l ine orradio (microwave) l ink

Fig. 13.3 Data Iink system

The data l ink is essentjally ground-controlled.Aircraft participating in the syitem are .polled, by theground station which transmits a sequence of digitalsignals split into messages each of which contains aparticular address (aircraft registration number) andsuitable text. The airborne equipment responds if i trecognizes its address in the poll ing sequ.n.. which isrepeated at a rate determined by the giound station.Messages wil l be either to/from ATC 6r to/from theairl ine- company and may relate to clearances,altitude changes, position reports, f l ight_plan change,weather data, etc. The text of the messages wil l berouted to/from terminals on the grouna ita apolnt-to-point communications networkinterconnecting the ground station and the terminals.

ADSEL/DABS

The problems of fruit and garbling were discussed inChapter 8; such problems increase with trafflcdensi ty . Address select ive SSR (ADSEL) anddiscrere address beacon sysrem (DABS) have beendeveloped in the UK and USA respectively in aneffort to postpone the date by which the iCAO SSRsystem would beconte saturated. In addition a new

b

Preamble 3.5us

t 2 r s I

Data Block 15'5 or 29'5rs

0.5 0.5 0.25

Sync.phasereversal

o'l------{ l* O'a j ".- o.rF S l l F S l l p S

ffi53 54 55 56 57H

0'8 rs '

rf = t03O MHz

Phase reversal- Position bit : ' l

Fig. 13.4 ADSEL/DABS interrogation format

before the sync. phase reversal. For an aircraft fittedwith a DABS transponder the received P5 amplitudewill exceed the amplitude of the data block hence thetransponder will not decode the d.p.s.k. modulatedsigral. An ICAO transponder equipped aircraft willnot reply to a DABS interrogation since the P2 pulsewill trigger the SLS suppression circuit.

A DABS transponder will generate ICAO replies(12 information pulses) in response to ICAOinterrogations and DABS replies in response to all-calland DABS interrogations; DABS transponders alsogenerate squitter at random intervals to allowacquisition without interrogation (similar to DMEauto-standby, Chapter 7).

A DABS reply is only similar to an interrogation inso far as it contains a preamble followed by a datablock of 56 or I l2 bits. The preamble consists of four0'5 ps pulses with the spacing between the first pulseand the second, third and fourth pulses being 1.0, 3.5and 4'5 prs respectively, measured from leading edgeto leading edge. The data block begins 8 prs after the

222

Phase reversalposit ion bit : 0

leading edge of the first preamble pulse and uses pulseposition modulation (p.p.rn.) at a data rate of I Mbit/s. In the I gs interval allotted to each data bit a0'5 gs pulse is transmitted in the first half i l the databi t is a ' l 'and in the second hal f i f a '0 ' . The datablock is thus 56 or I l2 ps long. The r.f. is 1090 MHzas for the ICAO SSR.

There are four types of interrogation from anADSEL/DABS interrogator all of which have thesame preamble. The all-call data block contains asequence of 28 ones in a 56-bit block, the all-call replycontains the aircraft address, details of datainterchange equipment on board and parity bits forerror-checking purposes. A surveil lance interrogationol 56 bits contains address and parity bits and also arepeat of the height in format ion received on theground. An aircraft recognizing the address in asurveil lance interrogation wil l reply with a 56-bit datablock containing the altitude or identity. Theremaining interchanges of data are in I l2-bit blocksboth ways, a comm.-A interrogation giving rise to a

56 or 112phase reversal posit ions

Preamble56 or 112 ps

Data Block

br o.5l+-) <H l ' ' , ' l ' , , r1

r-r-r : l -T- ' l

I ' l o l r l o l r l

o.5<H

o.5{+

P.P.M. cxamplel o l . . . . . 0 1 1

Fig. 13.5 ADSEL/DABS reply format

comm'-B reply and a comm''c^inte.rrogation giving. simplex system employing frequency modulationrise to a comm'-D reply' .The A-B inteichang! involves *orld b.-rr.d with ujt ir, l una ao*ntinr. frequenciesaltitude or identity as well as other data, anican be separated by between 4 and l0 MHz. Aircraftused lbr tracking purposes while the c-D interchange Saicom. unt.nnu, would be broadly directional,contans an extended-ren_gth message segment of g0 possibly with switchable lobes.bits in both directions' one thing not yet decided is The accuracy of any Satnav. system will depend onthe method by which data wil l be traniferred into the knowledge of satell i te position and so a numbe.fand out of the transponder from various sensors and tracking stations are required on the ground. Sinceto various displays via a suitable processor. Two. the airb"orne equipmenimust have th! oata relating to

T:tl99t are proposed for the interface, f irstly using the satell i te porit ion a l ink must be establishedARINC 429 digital information transfer ryrt*r between the tracking station and the aircraft, most(DITS) format, secondly.a synchronous I M bitisec probably via the satell i te. Knowing the position ofinterface which would allow data requested in an the satell i te the airborne equipment must establish theuplink to be contained in the next downlink. aircraft 's position relative to the satell i te in order toThe system has only been briefly described: the obtain a dx.reader is referred to ARINC 718 for further details. The various methods by which a fix can be

Satcom. and Satnav.

There are many satell i tes ih orbit around the earthbeing used for relaying telephone and televisionsignals,.weather sensing, observation and militarynavigation and communication. (For a comorehensrvereview see Flight,28 October 197g.) Unforiunatelynone so far are used by civil aircraft and it is notknown (by the author) when such use wil l occur.However, brief comments can be made on theprinciples involved.

A possible v.h.f. Satcom. system is described inARINC Characteristic 566. The satell i te is simply arepeater for voice and data communications beiweenair and ground. I f the sate l l i te is at synchronousaltitude (22 000 nautical miles) the service area wouldbe 4l per cent of the earth.s surface. a OouUle_cfrannel

obtained involve some combination of measurementof angular elevation of a satell i te, range of a satell i teand rate of change of range of a satell i te (Dopplershift). Direction of arrival of a signal at t ire saiell i temay be -found using interferonteter methods wherebythe satell i te antennas are mounted on long booms(say 50 ft) and the phase difference in signals arrrvinsat the antennas is measured. Range measurementmay be obtained in a similar fvay to that employed inDME. In a range-rate system the Doppler stri l t tf asignal from the satell i te is recorded ovlr a period ofsay l0 min, then the aircraft position can be computeclfrom the time of zero Doppler shift and the slope ofthe frequency/time graph at zero shift.

The range-rate m.ethod can provide an accurate fixabout once every lI h, using a satell i te in a 500-milecircular orbit, obviously for aircraft a large number ofsatell i tes must be used to reduce the tirne_intervalbetween fixes. The aircraft velocity and altitucle

must be accurately known during the time taken toobtain a fix as the satellite makes a pass over the lineof closest approach to the aircraft. The US Navy usesuch a system flor surface ship navigation.

Angle-only, rangeonly or angle-range methodsmay be used. In these systems, by using a two-waylink between the aircraft and a ground station via thesatellite, the computation of aircraft position can becarried out on the ground, the data being sent to bothaircraft and ATC. A range-only system which shouldcome on line in the 1980s is the Global NavigationSystem (NAVSTAR.) using 24 satell i tes;however usemay be restricted to military aircraft and groundpersonnel.

The frequencies involved for Satnav. are likely tobe v.h.f. or around 1.6 GHz. It wil l obviously beadvantageous if a group of satellites could be used forboth communication and navisation.

Microwave Landing System (MLS)

The long, controversial and heated argument aboutwhich system will be adopted as the successor to ILSended in 1978 with the choice of a time-referencedscanning beam (TRSB) system. The requirement wasfor a landing system that would allow for a variety ofcurved or straight-line approaches within a largevolume of airspace and that would not suffer to thesame extent as ILS from multipath effects.

The main contenders by the time the final decisionwas made were the USA with TRSB and the UK witha commutated Doppler. Demonstrations and

simulations showed that both systems would do thejob and the choiie must have been diff icult. Onefactor which helped swing the vote musthave been areduction in cost of the TRSB system brought aboutby the development of cost-minimized phased-arraytechniques (COMPACT) by Hazeltine. A conventionalelectronic beam scanning array consists of manyradiating elements eai,h of which is fed by anelectronic phase shifter. Changing the phase of ther.f. energy radiated by each element causes the farfield beam to scan. With COMPACT there is nearly a4: I reduction in the number of phase shifters eachof which feeds all radiating elements through apatented passive network. The result is accuratebearn scanning with low side lobes at a reduced cost.

The principles of TRSB are quite simple. Considera radio beam scanned rapidly to and fro; anappropriately tuned receiver on an aircraft withinrange of the beam source would receive two pulses inone complete scan, as the beam swept past twice.The pulse spacing is related to the angle madebetween the centroid of the scanned sector and theline joining aircraft to beam source. Note that thesystem as described is ambiguous since the computedangle could be either side of the centroid of thescanned sector. Ambiguity may be removed by,knowing which is the 'to' and which is the 'fro' scan,or by knowing the scan cycle start time, i.e. thecommencement of the 'to' half cycle. For accuracyprecision timing circuits must be used.

For lateral and vertical guidance azimuth scanningand elevation scanning beams are required. Preambleinstructions must be used to identify the beams which.

Scan cycle

224

Fig. I 3.5 Principles of TRSB

tt related to {l

are synchronized so that one time dift-erence for eachbeam is measured in a 150 ms frame. Back beam andflare measurements are also possible on a t'ull systemwhich will be used in conjunction with a high-accuracyDME. Reflected beam reception can be elinrinated bytime_ gating (echo suppression). The r.f. employedwill be in the Ku band.

The battle lbr ICAO recognition is not the wholestory of MLS, systems having been developed iorspecial applications; MADGE (microwave iircraftdigital guidance equipment) is an MLS-basedon-ground derived interferometry, developed by theBritish company MEL(see below); SCAIvILS (smallcommunity airport MLS) is a Hazeltine product usingtheir COMPACT antennas for the TRSRsystem.

Microwave Aircraft Digital GuidanceEquipment (MADGE)

IntroductionMADGE is a microwave,secondary radar systemwhich provides guidance to a landing site for anysuitably equipped aircraft within a range of abour l5miles. As well as providing flexibility in the approachpath, as one would expect in any successor to ILS, atwo-way data link is established which results indeviation and range information being available notonly in the air but also at the landine site.

The MEL Equipment Company llmited (APhilips Company) have won sufficient military ordersto assure the future of MADGE. The first extensivecivilian use will be in the North Sea oil fields where.one platform is already equipped.

Basic PrinciplesThe system provides both range and angular data.The range is derived in the same way as with DME,i.e. measurement takes place in the airborneequipment after an air-to-ground interrogation and asubsequent ground-to-air reply. The air-derived rangemay be contained in the interrogations, subsequent tothe recognition of valid replies, in order to make thisinformation available on the ground.

Azimuth and elevation angles are ground-derived,using interferometry or, in the case of elevation, radioaltitude and range. If the latter is the case both rangeand radio alt itude data must be transmitted fromair to ground in order that the elevation triangle maybe solved for elevation angle. The angular data iscontained in the reply to the airborne equipntent'sinterrogation.

and C2. The particular modc used depends on theaircral't l l t and the type of ground staiion.

Mode AB Angular and range information is availableusing a standard land-based MADGE station. Thedirection from which the aircral't is interrogatine isnreasured by the ground station interferom-eterf thesubsequent reply containing horizontal and verticaldeviation from an approach or overshoot pathdetermined by the sit ing of the ground uni.nno arrays.The azimuth deviation with respect to the approachpath centre l ine is displayed on a purpose-builtprecision range and azimuth nreter (pRAM). while ona standard cross-pointer deviation indicator azimuthand elevation guidance is given to an approach pathselected by the pilot. The overshoot path is selectedby the pilot in azimuth only, elevation deviation iswith respect to the ground-dellned overshoot path.

Range data, derived by measuring the elapsed timebetween interrogation and reply. is displayed on aPRAlvl. With an appropriate l ink in the wiring of theairborne installation or on receipt of a command inthe ground station reply the range data is sent to theground interlaced with subsequent interrogations.The two types of air-to-ground transmission arereferred to as A channel - interrogation: B channel -range data.

Mode C Two alternatives for Mode C are available,both providing guidance to a point offset horizontallyfrom the landing site, by at least 200 metres. Suchguidance is suitable for helicopters operating tooffshore platforms. In addition to azimuth indelevation and range data one of two arrows on thePRAM indicates to the pilot on which side of hisaircraft the landing site is situated.

Unlike Mode AB the pilot-selected angular offsetof the approach path is not available with Mode C.The particular approach path used is ground-defined,hence Mode C is known as the ground controlledmode (GCM). The platform installation is rotatedto the correct compass bearing within one of foursectors chosen by the pilot, the choice being relayedby r l t .

The two Mode C alternatives are Mode Cl and C2which differ in the way elevation guidance is derived.With Mode Cl radio alt itude and range aretransmitted to the landing site. where the elevationangle can be computed since the sine of the angle isequal to the ratio of the radio alt itude to the (slant)range. With Mode C2 elevation information is derivedas in Mode AB.

Modes of Operation Mode C I allows greater flexibiJity of approach.There are three modes of operation, namely AB, Cl The aircraft can be guided down a glideslope to a point

225

remote from the landing site (say 0'5 nautical milesaway) from where the final approach is completed inlevel flight. The parameters of the approach path areunder the control of the ground station for bothMode Cl and C2.

lnterferometryA basic interferometer consists of trvo antennasfeeding receivers, the outputs of which are comparedin phase. If the radiated wave arrives from a directionother than normal to the plane of the two'antennaarray then the energy arriving at one antenna willhave travelled further than that arriving at the otherby a distance d. The phase difference between theantenna signals will depend on d which, in turn,depends on the direction of arrival.

A two-antenna interferometer is of little use sincean ambiguous measure of the direction of arrival isobtained. For example consider the antennas spacedI cm apart and a wavelength ofthe radiated waveequal to tr cm. lf d = 0, I, 2),, etc. the measuredphase difference wil l be zero corresponding todirections of arrival 0, measured with respect to the

Fig 13.7 Two antenna interferometer

normal, given by sin 0 = 0, tri Q, 2ltlQ, etc. Thus withI = 100 cm, say, and tr = 6 cm, then for zero phasedi f ference 0 could be 0,3 'M,6 '89, etc . degrees.

To resolve the ambiguity several antennas must beused in a l inear array. Phase dilference measurementscan be made between any two antenna signals to givecollectively an unambiguous direction of arrival.With the spacing of the antennas chosen to be in theratio 2:4 : 8 : l6 : 32 the derived angle word can becoded directly in binary.

" .r" i' t , i

Fig. 13.8 MADGE, hardware (courtesy MEL Equipment

Co. Ltd)

226

Conbrlls and InstrumentationA Mode AB controller has the following controls:

l. 'OFF/STANDBy/ON'. When in standby the

. input to the modulator is inhibited wirh a3-min warm-up delay for the transmitter isinit iated.

2. GROUND/AIR control. Four thumbwheelswitches which select the ground and airaddress and the frequency of operation.

3. Angle offsets. Three rotary switches whichallow the pilot to select one of a variety ofelevation and azimuth angles for approach.For overshoot azimuth offset only is provided.

4. 'TEST'. A push switch which activatis thebuilt-in test equipment (b.i.t.e.).

In addition, if the controller is dual mode i.e. ModeAB and C an 'ANGLE

OFFSETS'/,SECTOR GCM'five-position switch is provided to allow the pilot toselect which of the four approach sectors hi desiresfor Mode C (GCM) operation or, if using a Mode ABstation, the angle offset controls may be enabled byselect ing 'ANGLE OFFSET' .

The PRAM and crossed-pointer deviation indicatorhave been mentioned previously. In addition threeoptional indicators may be fitted:

l. Overshoot warning. Only active during ModeAB operation. There are two active stites ofthis ntagnetic indicator. in one of whichindication of the serviceabil ity of the groundovershoot interferometer is given; the othergiving overshoot warning. .Off is displayedwhen the indicator is not active.

2. Low-fly warning. Similar to ( I ) above exceprthat the active states show either that theelevation failure warning flag is pulled in(elevation safe) or that the aircraft is low.

3. Excess azimuth deviation. Only used inMode C. A lamp which flashes when theaircraft is more than 60 metres from theapproach path and within t nautical mile ofthe landing site.

Block Diagram OperationThe installation comprises an interrogator set (logicuni t ) . an in terrogator set ( t ransmit ter- receiver) . icontroller, an antenna selector, two antennas and upto five dilferent indicators, possibly duplicated, asdescribed previously. All electronic circuitry iscorr ta ined wi th in the logic uni t and t ransmi i ter- receiver(ti r) with tl"re exception of that contained in thePRAM.

The length and message content of the interrogationword, which is assembled in the logic unit, depenison the mode of operation. For A channel (M;de AB)a 25-bit word requesting guidance information rstransmitted at a j ittering mean rate of 50 Hz.B channel, containing range and other data in a 60-bitword, can be interlaced with A channel interrogationsat a factory set rate of 2.5,5, l0 or 50 Hz. C modesuse only one interrogation word 60 bits long at ajittering mean rate of 100 Hz. -Both

Cl and C2interrogations contain guidance interrogation dataand range, but in addition mode Cl transmits radioaltitude information. Appropriate air and groundaddress codes, as selected on the controller, formpart of the guidance interrogat ion.

The interrogation word pulse train, suitablyprocessed in a l ine receiver and pulse modulattr,controls the grid voltage of a travelling-wave_tubewhich amplifies the r.f. from a solid-state sourcephase-locked to 56 times a reference oscillatorcrystal. A 4-bit parallel code, determined bv thefrequency selection at the controller, selecti which offour crystals is to be used as a reference. The pulsecode anrplitude modulated frequency of between5'1825 and 5.2005 GHz is fed v ia a low-pass f l i terand circulator in the microwave assembly to one oftwo antennas selected by a switch which iscontrolled by the logic unit.

Received signals are amplif ied and detected in adouble-superhet receiver, then passed to the logicunit. lncoming noise pulses are combated byreducing the receiver sensitivity as the rate ofreceivedpulses increases. Interference from multipath echoesis avoided by setting a threshold level in accordancewith the amplitude of the first pulse of the incomingword. The weaker multipath echoes wil l be unlikelyto exceed this threshold which returns to zero at theend of each received word.

The reply is clocked into a shift register and thenchecked for validity and parity. Validity isdetermined by comparing air and ground addresscodes received with those selected. A ranse clock.whrch was star ted when. the in terrogat ion Iook p lace,is stopped on completion of .a successful validitv andpar i ty check.

The system functions in either search or track.Unt i l the rate of va l idared repl ies is acceptable thelogic unit causes the t/r output to switch betweenforward and rear antennas at a 0.5 Hz rale. l lavinsacquired a re l iab le range value, a t rack ing gate isgenerated so that only those replies within I gsbefore. and 2 ps after, the expected time of arrivalare accepted. With replies regularly fall ing within thetracking gate the range of change of range is

227

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228

computed. If the rate exceeds 30 knots antenna-svitching ceases, the forward antenna being selectedfor decreasing range, the rear antenna for increasingrange. Each antenna has a polar diagram which is180" wide in the horizontal plane and 40o wide in thevertical plane.

With validity and parity confirmed range and angledata is fed to the output control logic circuits. Digitalrange and azimuth information is fed to the PRAM,the azimuth information being unaffected by theselection of angle offsets. Azimuth and elevationinformation is compared with the selected angleoffsets, the resulting digital differences are thenconverted into analogue guidance outputs which arefed via matching resistors in the interface junctionbox to the cross-pointer deviation indicator. The d/aconverter gain is progressively reduced as the rangedecreases from I nautical mile. This reduction in eainis known as beam-softening and results in meterdeflection being propoitional to lateral rather thanangular displacement from the approach path.

Warning signals are generated within the logic unit.Flag drives are fed to the cross-pointer instrumentbeing 28 V for valid information, 0 V during a faultcondition. A low-fly warning is provided in ModeAB, or an excess elevation deviation signal in Mode C.Azimuth warnings are the excess azimuth deviation inMode C and the overshoot signal in Mode AB. ThePRAM display is enabled by the azimuth flag signal.

For Mode Cl a radio alt imeter interface unit isrequired to digitize the analogue height signal fromthe radio alt imeter. As a check the digit ized height isconverted back into analogue form, then comparedwith the height input. A fault signal is fed to the logicunit if a discrepancy greater than 1 25 ft is detected.The fault signal is also generated if the radioaltimeter's f lag output shows a lault condition. Witha fahlt detected the elevation flag shows and theheight information present bit in the interrogationword is negated.

Selected System ParametersTransmitter peak power: 150 W nominal.Transmit ter f requency channels: 5 182'5, 5188'5,

5194 '5 .5200 '5 MHz .Receiver frequency channels: 5004'5, 5010'5, 5016'5,

5022'5. 5028'5. 5034'5 MHz.lnterrogation word rate: 50 Hz mean (A channel);

100 Hz mean (C channel).Word j itter: + 2 ms (A channel); + I ms (B channel).Message b i t rate: l '01125 MHz nominal .lnterrogation word length: 25 bits (A channel);

60 bits (C channel).Reply word length: 60 bits (all channels).

Message format: non-retum.to-zero (n.r.z.).Height data, down link: I I bits (binary).Range data, down link: l6 bits (b.c.d.).,Elevation data, up link: 9 bits.Azimuth and flag data, up l ink: l5 bits.Analogue azimuth guidance: selectable + 2'5 to t l0o

full scale.Analogue elevation guidance: selectable t 0'5 fo t 5"

full scale.Azimuth coarse output: t 45o full scale.Elevation coarse output: t25" (A channel): -5 to

+20' full scale (Cl channel).Range output: 25 nautical miles at l0 V;30 nautical

mi les at l2 V.Velocity output: I 200 knots full scale.Digital outputs: I '01 125 MHz, bi-phase n.r.z.Angles and flags: 28-bit word at reply rate.Range data: l8-bit word at interrogation rate.Digital resolution: azimuth 0'235'; elevation O'144":

range 9'26 metres.

Collision Arioidance

Up until 1979 coll ision avoidance has been theresponsibility of ATC tbr aircraft flying under IFRwhile pilots are responsible for their own safetyunder VFR. This situation is l ikely to continue lbrsome time in the future even though a workablecollision avoidance system (CAS) has been developed.

A self-contained system would protect theequipped aircraft regardless ofwhether other aircraftwere similarly equipped or an ATC service wasavailable. Such a system could be built measuringrange, range-rate of change, and direction of all aircraftwithin a certain volume of space around the protectedaircraft. Received data could be used to computeprojected paths so that the risk of collision could beevaluated and, if necessary, a warning given orautomatic manoeuvre init iated. The cost of such asystem providing the accuracy required is prohibitive,at least for the time being.

As an alternative, we may have a co-operativesystem and this is an area in'which work has beendone. Two possibil i t ies exist:

l. an interrogator/transpondersecondary radarsystem which could measure range andrange-rate in much the same way as DME doesbut with obvious problems in crowded airspacewhere the system is most needed;

2. a time multiplexed system in which all aircrafttransmit in turn without interrogation.

The calculation of the range at nearest approach

229

q

:g(miss distance) is complicated and requires prcsentrelative positions, ipcluding altitude, and the speedand track ofeach aircraft involved. One approach isto use the component of relative velocityperpendicular to the line joining the aircraft as shownin Fig. 13.10 where V1 , Va, Vn are the velocityvectors of aircraft A, aircraft B and B relative to Arespectively, while X is the measure of the risk ofcoll ision. Another, simpler method is to use therange divided by the range-rate, measure of the riskbeing known as tau (r). In both methods a minimumvalue of the risk measure is set, below which evasiveaction is taken. With the latter method, however,when the closing velocity is small r is not a goodmeasure of risk and a minimum range criterionshould be included in the system.

AircraftI

AircraftA - L

Fig. 13.10 Collision threat geometry

As an example of a system we wil l briefly considerthe Bendix CAS which was workable as long ago as1969. The system involves an airborne clock,computer and transmitter-receiver while on the groundsyncl.rroniz-ation and clock accuracy of the system isassured by means of an atontic clock. During a 3 sinterval known as att epoclt, each participatingaircraft transmits during one of 2000 time-slots eachof 1'5 ms duration: while one is transmitting the restl isten. Transmission of alt itude and other data (e.g.heading) is rnade.

The ground station keeps all aircraft synchronizedin time;and frequency of transmission and hencerange rnay be measured from propagation time andrange rate fiom Doppler shift. Thus r ntay becalculated once during each epoch for all reportingaircraft. Commands to the pilot based on thecomputer's evaluation of the risk are: aircraft above,prepare to d ive, d ive. a i rcraf t below, prepare to c l imb,clinrb and l ' ly level. The comrnands are given byback- l ighted legends on one indicator .

Any CAS which may eventually be adopted wil lnot necessarily be l ike the Bendix system, butcertainly some of the tactors and techniques discussedabove wil l be relevant.

230

The Current Generation of ARINCCharacteristics

ln 1979 a number of new ARINC Characteristics(700 series) were adopted by the AEEC. The systemscovered include radio alt imeter, DME, ILS, VOR,ADF, Selcal., p.a. amp., v.h.f. comms, weather radarand ATC transponder. These characteristics detail theairl ine standards for radio equipment in the 1980s.There are obviously many rninor differences betweennew and old characteristics and some major ones suchas the incorporation of DABS into the ATCtransponder. However, the most significant change isthe switch to serial digital signals for both systemoutputs and control. The details of the digitalinformation transfer system (DITS) are given inARINC Specification 429-2 published I March 1979.Additional information is given in project papers 453and 72A, the very high speed (VHS) bus and digitalfrequencyi function selection (DFS) respec tively,although these papers had not been adopted by May1979.

The DITS describes the standards for the transferof digital data between all avionics systenls, not just

radio. Data flow is one way only, via twisted andshielded pairs of wires at a rate of l00K bits persecond (Kbs) or l2 to l4 '5 Kbs (1000 Kbs for VHSbus). The system is based on 32-bit words. Theencoding logic for each bit is based on a voltagetransition, a 'Hi' state at the beginning of a bitinterval returning to a 'Null ' state before the end oft ha t i n te rva l r ep resen ts a l og i c ' l ' s im i l a r l y a ' Lo ' t o'Null ' represents a logic '0'. The transmitter voltagel e v e l s a r e + 1 0 ! I V . 0 1 0 ' 5 V a n d - 1 0 1 I V f o r t h e'Hi ' , 'Nul l ' and 'Lo ' s tates respect ive ly .

lnlornration is coded in one oi several ways: binarycoded decirlal (b.c.d.). two's contplement fractionalbinary (b.n.r.) or the International Standardsorganisation (lSO) alphabet No. 5. Code l 'or graphicsdata has not yet been defined. Discrete data such ason/off switching is simply a I -bit 'code'. Words aresynchronized by a gap of at least 4 bits between words,

a gap is recognized by the fact that there is no voltagetransition. Errcir checking is carried out b1' one parity'bit

per word, the required parity is odd. i.e. t lte totalnumber o t ' ' l s ' i n a wo rd i s odd .

The basic organization of each word is label.source/destinatic-rn identif ier (SDI), dirta field.s ign/status matr ix and f ina l ly , b i i 32, par i t l . The f i rs t8 b i ts of each word are assigned to a labeluhichidentif ies the information contained in the data fleld,

e.g. v .h. f . comm. f requency, DME distance, se lectedcourse. etc . Bi ts 9 and l0 are used for the SDI where

a word needs to be directed to a specific system of a

/

multi-system installation or the source system of a

multlsystem installation needs to be identified. The

SDI is not available for alphanumeric words (lSO

alphabet No. 5) or where the bits are used as part of

the data field when the resolution required demandsit. Bits 30 and 3l are used for the sign/status matrixwhich represents t, North, right, to, test, no

computed data, etc. If several words are used to

transmit a message too long for one word then the

signi status matrix is used to indicate final word,intermediate word, control word or init ial word.

The above ideas are best illustrated by examples:

l . ADF. typ icalword0 0 0 I I 0 I 0 0 0 0 0 0 I I 0 I 0 0 0 0

l 1 0 0 0 0 0 1 0 0 0The label '0 0 0 I I 0 I 0' is followed by SDI'0 0' the all-call code. Data field bits I I ,12 and

l3 are b.f.o. - on/off (0 for off), ADF/antennamode (0 for ADF) and spare respectively. Datafield bits 14-29 are the frequency select bits with

' bits 21-29 the 1000 kHz selectiod, bits 23-26 the

100 kHz, b i ts l5-18 the I kHz and b i t 14 the

0'5 kHz selection. The example shown readingfrom bi t 14. isl l 0 l 0 0 0 0 1 1 0 0 0 0 0 10 . 5 + 5 + l 0 + 8 0 0 + 1 0 0 0= l 8 l 5 ' 5 k H z .Sign/status is not applicable here but in any case

00 can be read as positive. The final bit is set to

0 to make total one count odd.

2. ATC transPonder, tYPical word0 0 l I I 0 l I 0 0 0 0 1 0 0 0 0 0 0 0 I0 1 0 1 0 1 0 0 0 0 1The data field bits I l '17 in ascending order arealtitude reporting on/off (0 for on), inertialreference system/fl ight management computer,i.e. IRS/FMC inputs which are reserved for futureuse (0 for IRS), ident. on/off (1 for on)' alt itudedata source select (0 for No. l), IFR/VFR whichis reserved for future use (0 for IFR) and X-pulseon/off (0 for off). Data field bits l8 to 29represent pilot-selected code for Mode A replieswith bits l8-20 the D code group select, 2l-23the C,24-26 the B and 21'29 the A code group

select. The example shown reading back from

0 l 0 l 0 0 05 0 = c o d e

3. DME distance (b.c.d.)1 0 0 0 0 0 0 1 0 0 0 1 1 0 0 0 0 1 I I I0 1 0 1 0 0 1 0 0 0 0The data field is used exclusively for the DME

distance with the most significant bit (m's.b') of

the most significant character (m.s.c.) being bit

29. The eximple shown reading back from bit 29

is0 1 0 o l 0 l 0 l l l 1 0 0 0 0 l l 0

2 5 7 8 6nautical milesDITS words reprcsenting parameters are generated

in units such as VOR receivers, DME interrogators'

etc. and will terminate in navigation computers'

display drivers or indicators. The rate of

transmission of such words varies, for example the

minimum rate for DME distance is 6 times per

second while radio height is 20 times per second;

DITS radio control words such as examples (l)

and (2) above are transmitted 9 times per second'

Concluding Remarks

This book has considered current radio systems with

examples of a range of technologies. The final

chaptir has attempted to show directions in which

radio systems may progress. We will certainly see

Ut-S and most probably ADSEL/DABS' Flight decks

will make full use of f lexible electronic displays but a

minimal complement of conventional instruments and

controls wil l remain indefinitely.Modulating signals will probably remain analogue

in propagating systems, since to go digital would

requlre ishifi up the frequency scale to accommodate

the wider bandwidths required. Even though the

basis of the systems will be analogue sigrals, they will'go digital 'as early as possible in the circuitry'

Cornputlng power will increase beyond that which we

can employ; this has happened already, the- power is

there, all we need to do is think of the applications'

after which there will be yet more computing powei'

Perhaps the biggest question mark is ov-er the

future of VOR/DME as the standard ICAOnavigational aid. Possible replacements are v'l'f'/Omega'

Satniv. in one form or another' or even Loran C; the

advantages of these being that they are lo.ng.range'

A poisible scenario is an aircraft f itted with a

variety of dead reckoning and position fixing nav'

aids in a minimum number of boxes backed up by

accurate ground tracking, immensely powerful

ground computers and a corpprehensive data l ink'

ivith u -ult iple CAS and MLS of the required

accuracy the aircraft of the future could go from

ramp t; ramp without the intervention of the pilot

andln almost perfect safety. This could be

developed now, but would be cost prohibit ive;

and anyway who wants a robot f lying the'plane?

bit 29 is0 0 r 0 l

r aI J

Recommended reading

As the reader is probably aware there is a dearth ofbooks on avionics; those that exist and concernthemselves with aircraft radio are perhaps a littledated. In contrast the number of books ivailable onelectronics, computers and radio is staggering.

I have chosen to list most books available onavionics which I think are worthy of the reader'sattention. For sources of background material onbasic theory of electronics and radio the reader isperhaps best advised to, visit a suitable l ibrarv orbookshop and pick out those which seem to suit himor her best. Since this is a fbrmidable task. I havelisted some books which I think may be useful.A briefnote is given as a guide to content and levelof each.

l. M. Kayton and W.R. Fried (Editors\, AvionicsNavigation Systems. John Wiley and Sons (1969).

Authoritative book which.is sti l l ofconsiderable interest. Covers radio andnon-radio aids. Postgraduate engineer level.

2. G.E. Beck (Editor),Novigation Systerns. yanNostrand Reinhold ( 197 1\.

Similar to Kayton and Fried. also considersmarine navigation systems. Equally worthybut not, perhaps, as comprehensive since it hasalmost 300 l 'ewer pages.

3. B. Kendal. Mqnual of Atktnics. Granada ' l9791.Biased towards air tral 'f ic control aspects of

'

aircralt operations. Complementary to thisbook.

4. E.H.J. Pallert, Airuaft Electrical Systems.Pi t rnan (2nd edi t ion. 1979).

5. E.H.J. Pallett, Aircraft Instruments. pitman(2nd ed i t i on . t qS l ) .

6. E.H.J. Pallett, Automatic Flight Control.Granada ( 1979)'.

All of Pallett 's books are well written withgood il lustrations. Very useful to avionicsengineers. May be considered as comDanionvolumes to th is book.

7. N.H. Birch and A.E. Branrson , night Briefing J'rtr.&/ors, Volume 3,Radb Aids to Air Nayisatiort.P i tman (4th edi r ion. 1979).

232

A very good book but the emphasis is onoperational use so the theory given is brief.Complementary to th is book.

8. S.E.T. Taylor and H.A. parnrer. Grountl Stutl iesfor Pilots. Volurne l, Radio Aids. Granada(3rd edi r ion, 1979).

Covers the needs of prospective contnrercialpilots in so far as the use of raclio aids isconcerned.

9. J.L. McKinley and R.D. Brenr. Electricitt, andElectronics for Aerospatc Vchit.les. MeGruw-t{ill( 2nd ed i t i on , t 97 l ) .

Covers fundamental theory and briellydescr ibes av ionics systcn)5. Bts ic .

10. D.C. Green, Transmissiott,st 'slarrs, l l . pitnran(1978'1.

I l . D.C. Green, Radio Systenrs, l l . p i rnran ( l97g) .12. D.C. Green, Radio Sl ts tems, I l l . P i t r r ran ( 1979).13. D.( ' . Green, Electronics, l l . p i tman ( l97g) .14. D.C. Green, L ' le t , t rot i lc 's , l l l . p i t rnan (197g).

Al l o l 'Green's books are wel l wr i t ten andi l lust rated. Technic ian level .

15. J .E. F isher and I l .B. Gat land, L lec. tntn ics f r i tmTheory inkt Prac't ice. I)ergrrnon ( lrrd edii ion.t 9761 .

Mainly concerned with design tif circuits.Undergraduate and pract is ing engineers.

16. G.A. Streitrnatter arrd V. I: iore.MicroprocessorsThcory anl Applit 'utr, lrs. Reslorr ( lg79).

One of thc best o l 'a rccent nunrber of bookson mlcroprocessors.

f 7 . A.J. Baden Ful ler , Microwaves. pergamon( lnd ed i t i on . l t ) 7 t )1 .

L lndergraduale lc 've l . Usefu l descr ipt ions ofcolnp()nents and devices.

18. Telec'onntwic'utlons S-r,s/enls Llnits, l-6. Course(-hai r rnan: G. Smol . The Open Univers i ty press( l ( r 7 6 ) .

19 . L.' lec t rr t rnag,t c t i( s a nd L' I ec t ro n ics. Co u rseChair rnan: J . J . Sparkes. The Open Univers i t l ,P ress ( l 97J ) .

Both l5 and l(r are highly recomnrended ascourse ntatcr ia l .

' I am sure I have omitted many books which are

equal in merit to those listed. The editions referredto ( I st unless otherwise indicated) are those withwhich I am familiar; the reader is rCvised to checkthat these are the latest editions.

I have not mentioned any mathematics textbooks.but for those who wish to study aircraft radiosystems in depth, considerable mathematic maturitvis needed. Many books have titles which are variationson the theme 'Mathematics,for

Technicians' most ofwhich will be useful. The Open University is again a

useful source for those requiring applied mathematicsat undergraduate level.

A huge source of material on avionics comes fromorganizations which are not publishing houses. Thereader is advised to consult the publications l ists ofnational aviation authorit ies, such as the CAA andFAA, and also ARINC and ICAO. Aircraft andequipment manufacturers produce comprehensivemanuals of varying standards which will be consultedby the reader, as a matter of course, in the executionof his duties in the aircraft industrv.

83

Glossary

a.c. - Alternating current: current flow whichchanges direction periodically.

Acquisition - The recognition of a signal.ACU - Antenna Coupling Unit.A/D - Analogue to Digital conversion.,,Address - A"Brq,qp of pits which identify a particular

location in memory oi'idiri'e other data source ordestination.

ADF - Automatic Direction Finder: a system capableof automatically giving the bearing to a fixed radiotransmitter.

ADI - Attitude Director Indicator: an instrumentwhich demands attitude changes which, ifexecuted, cause the aircraft to fly a pathdetermined by radio or other sensors.

ADSEL - Address Selective SSR: Britishdevelopment of SSR, compatible with DABS.

a.f.c. - Automatic frequency control: automatictuning of a radio receiver.

AFCS - Automatic Flight Control System.a.g.c. - Automatic gain control of a radio receiver.AID - Aircraft Installation Delay: the time elapsed

between transmission and reception in a RadioAltimeter installation when the aircraft is in thetouchdown position.

Air Data Computer - A unit which senses, evaluatesand outputs quantities associated with altitude,airspeed, vertical speed and Mach number.

Air speed - The speed of an aircraft relative to the airmass through which it is flying.

AIS - Audio Integrating System: the electronicinterface between crew members and audio sourcesand destinations.

Algorithm - A sequence of steps which always leadsof a conclusion.

Alphanumerics - Displayed characters which may beletters or numerals or both.

Altimeter - Pneumatic: a pressure measuring devicecalibrated in feet; also barometric, pressure andservo altimeter. Radio: a system which measuresthe height of an aircraft above the earth's surface.

Altitude hole - [,oss of signal in an f.m.c.w. radar dueto round trip travel time being equal to themodulation period: of importance in Doppler

234

navigation rystems.Altitude trip - A discrete signal from a radio

altimeter which changes state as the aircraft passesthrough a pre-determined altitude.

a.m. - Amplitude modulation: meaningful variationof the amplitude of an r.f. carrier

Analog, analogue - A quantity or signal which variescontinuously and represents some othercontinuously varying quantity;hence an analogcircuit which processes such signals, an analogcomputer which performs arithmetic operations onsuch signals.

AND gate - A logic circuit which gives an output ofI if, and only if, all its inputs are l.

Angle of cut - The angle between two hyperbolic orcircular l.o.p. at their point of intersection.

Antenna, aerial - A device specifically designed toconvert r.f. current flow to electro-magneticradiation, or vice versa.

a.o.c. - Automatic overload control: a circuit whichprevents an excessive rate oftriggering ofatransponder.

APU - Auxiliary Power Unit: a motor-generatorfitted to an aircraft for the purpose of providingground power and starting the main engines.

A/R - Altitude Reporting: automatic codedtransmission of altitude from aircraft to ATC in anSSR system.

Array - A group ofregularly arranged devices, forexample, antennas or memory cells.

ASP - Audio Selection Panel.Associated identity - The identification of co-located

VOR and DM!. beacons by synchronizedtransmissions of the same Morse code charactersfrom each beacon.

Astable - Having no stable state.ATC - Air Traffic Control.ATE - Automatic Test Equipment.ATU - Antenna Tuning Unit.A-type display - A c.r.t. display in which the timebase

deflection is horizontal and the signal deflection isvertical.

Automatic standby - see Signal controlled search.

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Backlash - PleyGdfrdrani:l linkage such asscanner driles,

Balun - Balanced to unbelanced transformer: used.for example. rn conncstint a co.axial feeder(unbalanced) to a (xntrc fed half wave dipoleantenna (balancedl.

Barometric di - The reference pressure levql of abarometric dtimeter as set by the pilot.

Base - The integral value of the number of sym[olsin a countrng system. The central region of abipolar transistor.

Baseline - The line joining two ground stations in ahyperbolic navigation system.

b.c.d. - Binary coded decimal: a positional code inwhich each decimal digit is binary coded as a 4-bitword.

Beam softening - Progressive reduction in gain ofdemand signal channel in landing systems.

Bearing - The angle, measured in a clockwisedirection, between a reference line through theaircraft and a line joining the aircraft and theobject to which bearing is being measured. Thereference line may point to magnetic or true Norfhor be in line with the aircraft's longitudinal axis formagnetic, true or relative bearing respectively.

Beat frequency - The difference frequency resultingwhen two sinusoids are mixed in a nori lineardevice.

b.f.c. - Beat frequency oscillator: an oscillator, theoutput of which is mixed with an incoming c.w.signal in order to produce an audible beatfrequency.

Bidirectional - Refers to an interface port or busline which can transfer data in either direction.

Binary number system - A counting system using 2 asits base and employing the symbols 0 and l.

Binary signal - A signal which can take on one oftwo states, one lepresenting the bit 0, the otherthe bit l.

Bipolar transistor - A solid state device utilizing twotypes of current carriers: holes and electrons;capable of amplifying or switching functions pvhenused in suitable circuits.

Bistable - Having two stable states.Bit - A single binary digit, i.e. the symbol 0 or l.BITE - Built In Test Equipment.Blade antenna - A rigid quarter wave antenna, the

blade shape of which gives operation over a wideband of frequencies; electrical componehts may behoused within the blade for the purpose ofimproving the performance.

BNR - Signal representing a binary number.Bonding - Electrical: interconnecting metal parts

with conductors in order to eliminate potential

differences. Mechanical: joining parts to oneanother by methods other than those involvingbolts, screws and rivets.

Boolean algebra - The algebra of two state, or binary,variables.

Buffer - A circuit used for isolating or matchingpurposes.

Bug - A fault, usually in software. A mark, fixed orset, on a meter face.

Bus - One or more conductors used as aninformation path. A conductor used to cany aparticular power supply to various userequlpments.

Byte - A specific number of bits (usually 8) treatedas a group: 8-bit word.

c - Standard notation for the speed ofpropagation ofe.m. waves in free space: c = 3 x 108 metres persec = 186 000 miles per sec = 162 000 n.m. per sec.

CADC - Central Air Data Computer.Capsule - An evacuated airtight container used to

detect changes in pressure.Capture - The sensing of a radio beam such as occurs

in ILS.CAS - Collision Avoidance System.CDI - Course Deviation Indicator: an instrument

which presents steering signals to the pilot which,if obeyed, cause the aircraft to follow a particularflight path.

CDU - Control Display Unit.Cell - A circuit or device used for storing one

character or word, the location being given by aparticular address. A single chemical source ofelectro motive force (e.m.f.).

Chip - A collection of interconnected electroniccomponents formed on a single silicon wafer.

Choke flange - A type of waveguide joint which. byuse of a short-circuited half-wave stub, gives a goodelectrical connection across the joint.

CIWS - Central Instrument Warning System.Clarif ier - Tuning control for the inserted carrier

oscil lator needed for s.s.b. reception.Clear - To place one or more storage locations in a

particular state, usuallyO.Clock - The basic synchronizing circuit in a system;

the waveform from such a circuit.Clutter - Unwanted radar returns.Co-axial cable - A pair of concentric conductors

separated by an insulating material and used forl ine transmission of r.f. currents up to about4 GHz.

Code - A system of symbols and rules used forreprese nting information such as numbers, lettersand control signals.

2!F

Colocation - VHF navigation and DME beaconssharing the same geographical site; such beaconswill operate on paired frequencies and useassociated ident i ty .

Commutator - A mechanical or electronic rotatingcontact device.

Compass rose - A circular scale marked in degreesand used for indicating aircraft heading.

Computer - A machine or system which performsarithmetic and logical functions;may be analogueor digital, electronic or mechanical.

Contour - Blanking of the strongest signals in aweather radar.

Control bus - A bus used to carry a variety ofcontrol signals.

CPU - Central Processing Unit: a device capable ofexecuting instructions obtained from memory orother sources, a term often used for amicroprocessor.

Cross modulation - Modulation of a desired signal byan unwanted signal.

Crosstrack deviation - The perpendicular distancebetween aircraft position and the desired track.

c.r.t. - Cathode ray tube: an evacuated thermionicdevice which has an electron gun at one end and afluorescent screen at the other; the electron beamfrom the gun writes a pattern on the screen whichis a function of analogue signals applied to thedevice.

Crystal - A frequency sensitive device used todetermine and maintain the frequency ofoscil lators or establish narrow pass or stop bandswithin close l imits. A point contact diode whichmay find use as a mixer or rectifier in microwavesystems.

CVOR - Conventional VOR (beacon).CVR - Cockpit Voice Recorder.c.w. -Continuous wave: continuous transmission of

unmodulated r.f. during the time the transmitter iskeyed.

Cycle - One of a recurring series of events.

D/A - Digital to Analogue conversion.DABS - Discrete Address Beacon System: American

development of SSR; compatible with ADSEL.Ilata - That on which a computer or processor

operates; singular: datum.Data bus - Usually either 8 or l6 bi-directional l ines

capable of carrying information to and from theCPU, memory or interface devices.

Data save memory - A memory device which doesnot lose the data stored in it when power isswitched off. May require a battery.

dB - Decibel: unit of relative power or voltage

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measured on a logarithmic scale with multiplyingfactors of l0 and 20 respectively.

dBm - Unit of power: decibels relative to I rnW.d.c. - Direct current: current f low in one direction

only.d.d.m. - Difference in depth of modulation: refers to

the 90 Hz and 150 Hz modulating frequencies usedin ILS.

Dead reckoning - Calculation of position usingvehicle speed or acceleration, time in motion,direction and the known co-ordinates of the initialposition. The absolute error in a dead reckoningsystem increases, without bound, with distanceflown.

Decca navigator - A c.w. hyperbolic navigationsystem.

Decimal number system - A counting system usingl0 as its base and employing the symbols O,1,2,3 , 4 , 5 , 6 , 7 , 8 , 9 .

Decometer - A phase meter used in the Deccanavigation system; three are employed: purple,red and green.

fletector - An electronic circuit which de-modulatesamplitude or pulse-modulated waveforms.

D/F - Direction Finding.DH - Decision Height: the height at which the

runway should be in view when on an approach.Differentiator - A device which gives an output

proportional to the rate of changb of input.Digital - A system or device using discrete signals to

represent particular values of a varying or fixedquantity numerically; a signal in such a system.

Diode - A semiconductor or thermionic device whichprevents flow of current in one direction.

Dipole, half wave - An antenna consisting of twoco-linear lengths of conductor each one quarterwavelength long at the desired frequency ofoperation. The two poles may form a 'V' shape ifa more omni-directional polar diagram than a figureof eight is required.

Direct access - Capability of reading data from aparticular address in memory without having toaccess through preceding storage area.

Disc - Storage device giving large capacity and almostdirect access.'

Discrete signal - A signal characterized by beinge i t h e r ' o n ' o r ' o f f ' .

Discriminator - A circuit which converts frequencyor phase differences jnto amplitude variations.

DITS - Digital Informition Transfer System:ARINC 429.

DME - Distance Measuring Equipment: a secondaryradar system capable of measuring the slant rangeof a fixed transponder.

Doppler effect - The change in frequency noted ,when a wave source is moving relative to anobserver.

Doppler navigation system * A dead reckoningsystem consisting of a Doppler radar and acomputer which, with heading information from acompass, calculates the position of the aircraft.

Doppler radar - A primary radar system whichutilizes the.Doppler effect to measure two or moreof ground speed, drift angle, longitudinal velocity,lateral velocity, vertical velocity.

Doppler shift - The difference between transmit and. receive frequencies in a system subject to theDoppler effect.

Doppler spectrum - A band of Doppler shiftfrequencies produced by a Doppler radar with afinite beam width.

DPSK - Differential Phase Shift Keying: a form ofdigital rnodulation in which a phase reversalindicates the binary digit ' l ' .

thift angle - The angle between heading and track.d.s.b. - Double side band: transmission of both side

bands of an a.m. wave, the carrier being suppressed.Duplexer - A device which permits sharing of one

circuit or transmission channel by two signals inparticular use of one antenna for reception andtransmission.

DVOR - Doppler VOR (beacon).d.v.s.t. - Direct view storage tube: a type of c.r.t.

with a high intensity display.Dynamic RAM - A type of RAM in which data

stored will fade unless periodically refreshed.

EADI - Electronic ADI: similarly ECDI and EHSLEAROM - ElectricallyAlterable ROM:see EPROM.Earth - see Ground.ECL - Emitter Coupled Logic: logic circuits employing

.bipolar transistors giving very fast operation,reasonable fan-in and very good fan-out.

e.h.t. - Extra high tension: a source of e.m.f., usuallymeasured in kilovolts, used as a supply for c.r.t. 'sand high power transmitters.

Elapsed time - The time between transmission andreception in a radar system.

Electroluminescent - A property of devices whichconvert electrical energy to light.

e.m. wayes - Electromagnetic waves which includeradio and light waves.

e.m.f. - Electromotive force.Enable - A signal which allows a circuit to give an

output; alternatively a gate (signal).Encoding altimeter - A pneumatic altimeter with a

parallel coded output of9 to I I bits representingthe aircraft's height above mean sea level to the

nearest 100 ft.EPROM - Erasable Programmable ROM: a ROM

which, using suitable equipment, can be erased andre-programmed.

Fan-in - The number of inputs that can be handled,usually by a logic circuit.

Fan marker - A position fixing aid for en routeairways navigation: also Z marker.

Fan-out - The number of circuits which can be drivenfrom an output terminal, usually of a logic circuit.

Fast erect - The application of a larger voltage thanrequired for normal running to a gyro in order toreduce the time taken to achieve operating speed.

f.e.t. - Field effect transistor: a solid state deviceutil izing one type of curretrt carrier (c.f. bipolartransistor).

Fetch - The part of a digital computer cycle duringwhich the location of the next instruction isdetermined, that instruction is taken from memoryand entered into a register.

Filter - A circuit which selects wanted or rejectsunwanted signals, usually on the basis of frequency.

Firmwave - Instructions stored in ROM and hencenot easily amended.

Flag - A signal which has two discrete states, one ofwhich (low) usually indicates failure.

Flag bit - The software equivalent of a flip flopwhich may be used as an indicator, for example, toindicate the beginning or end of a piece of data.

Flare - The final phase of a landing during which therdte of descent is reduced with height.

Flash over - Discharge through air betweenconductors across which a large potential exists.

Flight level - With a pneumatic altimeter referenceset at 1013.25 mbar or 29.92 in .Hg the indicatedheight, to the nearest hundred feet, is thc fl ightlevel;the reply from an ATC Transponder to modeC interrogations.

Flight log - A device which records an aircraft'sfl ight path in the horizontal plane, for example,aroller map.

Flip flop - A circuit having two stable states, oftenreferred to as a bi-stable. The circuit remains inone state unti l tr iggered, two triggers beingrequired to revert to the original state. Amono-stable may be referred to as a flip flop.

Flowchart - A diagrammatic representation of asequence of operations which are often algorithmic.

f.m. - Frequency modulation: meaningful variationof the frequency of a carrier.

Fruit - Unwanted SSR replies.Frequency pairing - The permanent association of

frequencies in different systems such as VOR/DME

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and Localizer/GlidesloPe.Freeze - Not allowing updating of a weather radar

picture.f.s.d. - Full scale deflection.

Garbling - Received signals overlapping in time.

Gate - A circuit, the output of which depends on

certain input conditions being met; for example

AND, OR, NAND, NOR gates. A switchingwaveform. The region of an f.e.t. which controlsthe output current.

Gimbal - A frame in which a gyro is mounted so asto allow freedom of movement about an axisperpendicular to the gyro spin axis.

Glidepath/glideslope - The vertical plane approachpath to a landing site. That part of ILS whichprovides vertical guidance.

GPWS - Ground Proximity Warning System.Gray code - A one bit change code.Grey region - A condition of uncertainty.Ground - A point of zero potential; also earth.Ground plane - A surface which completely reflects

e.m. waves and which, at the frequency of interest,behaves as if it extends to infinity in all directions.

Ground speed - The speed of an aircraft projected onto the earth's surface.

Ground wave - A radio wave which follows theearth's surface.

Gunn diode - A solid state device utilizing thebunching of current carriers and finding '

application as an oscillator in microwave systems.Gyroscope, gyro - A spinning mass free to rotate

about one or both of two axes perpendicular toone another and the axis of spin. In the absenie ofexternal forces the spin axis direction is fixed inspace.

Handshake - Electrical verification that a datatransfer has taken place.

Hard data - Data which remains in memory withpower removed.

Hardware-- The sum total of componcnts of a systemwhich have a physical existence.

Hard uV - Hard micro volts: the voltage across anopen circuit load.

Head up display - Equipment which allowsinformation to be visually presented to the pilotwhile looking through the windscreen.

Heading - The tail to nose direction of the aircraftlongitudinal axis measured in degrees clockwisefrom either magnetic or true North.

Height ring - The ground return from a verticalsidelobe in a Weather Radar gives a bright heightring on the p.p.i. centred on the origin.

238

Hexadecimal number system - A counted systemusing l6 as its base and employing the symbolso , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , A , B , C , D , E , F .

High level language - A vocabulary, together withgrammatical rules, in which each statementcorresponds to several machine code instructionsso making the task of programming leSs tedious;examples are BASIC, FORTRAN, ATLAS,PASCAL. etc.

Hot mic. -'A microphone which is permanently liveirrespective of crew operated switch positions;live output is fed to the CVR.

HSI ' Horizontal Situation Indicator: an instrumentdisplaying information from the compass and v.h.f.navigation aids, the latter being in the form ofdeviation signals which, in the case of VOR, relateto a course selected on the same instrument.

Hyperbolic navigation - A means of navigation usinga co-ordinate system of hyperbolic lines defined byground based radio transmitters.

i.c. - Integrated circuit: a device containingelectronic circuits inseparably fabricated as anintegral part of the device itself; often referred toas a chip.

i.f.- Intermediate frequency: the fixed frequency atwhich most of the amplification and selectiontakes place in a superhetrodyne receiver.

IFF - Identification Friend or Foe: military versionof SSR.

IFR - Instrument Flight Rules: apply when VFR areexcluded by virtue of flying in controlled airspaceor lack of visibility.

IIS - Instrument Landing System: the currentstandard ICAO approach aid.

Impatt diode - Impact avalanche transit time diode:a silicon p-n junction reverse biased to itsavalanche threshold; can be arranged to act as anegative resistance in microwave circuits, hence itsuse in oscillators.

Impedance - The total opposition to the flow ofcurrent;in general impedance varies withfrequency.

in.Hg - Inches ol mercury: a unit of pressuremeasurement; the height of a column of mercurysupported by the pressure being measured;29.92 in.Hg= 1013.25 mbar = I standardatmosphere, i.e. the pressure at mean sea level.

Inhibit - A signal which prevents,a particular circuitfrom performing its function.

INS - Inertial Navigation System: a non radionavigation aid which computes the aircraft's position by dead reckoning using the measured accele'ration of an airborne gyro stabilized platform.

trctmction - Coded rnformation which causes acomputer to perform an operation usually on dataavailable at an address which forms part of theinstruction.

lnstruction set - The sum total of instructions whichcan be executed by a particular computer.

lntegrator - A device, the output of which isproportional to the sum of past inputs.

Intensity Modulation - Variation of the velotity ofthe electron beam in a c.r.t. so as to cause acorresponding valiation in intensity of thebrightness of the display.

Interface - The point at which two parts of a systemor two systems meet.

lntprferometer - An antenna array, together withphase discriminators, capable of measuring thedirection of arrival of an e.m. wave.

lnterlace - Time multiplexing of modes ofinterrogation in a secondary radar system; inparticular modes A and C may be interlaced inSSR.

Interrogator - The independent part of3 secondaryradar system.

lntemrpt - The suspension of the execution of acurrent routine while a computer carries out analternative routine; the signal which triggers suchan actron.

I/O - lnput-Output.IRS - Inertial Reference S)'stem: the heart of INS'Isodop - The line joining those points on the earth's

surface from which reflected e.m. waves.originating from an airborne transmitter, suffer thesame Doppler shift.

Jitter - Random variation of frequency; employed inDME and MADGE in order that wanted repliesmay be recognized.

Key - To turn on a radio transmitter.Keyboard - A device which allows an operator to

input- information into a computer; the keys orswitches may represent alphanumeric characters ordedicated functions.

Klystron - A thermionic device employing velocity.nrodulation of an electron beam and capable ofoscillating or amplifying continuously atmicrowave frequencies.

Knot -- The unit of speed used in air and marinenavigation: I knot = I nautical m.p.h.

Lagging edge - The edge of a pulse which occurs lastin time, i.e. the right-hand edge if the pulse isviewed on an oscil loscope or drawn against a timescale increasing to the right.

Lenc - A region bounded by lines of equal phase inthe Decca or Omega navigation systems.

latch - A circuit that may be locked into one of twoparticular stable conditions.

Leading edge - The edge of a pulse which occurs firstin time; cf. lagging edge.

leakage - Unwanted coupling between transmit andreceive antennas.

LED - Light Emitting Diode:'a semiconductor diodewhich emits light when forward biased.

Limiter - A circuit which limits the voltage excursiorof a waveform.

Linear array - A one-dimensional array of antennasarranged to produce a beam which is narrow inone plane, for example, a slotted waveguide.

l.o. - Local oscillator: the circuit used to provide anoutput which is mixed with an incoming r.f inorder to produce the i.f. in a superhetrodynereceiver.

Load - A device which draws current; to connectsuch a device.

Localizer - That part of ILS giving azimuth guidance.

Logic circuit - A circuit which processes binarysignals in accordance with the rules of Booleanalgebra;extensively used in digital systems.

L,ook-up table - A circuit, the output of which is thefunction value corresponding to the input whichrepresents the argument;usually a ROM which, forexample, might store the sine (function values) or alarge number of different angles (arguments).

Loop antenna -' An antenlta consisting of a coil ofwire, usually wound on a ferrite core, which,ideally, reacts only to the changing magnetic fieldin an e.m. wave; an ADF or an Omega loop antennahas two mutually perpendicular coils.

l .o .p. - L ine of posi t ion.l.oran - Long range aid to navigation: a pulsed

hyperbolic position fixing system; Loran A -

obsolete, Loran C - current, Loran D - shortrange version of Loran C (Loran B - neveroperational).

LSA diode - Limited Space - charge Accumulationdiode: similar to Gunn diode.

l.s.b. - Least significant bit in a binary word- Lowersideband: the sideband of an a.m. transmissionwhich is of lower frequency than the carrier.

ISI - Large Scale Integration: a large number of

circuits (usually 1000 or more) on a single i.c.,similarly SSI (small), MSI (medium) and VLSI(very large).

Lubber line - A reference line against which a movlngscale is measured.

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Machine language - The basic instructions, in binarycode, executed by a computer;may be writtendown in hexadecimal or octal code.

MADGE - Microwave Aircraft Digital GuidanceEquipment: a secondary radar system with civilapplication as an approach aid to offshoreplatforms.

Magnetron - A thermionic device employing velocitymodulation of an electron beam in a magnetic fieldand capable of oscillating at microwave frequencieswith very high power output for short periods.

Magslip - A four wire synchro resolver commonlyused to resolve a timebase waveform into sine andcosine components with respect to the azimuthangular position of a radar scanner.

Main bang - The transmission from a pulsed radar.Main lobe - The predominant cigar-shaped part of a

directional antenna polar diagram.Marker beacon - A radio aid transmitting a vertical

directional beam so allowing a pilot to fix hisposition on an app.roach path or airway.

mbar - Mil l ibar: a unit of pressure measurement;1013.25 mbar = I standard atmosphere i.e. thepressure at mean sea level.

Memory - A device rvhich stores information forfuture use.

Microcomputer - A contplete digital computingsystem the hardware of which comprises amicroprocessor and other LSI circuits such asmemory and input/output ports; a single chip withcircuits capable ol' providing control. arithmetic/logic operations. nlentory and input/output.

Microphone, mic. - A transducer which convertssound waves to electrical signals.

Microprocessor - A chip which provides the controland arithmetic/logic operations required by adigital conrputer; a chip caprble of processinginformation in digital tbrrn in accordance with acoded and stored set of instructions in order tocontro l the operat ion o l 'o ther c i rcu i t ry orequlpment.

Microstrip - Transntission lines and p3i\ivecomponents formed bv depositingmetal srrips ot'suitable shapes and dinrensidns on one side of adielectric substrate. the other side of which iscompletely coated with metal acting as a groundplane.

Microwaves - A.vague term used to describe radiofrequencies above 1000 MHz.

MLS - Microwave Landing System: replacement forILS.

m.o. - I\'laster oscillator: the circuit which providesthe frequency reference in a radio transmitter.

Modulation - Variation of one or more

240

characteristics of a carrier wave in order to impressinformation on it.

Monostable - A device or circuit with one stablecondition to which it will return, after a specifieddelay, when disturbed.

Morse code - Combinations of dots and dashesassigned to letters of the alphabet and to numerals.

MOSFET - Metal-Oxide-Semiconductor FET: anf.e.t. where the gate connection is insulated fromthe drain to source channel by an oxide of silicon.

m.p.e.l. - Maximum permissible exposure level: inrelation to microwave radiation.

MP mode - Multipulse mode: a transmission sequencewhich allows lane ambiguities to be resolved in aDecca Navigation System.

m.s.b. - Most significant bit in a binary word.m.s.l. - Mean sea level.MTL - Minimum Triggering Level; the signal

amplitude required to init iate an action;ofsignificance in secondary radar systems.

Multiplexing - Transmitting more than one signalover a single link; signals may be separated in timeor frequency.

Multivibrator - A circuit which can be rn one of twostates, neither, one or both of which may be stable,hence astable. monostable and bistablemultivibrators.

NAND gate - A logic circuit which gives an output of0 if, and only if, a.ll its inputs are l.

NDB - Non-Directional Beacon: a radio transmitterat a known geographical site for use by ADF.

Negative logic - Representation of the bit I by a low-voltage and the bit 0 by a high voltage.

n.m. - Nautical mile: the length of a minute of arc otlongitude on the equator;approximately 6076 fto r 1852 m.

Noise figure - The s.n.r. at the input of a receiverdivided by the s.n.r. at the'output: a measure ofreceivey,'noisiness'.

Nor\-retum to zero - A binary signal which makes atransition only when the bit I needs to berepresented.

Non-volatile membry - A memory that holds dataafter power has been disconnected.

NOR gate - A logic circuit which gives an cutput of Iif, and only if, all i ts inputs are 0.

Null - A no error signal condition in servo systems.

OBI - Omni-Bearing Indicator.OBS - Omni-Bearing Selector: the control with

which the pilot selects the desired VOR radial.Octal number system - A counting system using 8 as

its base and emplo.,ing the symbols 0, I , 2, 3, 4, 5 ,.6 . 7 .

Omega - A c.w. hyperbolic navigation system givingworldwide cover.

Omnidirectional antenna - An antenna which radiatesin or receives from all directions equally; impossibleto achieve in three dimensions.

Omni-station - A VOR ground transmitter.One bit drange coile : A binary code in which only

one bit in the word changes with each count, forexample, Gray code.

One shot - A monostable multivibrator.On-line - Refers to equipment in direct interactive

communication with a computer. Refers to theinstallation and commissioning of a system or thestate of a system when operational.

ONS - Omega Navigation System.OBen centre - A p.p.i. display in which the radial

timebase line has its origin on an arc of non zeroradius, the arc representing zero n.m. range.

OR gate - A logic circuit which gives an output of 0if, and only if, all its inputs are 0.

Orrgn - A point with zero co-ordinates; theilluminated spot on the screen of a c.r.t. at thestart of the timebase.

Orthogonal - At right angles (.in two or threedimensional spaces).

PA - Passenger Address system. Power amplifier.Page - A number of words treated as a group; within

memory typically 4096 consecutive bytes; fordisplay purposes a portion of memory which canconveniently be displayed on a VDU.

Paired frequencies - see Frequency pairing.Parallax eror - The reading error resulting from

viewing an instrument or display from other thanhead on.

Parallel operation - Used by a digital system inwhich one line or circuit deals with only one bitin a word.

Parity - A bit or bits added to a group of bits such asto make the sum of all bits odd or even, hence 'oddor even parity which can be checked for errordetection or correction.

p.e.p. - Peak envelope power: a measure of power ins.s.b. systems; since carrier power cannot be quotedthe r.f. power dissipated at the peak of themodulating waveform is given in specifications.

Performance index - A global measure of the qualityof a Weather Radar system; related to maximumrange.

Peripherals - Units or devices that operate inconjunction with a computer but are not part of it;more generally, units or systems connected to asystem under consideration but not part of thesystem.

Phantom beacon - A waypoint, in an RNAV systembased on VOR/DME, at which no actual radiobeacon exists, its position being defined in termsof bearing and distance from the nearest in-rangebeacon.

Photosensitive - A device which changes its electricalcharacteristics when exposed to light; for exarnplephotocell, photodiode, phototransistor.

PIN diode - P-type/lnsulator/N-type diode: may.beused as a higtr power switch at high frequencies.

Pitot pressure - The dynamic air pressure on anaircraft caused by its movement relative to the airmass surrounding it; dependent on both air speedand static pressure.

Planar array - A two-dimensional array of antennasarranged to produce a beam which is narrow intwo planes, for example a flat plate slotted array asused in Weather Radar sYstems.

p.l.l. - Phase lock loop: a circuit which, by using aphase discriminator to generate an error signal,controls an oscil lator so as to make its outputequal in phase and hence frequency to an input ordemand signal.

Polar diagram - A plot of points of equal fieldstrength which gives a diagrammatic representationof the directional properties of an antenna.

Polarization - The plane of the e-field in an e.m.wave.

Polling - lnterrogation of circuits, units or systems todetermine their state of readiness to receive ortransmit information; scanning interrupt l ines todetermine which, if any, require servicing by acomputer.

Port - A circuit providing electrical access (in or out)to a system, usually a computer system.

Position feedback - A signal representing the position

of the output of a position control servo systemwhich may be compared with an input or demandsignal so as to produce an error signal.

Position fixing - Finding the position of a vehicle inrelation to a ground feature such as a radio beacon.

Positive logic - Representation of the bit I by a highvoltage and the bit 0 by a low voltage.

Potentiometer - A three'terminal variable resistor;the resistance between the wiper terminal andeither of the end terminals varies with adjustment;the resistance between the end terminals is fixed'

p.p.i. - Plan position indicator: a radar display whichshows the relative position of targets in a plane;

targets on the same bearing wil l be superimposedon the display if they have the same slant range.

p.p.m.- Pulse position modulation: transmission of

information by varying tEe position (in time) ofpulses within a grouP'

241

p.r.f. - Pulse repetition frequency: the number ofpulses per second; may be used to quantify pulsegroups per second.

Primary radar - A radar (radio detecting and ranging)system which detects the reflections of its owntransmissions from an uncooperative tardet.

Program pins - A group of connector pins Jome ofwhich may be grounded to select particular modesof operation of a system from several optionsavailable.

hogrammable - The capability of accepting datawhich alters the electrical state of internal circuitryso as to be able to perform one of several possibletasks.

hogramme counter - A CPU register which holds theaddress of the next instruction to be fetched frommemory ; automatically incremented after a fetchcycle.

hogramme, progrtrn - A set of instructions.arranged in an ordered sequence, which determinethe operations carried out by a computer.

PROM - Programmable ROM; programmed aftermanufacture according to the user's silecifications;generally not reprogrammable.

p.r.p. - Pulse repetition period: the reciprocal ofp.r.f.

p.t.t. - Press to transmit or press to talk.Pr.rlse compression - A technique used in radar

systems which allows a relatively wide pulse to betransmitted and a narrow pulse to be processed inthe video circuits.

Q+ode - A code used in R/T operations to identifythe nature of commonly used messages, forexample: QFE - atmospheric pressure at airfieldlevel;QTE - true bearing from ground station;QNH - atmospheric pressure at local sea level.

QE - Quadrantal Error: the error introduced in ADFdue to re-radiation from the airframe.

Qfactor - A measure of the selectivity of a tunedcircuit.

Quadrature - At right angles: a 90o phase differencebetween signals.

Quarter-wave antenna - One of the conductors of ahalf wave dipole mounted on a ground plane whichserves to 'reflect' the quarter-wave conductor so asto produce, effectively, a dipole.

Radar mile - The time taken for an e.m. warc iotravel I n.m. and back, approximately 12.36 ps.

Radial - One of a set of straight half lines terminatingat a fixed point; a line of radio bearing from a'VOR stat ion.

Radio, radar altimeter - see Altimeter.

242

Radome - A detachable aircraft nose cone'made ofdielectric material; more generally any dielectricpanel or antenna cover.

RAM - Random Access Memory: a memory whichaffords immediate access to any location wherebyinformation may be written in or read out.

Raster - The pattern traced by the electron beam ina c. r . t .

RBI - Relative Bearing Indicator: displays therelative bearing of an NDB.

Read - To sense information contained in somedevice such as memory or an input port.

Real time - Computation relating to a process duringthe time that the process occurs; the results ofsuch computation may be used to control therelated process.

Refreshing - The process of restoring the charge ofcapacitors which store the contents of dynamicRAM: at intervals of, for example, slightiy less thanI ps cells are automatically read, the results thenbeing written into the same cells.

Register - A memory device with minimal accesstime used for the temporary storage of binarycoded information; usually a collection of flipflops,one for each bit which can be stored.

Resolver - A device which can give signalsrepresenting the sine and cosine ofan angle.

r.f. - Radio frequency.Rho-Rho-Rho - A position fixing system which

relies on measurement of distance to fixed points:rho-rho systems give ambiguous fixes.

Rho-theta - A position fixing system which relies onmeasurement of distance and bearing of a fixedpoint.

Rising runway - A runway symbol on a flightdirector driven laterally by localizer signals andvertically by radio alt imeter signals.

RMI - Radio Magnetic Indicator: an aircraftinstrument which indicates relative and magneticbearings derived from VOR and ADF.

RNAV - Area navigation: navigation which does notnecessarily confine the aircraft to a fixed airwayssystem.

ROM - Read OnlyMemory: contains permanentlystored information written in during manufacture;random access is available to all storedinformation.

Routine - A list of correctly sequenced computerinstructions; the terms routine and program areoften interchangeable but the former is usuallyapplied to a commonly used set of instructionswhich may be called by other programs.

R/T - Radio telephony: speech communication bymodulated radio waves.

Scan conversion - In position: the conversion ofco-ordinates fronr rho-theta (angle and distance) toX-Y (orthogonal grid). In time: the conversionbetween rate of receipt of data and (usually faster)rate of display.

Scanning - The process of causing a directional beamof e.m. radiation to sweep through a sector ofspace. The process of polling interrupt lines ordevices.

Scott-T transformer - Used in synchro systems forthree-wire to four-wire conversion or vice versa.

Scratchpad - Memory, often on the CPU chip, inwhich data needed for subsequent operations maybe temporarily stored. Display on which data maybe checked before being entered into the mainmemory.

Search - The process leading to acquisition.Second trace echo - An echo or return of a radar

transmission which gives a false range reading sinceit arrives at the receiver after a transmissionsubsequent to the one giving rise to the echo.

Secondary radar - A radar system whi,ch requires acooperative target; a radio l ink is established by aninterrogator, the return or reply being supplied bya transponder on receipt of the interrogation.

Selcal - Selective call ing: automatic alerting systemusing a v.h.f. or h.f. ground to air l ink to aparticular aircraft or group of aircraft.

Sequential access - Reading data from a particularaddress in memory having gone through precedingstorage area in order to find that address; forexample. storage on magnetic tape.

Serial operation - Used by a digital system in whichone line or circuit deals with all the bits in a wordsequentially.

Servo loop - A system in which a signal representingthe output is fed back for comparison with aninput reference, the amplif ied difference, or errorsignal, being used to control the output.

Seven-segment indicator - Seven small bar-shapedlight sources arranged in a figure of eight pattern,sucli that activating particular combinations of theseven sources causes a character to be displayed'

Shadow mask c.r.t. - A type of c.r.t. commonly usedfor colour displays.

Shift register - A register which is accessed seriallyboth in and out; variations can give parallel to serialor ser ia l to para l le l convers ion.

Sidebands - Those bands of frequencies. either sideof the carrier frequency, produced by modulatioit.

Sidelobe - Those parts of a directional antenna polar

diagram either side of the main lobe.Signal controlled search - Allowing DME to

commence searching and hence transmitting only

when squitter is received; known also as automaticstandby.

Skin effect - The tendency of a.c. at h.f. and aboveto avoid the centre of a conductor so reducing theuseful cross sectional area and hence increasingresistance to'current flow.

Skywave -- A radio wave refracted back to earth bythe ionosphere.

Skywave contamination - Reception of a skywaveand ground or space wave simultaneously;anexample of multipath propagation.

Slant range - The actual range of a target in a planewhich is not necessarily horizontal.

SLS - Side Lobe Suppression: a technique used inSSR to prevent interrogation by sidelobes.

s.m.o. - Stabilized Master Oscillator.s.n.r. - Signal to noise power ratio.Soft data - Data which is lost when power is removed.Software - Programs, languages and procedures of a

computer system: no part of software has a' physical existence other than as written down on

paper or stored in code as represented by the stateof a signal or device.

Spacewave - A radio wave which travels in a straightline being neither refracted nor reflected.

Spectrum - The sum total offrequency compon€ntsof a signal.

SPI - Special Position Indicator: an additional pulseof r.f. which may be radiated by an SSRtransponder for identif ication purposes.

Squitter - Random transmission of pairs of pulses ofr.f. from a DME beacon as required for theoperation of signal controlled search.

'

s.s.b. - Single sideband: the transmission of onesideband of an amplitude modulated wave.

SSR - Secondary Surveil lance Radar: a secondaryradar system employing an airborne transponderwhich transmits information relating to identityand/or alt itude; range and bearing is available bymeasuring elapsed time and using a directionalinterrogation.

Stable - A mechanical or electrical state which isautomatically restored after a disturbance.

Static memory - Memory which stores informationin such a way that it doe's not need refreshing.

Static pressure - The air pressure due to still air;decreases with height.

s.t.c. - Sensitivity t ime control: variation of receivergain with time so as to make the output amplitudeindependent of the range of the received signalsource.

Subroutine - A small program or routine which maybe called by a larger program or routine to performa specific operation.

, a r (

243

Super flag - A high level warning signal providingsufficient current at 28 V d.c. to energize a relay soindicating valid or no-warning status.

Superhetrodyne receiver, superhet - A radio receiverin which the received signal is mixed (hetrodyned)with a tuneable locally generated signal in order toproduce a constant i.f.

Swept gain - An alternative term to s.t.c.s.w.r. - Standing wave ratio: see v.s.w.r.Synchro devices - A type of transducer which

converts angular position to an electrical signal orvice versa; all synchros are transformers with bothrotatable and stationary coils. Three wire devicesestablish a unique relationship between the rotorangle and the voltage distribution in the three coilstator; four wire devices establish voltages whichdepend on the sine and cosine of the rotor angleand are thus termed synchro resolvers.Designations are STRX or TR: synchro torquereceiver;STTX or TX: synchro torque transmitter;DTTX or TDX or CDX: differential synchro torquetransmitter; CT: control transformer; RS: synchroresolver.

Synchronization - Changes related in frequency, timeor position.

TACAN - Tactical Air Navigation: a military sysremwhich gives rho-theta navigation;the ranging parthas the same characteristics as DME.

Tape, magnetic - Storage derrice using sequentialaccess.

Telephone, tel. - A transducer which convertselectrical signals to sound waves.

Time base - A waveform which changes linearly withtime; the term is normally applied to the waveformwhich causes regular deflection of the electronbeam in a c.r.t. so as to trace a line representing atime axis, the line also being referred to as atimebase.

Time constant - A measure of the degree ofresistance to change: if a system is subject to anexternal influence which causes it to change fromone state to another and it were to execute thatchange at a rate equal to the initial rate it would' complete the change in a time equal to the timeconstant.

To/From - Refers to selected VOR radials oromni-bearings; if the pilot complies with VORderived steering commands he will be flyingtowards the beacon if a 'to' flag is in view andaway from the beacon if a 'from' flag is in view.

Topple - The effect of allowing the angular velocityof a VRG to fall below that at which it exhibitsthe properties of a gyro.

24

Track - The actual direction of movement of anaircraft projected onto the earth and measured indegrees clockwise from magrretic Nsrth.

Track angle etror - The angular difference betweentrack and desired track.

Transducer - A device which converts input energyofone kind into output energy of another kindwhich bears a known relation to the input.

Transponder - The triggered part of a secondaryradar system.

t.r.f. - Tuned radio frequency: a basic radio receiverin which selection and amplification of themodulated signal is carried out in the r.f. stages,there being no i.f.

Trigger - A signal, usually a pulse, which initiates acircuit action.

Tri-state buffer - A buffer which can assume one oithree states: 0 or I when required to feed a load,high impedance otherwise; the high impedancestate exists in the absence of an enable signal.

TRSB - Time Referenced Scanning Beam: adoptedby the ICAO as the technique to be employed byMLS.

TTL - Transistor-Transistor Logic: logic circuitsemploying bipolar transistors fabricated on i.c.s.giving fast operation and good.fan-in and out.

Tunnel diode - A type of semiconductor devicewhich can be made to exhibit a negative resistancecharacteristic under certain conditions.

TVOR - Terminal VOR: a low power VOR stationsituated at an airfield.

Two.from five code - A code commonly used forfrequency selection; any two from five wires maybe grounded giving ten possible combinations, ohefor each of the digits 0-9.

USART - Universal Synchronous/Asynch ronousReceiver/Transmitter: a device which interfacestwo digital circuits the timing of which may ormay not be related;similarly UART and USRT.

u.s.b. - Upper side band: the sideband of an a.m.transmission which is of higher frequency than thecarrier.

WEROM - Ultra-violet Eraseable ROM: an EPROM.

Varacter diode - A voltage controlled variablecapacitance; the capacitance varies with reversebias.

v.c.o. - Voltage controlled oscil lator.Velocity feedback - A signal which is proportional to

the rate of change of position of the output of aservo system; in position control systems suchfeedback is used to limit hunting, i.e. an excessivenumber of overshoots.

VFR - Visual Flight Rules: apply in uncontrolledairspace when visibility allows; limitations on pilotqualif ications and equipment fitted are minimalunder VFR.

Video signal - The post detector signal in a radarreceiver.

Volatile memory - A memory that loses stored datawhen power is disconnected.

VOR - VHF Omni-Range: a system giving the bearingto a fixed ground radio beacon.

VORTAC - VOR and TACAN beacons on the samegeographical site, i.e. co-located, are termedcol lect ive ly a VORTAC beacon.

VRG Vertical Reference Gyro: a gyro to whichgravlty controlled fbrces are applied by an erectionsystem such as to maintain the spin axis in lhevertical plane; used to give signafs proportional topitch and roll.

v.s.w.r. - Voltage standing wave ratio: the ratio of themaximum to minimum voltage of the standingwave set up on a mismatched line;v.s.w..r. = (l + (pr/pr)o.t y(l _ prlpr)o.s) > Iequality indicating a perf'ect match, p, and p1being the reverse (or reflecteci) power and forward(or incident) power respectiveli.

Waveguide - A hollow, round or rectangular, metaltube which is used to transmit e.m. energy atmicrowave frequencies.

Wavelength - The distande between points ofidentical phase angle; wavelengrh ), = c/f.

Waypoint - A significant point on a route.Wheatstone bridge - An electrical measuring circuit

consisting of four impedance .rrn, .onn.-.ted in aclosed chain; with an excitation supply, connectedto two opposite terminals in the chain the currenrdrawn from the other two terminals is determinedby the ratio of the impedances.

Whip antenna - A quarter-wave antenna made from ath in rneta l rod.

Word - A group of bits treated as an entity; it mayrepresent an instruction, address or quantitv.

Write - To record information in some device such asa memory or output port.

X-Y display - A p.p.i.display on which rargetposition is determined in terms of horizontal (X)

rand vertical (Y) diSplacement from a datum point:it may be referred to as t.v. display.

7*nner diode - A diode operated with reverse bias sothat breakdown occurs, the breakdown voltageremaining constant for a wide range of ,.urrricurrents.

Zone - A region bounded by hyperbolic linesseparated by a distance equal to half a wavelengthof the fundamental frequency of a Decca chain.

245

Exercises

The following exercises are given to test the reader's

knowledge aid understanding of the cont€nt of this

volumei ihis aim will be best achieved if the questions

are not read until one is ready to attempt them'

Having worked through each chapter the exercises

associited with that chapter should be attempted'

Six test papers are given which should be attempted

only aftir ihe whole book has been read' It is

recommended that one hour be devoted to each test

oapert the answers are not given but are to be found

*iittin ttte relevant chapters' Ideally the t€st papers

Joufd be marked independently, however' the reader

strould be able to give an assessment, albeit subjective'

of his attempt at the written answer-type papers'

An accurate assessment can be achieved from test

oaper 6 by giving one mark for each correctly

utt.rnpt.a qu.ttion, deducting half a mark for each' incorrectly attempted question, leaving the-score

unchanged for each question not attempted, then

multiplying the result by 10/6 to give a Percentage'Some of the questions can be used to Senerate

others, for example, those concerned with drawing

block diagrams, ramp tests, l isting controls, etc' could

apply to any of the systems described herein' Even

*iittun extlnded set of questions, as suggested, it is

unlikely that the syllabus for any course will be

completely covered. For example, prospective

aircraft radio maintenance engineers will be required

to satisfy examining bodies and/or employers in the

following areas:

basic electrical and electronic principles

. legislationramp, hangar and workshop practices

reading wiring and schematic diagrams

fault f inding skii ls, etc.

In addition they must show evidence of having had -sufficient experience to assume the responsibilities of

an engineer.

2. Draw a block diagram of an f'm' transmitter'

3. Draw a block diagram of a superhet receiver'

4. Describe four different ways in which binary

digits may be represented in electronic circuits'

5.- Desiribe fwo codes commonly used for airborne

radio frequencY selection.6. Compare analogue and digital representation of

data.7. Discuss briefly the following: I 'C'A.O', ARINC'

ATA 100, national aviation authorities'

Ghapter 2

l . Describe typical antenna tunitrg arrangements io

a general aviation 20 channel h'f ' comms system'

t: List and state the function of typical audio

systems on a large passenger aircraft'

i. Describe *hui hupptnt when a crew member

transmits on v'h.f. comms'4. Draw the block diagram of a CVR showing

clearly the sources of the inPuts'

5. Discuss typical v.h.f ' comms antennas'

e:. itr- to mii ' feeduack in an AIS leads to a howl;

how would You isolate the fault?

Chapter 3

l . List the soulces of errors affecting ADF

operation'2'. Describe quidrantal error and explain how it

maY be ctlrrected.3. Describe how an ADF ground loop swing ts

carried out.4. Draw a situatton diagram and a dual pointer RMI

piesentation for an aircrait on a heading ol 200"(M)

*lth un NDB due north of the aircraft and another at

:OO; ,.tutiu. to which numbers I and 2 ADFs are

Chapter 1

1. Comment oninformation link.

246

tuned resPecttvelY'

the significance of bandwidth in an 5' Exp[ain the basic principles of ADF'

6. Draw an ADF block installation diagram'

Chapter 4

l. Describe the differences between the radiatedsignals from Doppler and conventional VOR stationsand explain why airborne equipment operation is notaffected.

?. Explain the terms automatic and manual VOR.3. Draw a situation diagram for an aircraft on aheading of 090'(M) with i selected radial of 2g0; andwith a fly right demand and from flag showing on theflight director.4. Draw a dual VOR block installation diasram.5. Describe how information derived from"a VORreceiver is presented to the crew.6. Discuss typical VOR antennas.

Chapter 5

l. Explain why a marker sensitivity switch isrequired.2. Describe the need for and a typical implementa_tion of loading compensation fo, an ILS installation.1

Draw a block diagram of a glideslope receiver.t. Describe the outputs from iLS to the aircraft,sinstrumentation and to other systems.5. Descr ibe ILS and marker ihannel l ingarrangements stating how selection is made.6. Describe, in general terms, how you would carryout a ramp test of an ILS.

Chapter 6

l. Explain how, in distance related phase measuringnavigation systems, errors due to changes in clockoffset can be minimized2. List the factors affecting propagation of Omegasignals stating for each, how, if ati l l , compensation ismade.3. Describe briefly the general procedure for skinmapplng prior to deciding the position of an Omesaantenna.4. Describe how Decca chains are designated.5. Explain how lane ambiguities in De"cca may beresolved by using the Mp mode.6 Describe the characteristics of the radiated signalsf r o m a L o r a n C c h a i n .

Chapter 7

l . Describe the four possible modes of operation of

a DME ihterrogator (assume switched on and anywarm up time expired).

?. Explain how echo protection can be achieved inDME.

?l^ Why might a DME interrogator receive less thanIOO% replies'!4. Describe the arrangements for colocatedbeacons.5. Describe, in general terms, how you would carryout a ramp test of DME.6. Draw a simplified DME block schematic diagram.

Chapter 8

l, Explain the need for and the implementation ofSLS in SSR.2. Explain the terms fruit and garbling as applied toSSR.3. What is successive detection and whv is itnecessary in an ATC transponder?4. Draw a block schematic diagram and explain theaction of a decoder in an ATC transponder.5. Describe how barometric alt ituie may beencoded into a form suitable for selecting the reply toa mode C interrogation.6. Draw a typical ATC transponder controller.stating the purpose of each control.

Chapter 9

l . Compare platform and line of sight stabil ization.2. Describe, briefly, video signal processing in adigital weather radar.3. Describe how a p.p.i. display can be used topresent information relating to weather ahead of theaircraft,4. How does a weather radar flat plate antennaachieve a narrow directional beam?5. Describe the safety precautions to be observedwhen operating weather iadar, stating the possibleconsequences ofnot doing so.6. Describe how you would check a waveguide runfor condition7. Discuss contour, STC and AGC in a weatherradar.8.

. Describe how range and bearing resolution maybe improved in a weather radar statine thedisadvantages of tak ing such measurei to g ivermprovement.9 Explain the basic principles of operation of aRyan Stormscope.

247

Chapter 10

l. Describe the Doppler effect as utilized in an

airborne DoPPler radar.2. Explain how a moving antenna Doppler radar

measures drift angle'3. Discuss factors leading to a choice of f'm'c'w'

for Doppler radars.4. Explain the need for a land/sea switch'

5. Driw a simplified block diagram of a Doppler

navigation sYstem.6. Describe, in general terms, how you would carry

out a ramp test of a Doppler navigator.

Chapter 11

l. Distinguish between barometric and radio

altitude commenting on the usefulness of both.

2. Explain the basic principles of an f.m.c.w.

altimeter.3. Why does Doppler shift have a negligible effect

on a radio alt imeter?4. List the sources of error in radio alt imetersystems.5. Explain the main advantages of using constantdifference frequency altimeters over conventionalf.m.c.w. alt imeters.6. Draw a block schematic diagram of a pulsed radio

altimeter.7. Which systems require signals fronl a radio

altimeter? What are the signals involved?

Chapter 12

l . Draw the block diagram of a general areanavigation system.2. Explain the basic principles of RNAV based onVOR/DME beacons.3. Draw and label typical RNAV, deviation andslant range triangles.4. Describe the functions performed by a typical

digital navigation computer being part of a VOR/DN{E

based RNAV system.5. Explain the action of a typical data entry/recorduni t .6. Describe. in general terms, how you would carryout a ranlp test of a VOR/DNIE besed RNAV system.

Chapter 13

l. Describe in general tertns, the instrumentation of

244

the airl iner of the 1980s and beyond, paying particular

attention to the presentation of information from

radio sensors.2. Explain the basic principles of how an automaticdata l ink using h.f. and/or v.h.f. comms could be set

up.3. Compare ADSEL/DABS with current SSR.

4. Explain one way in which satell i tes could be

used for navigation Purposes.5. Describe, briefly, a TRSB MLS'6. Explain how a collision risk measure may bearrived at.7. Compare DITS with current methods ofinformation transt'er.8. Explain the principles of interferometry.

Test Paper 1

l . Compare, briefly, the different types of antenna

which may be found on aircraft.2. Draw a simplif ied block diagram of a computer

and state briefly the function of each block.3. Draw and explain a simple anti 'crosstalknetwork.4. With the aid of a block diagram explain the

action of an h.f. ATU.5. Describe how information from an ADF is

presented to the pilot.6. Explain how displayed noise is reduced in a

digital weather radar.

Test Paper 2

l . Describe two ways of modulating a c.w. carrier'

2. Discuss navigation using radio aids under the

headings, dead reckoning, rho'theta, rho'rho-rho,

theta-theta and hYPerbolic.3. Draw a simplif ied block diagranr and explain the

action of a frequency synthesizer paying particular

attention to se,lection.4. Draw a block diagram of a VOR receiver'

5. Define the terms jitter and squitter.6. Describe, in general terms, how you would carry

out a ramp test of a l ine of sight scannerstabil ization sYStem'

Test Paper 3

l. Describe the modes of propagation used with

airbclrne radio equiPment.

2. Define the terms hardware and software.3. Describe, with the aid of a sketch, a typical h.f.wire antenna installation paying particular attentionto safety features.4 Sketch a typical error curve for ADF stating eE,loop and field alignment errors for your curve.5. Draw and explain a simplified ATC transponderblock diagram.6. Draw a typical weather radar control panelstating purpose ofeach control.

Test Paper 4

l. Describe how a capacitive type antenna operates;list systems which might use such an antenna.2. Explain how an interrupt signal might be used toachieve a data transfer from a radio sensor to anavigation computer.3. Describe briefly the basic principles of ILS.4. List the facilities provided by a typical generalaviation AIS.5. Draw a simple interlock arrangement for a dualh.f. installation.6. Describe how the possibility of lnterference isminimized in a multiple radio altimeter installation.

Test Paper 5

l. Describe, briefly, the fetch-decode-increment.execute cycle of a computer.2. List sources of interference to aircraft radiosystems and state methods used to minim2e theeffects of such sources.3. Discuss the term squelch.4. Explain the principles of lane ambiguityresolution in ONS.5. Describe, in general terms, how you would carryout a ramp test of a VOR.6. At a point in a 180 n.m.leg of a flight thefollowing situation exists:

headingdriftdistance to go

090"(M)10" port80 n.m.

desired track 085"(M)Draw the situation diagram and calculate the acrossdistance reading if the wind and heading haveremained unchanged for the leg so far. (Assume0.017 radians/degree and that sin 0 = 0 if 0 < 0.2radians).

Test Paper 6

l. An e-m. wave of frequency 30 MHz will have awavelength of (a) l0m, (b) lOcm, (c) l0 ft.

2. A loop antenna is used f or (a) VOR and ADF,(b) ADF and Omega, (c) Omega and VOR.

3. Above 30 MHz propagation is by (a) spacewave, (b) sky wave, (c) ground wave.4. Fading at l.f. and m.f. may be due to (a) poor

receiver sensitivity, (b) atmospheric attenuation,(c) simultaneous reception of sky and ground wave.

5. A carrier of amplitude 5 V is amplitudemodulated by a signal of amplitude 3 V, the percentagemodulation is (a) 157o,(b) tbJ%,(c) 60Io.6, A constant amplitude modulating frequency of

500 kHz causes a carrier to vary between 8798.5 MHzand 8801.5 MHz, the modulation index is h\ l l3.(b) 3, (c) 6.

7. Which of the following is not equivalent to23 ' s? (a ) l 0 l I 12 , ( b )27s , ( c ) 15 ,u .

8. The b.c.d. equivalent of 3C16 is (a) 0l l0 0000,(b) I I I 100, (c) 001 I I 100.9. Which of the following, where the l.s.b. is an

odd parity bit, represents 68,e? (a) 10001001,(b) I 1000100, (c) 10001000.10. An address bus usually consists of (a) l6bi-directional lines, (b) l6 unidirectional lines,(c) both bi- and uni-directional l ines.I l. Rho-theta navigation is the basis of(a) VOR/DME, (b) Omega, (c) ADF.12. To avoid earth loops in audio systems cablescreens should be (a) earthed at both ends,(b) not earthed at either end, (c) earthed at one endonly.13. An aircraft v.h.f. communications transceiverwill provide (a) 720 channels at 50 kHz spacing,(b) 360 channels at25 kHz spacing, (c)720 channelsat 25 kHz spacing.14. An aircraft at flight level l0O will be able tocommunicate with a v.h.f. ground station at 100 ftabove m.s.l. at an approximate maximum range of(a) 123 n.m., (b) 12.3 n.m., (c) 135 n.m.15. The minimum 1000 Hz, 30% modulated signallevel to achieve an output s.n.r. of 6 dB from anairline standard v.h.f. receiver is (a) I pV, (b) 3 gW,(c ) 0 .18 x l 0 - r2W.16. A typical a.f. response of a v.h.f. transceiver is(a) 500 to 2000 Hz, (b) 300 to 25OO Hz,(c) 300 to 4000 Hz.17. Typical radiated power from an airlinestandard v.h.f. comms transmitter would be(a) .10 W, (b) 30 w, (c) 50 w.18. In an airline standard h.f. installation the ATUwould reduce the v.s.w.r. of the antenna and ATU

249

19. An ARINC standard h.f. comms system has a l2l3 MHz.typical power output of (a) 400 W p.e.p., (b) 700 W 38. DME gives (a) range, (b) slant range, (c) ground

combined to better than (a) l. l : l , (b) 1.3: I,( c ) 1 .5 : l .

p.e.p., (c) 1000 W p.e.p.20. A Selcal transmission is coded by (a) thenumber of r.f. bursts, (b) the pulse spacing,

(b) fly right;to, (c) fly left; to.26. The frequency range of a VOR receiver is(a) 108 to I 17.95 MHz, (b) 108 to 1 1 1.95 MHz,(c) I l8 to 135.95 MHz.27. The VOR audio identification tone is at

37. TACAN beacons transmit in the range (a)962to l2l3 MHz, (b) 1030 to 1090 MHz, (c) 978 to

speed.39. If a DME is in track subsequent loss of signalwill cause the equipment to (a) search,

(c) Pl ( P2.45. An X-band weather radar will operate at(a\9375 MHz, (b) 5400 MHz, (c) 8800 MHz.6. Second trace echoes are avoided by(a) choosing a p.r.f. greater than some minimum,

(c) the modulating tones used.' (b) automatically standby, (c) go into memory.

ii. An anti-crosstalk network (a) reduces radiated 40. Mode A and C pulse spacing are, respectively

interference, (b) reduces conducted interference, (a) 8 and 2l gs, (b) 12 and 36 ps, (c) 8 and l7 ps.

(c) prevents transmission on both h.f. systems 41 . Selection of 5237 on an ATC transponder will

simultaneously. give the following pulses, in order of transmission

22. Ai r l inestandardADFswi l l ,a f terQEcorrect ion, (a)Fl Al A4B.2ClC2DlD2D4F2,have an error bound of (a) 3', (b) 5", (c) 8". (b) Fl cl Al c2 A4Dl B2D2D4 F2,

23. The average of the absolute values of the peaks (c) Fl Cl C2 Al A4B2Dl D2D4 F2.

of an ADF error curve give (a) field alignment error, 42. The output of an encoding altimeter for an

(b) loop alignment error, (c) QE correction. altitude of 7362 ft would give the code

)4. Itthe varlable phase leads the reference phasc (a) Al A2 A4 Bl B2 C2C4D2,(b) Al A2 A4B/.,

by 30" the magnetic bearing to the VOR station will (c) A2 ,44 Cl C2.Ue 1a) :O', (b) 210", (c) 150b. 43. The -3dB bandwidth of an ATC transponder

25. With a selected omni-bearing of 090' and the receiver is (a) 6 MHz, (b) 3 MHz, (c) 12 MHz.

variable phase lagging the referenci phase by 280'. 4. An ATC transponder should not reply if

the flighi director will show (a) fly right; from, (a) Pl > Y2. + 9 dBs, (b) Pl > P2 + 4.5 dBs'

(a) 1350 Hz, (b) 1000 Hz, (c) 1020 Hz. (b) choosing a p.r.p.greater than some minimum,

)6. Which of the following is a localizer frequency? (c) increasing either or both of the receiver

(a) I10.20 MHz, (b) 109.15 MHz, (c) I12.10 MHz. sensitivity and transmitter power.

)9. In which oi tire following bands does glideslop e 47 . The pilot reports pronounced ground returns tc

operate? (a) h.f., (b) v.h.f., (c) u.h.f. one side of the display, the most likely cause is

3b. If the 90 Hz tone predominates in a localizer (a) system permanehtly in the mapping mode,

receiver the deviatiorr indicator will show (a) on (b) scanner tilt faulty, (c) gyro toppled.

course, (b) fly left, (c) fly right. 48. Broken radial lines are observed on the weather

31. The v.j.w.r. of a localizer antenna should be no radar indicator, the most likely cause is (a) a.f.c.

more than (a) 5: I , (b) 3: I , (c) I .5: I . circuit sweeping, (b) dirt in the magslip,

32. An ONS, using software correction for (c) interference from another radar'predictable errors, will give aircraft position to an 49. A weather radar with a p.r.f. of 200 and a duty

u..uru.y (r.m.s.) of (a) l-2 nm, (b) 0-l nm, cycle of l0 x 10-3 would have a bandwidth of

(c) 2_inm. approximately (a) I MHz, (b)_500 kHz, (c) 3 MHz.

3t. Omega gives worldwide navigation facilities 50. A typicalmemory size for a digital weather

using (a) five stations transmitting on 10.2, I 1.33 and radar employing an X-Y display is (a) 4 kbit'

13.6 kHz, (b) eight stations transmitting on.10.2, (b) 8 kbit, (c) 32 kbit.11.33 and 13.6 kHz,(c) strategically placed stations 51. If Pand R are theVRG pitch and rollsignals

transmitting frequency multiplexed signals. respectively and 0 is the azimuth angle then the

34. A Decca chain usually consists of (a) a mastet demand signal for a line of sight stabilization

and three slaves, (b) a master-slave pat, (c) independent system is (a) Psin0 + Rcos0, (b) Pcos0 + Rsin0,

statlons.35. The usable night range of Decca is about(a) 120 nm, (b) 240 nm, (c) 360 nm.36. Loran C radiates (a) pulsed r.f' at 100 kHz,(b) pulsed r.f. at 14 kHz, (c) c.w. at 100 kHz.

250

(c) Pcos0 x Rsin0.52. An X-band Doppler radar shows a ground

speed of 400 knots, a reasonable estimate of the

Doppler shift would be (a) 5 kHz, (b) 500 Hz,

,c) 12 kHz.

53. An f.m.c.w. Doppler radar operating at afrequency of 8800 MHz, modulated at 500 kHz witha depression angle of 60' will have altitude holes atmultiples of approximately (a) 500 ft, (b) 2000 ft,(c) 8000 ft.54. Wobbulation in a Doppler radar is used toovercome the effects of (a) reflections from thedielectric panel, (b) overwater calibration shift errors,(c) altitude holes.55. A radio altimeter would not be connected to(a) MADGE, (b) a flight director, (c) an ATCtransponder.56. The DH lamp comes on when the aircraft is(a) over the outer marker, (b) below a pilot setbarometric altitude, (c) below a pilot set radioaltitude.57. It is found that the most suitable positions for aradio altimeter transmitter-receiver and antennas

leads to a minimum total feeder length of 9 ft and anantenna€round-antenna path length of 8 ft; whatwould be a suitable AID setting? (a) 20 ft, (b) 40 ft.(c) 57 ft.58. A phantom beacon is (a) a co-locatedVOR/DME beacon with no identity transmission,(b) a TACAN beacon, (c) related to a VOR/DMEbeacon by pilot set distance and bearing.59. An aircraft is 20 n.m.'and 045'(M) from aVOR/DME beacon; the range of the current waypoint,which is due east of the beacon. is shown as 20 nm;approximately how far is the waypoint from thebeacon? (a) 20 nm, (b) 30 nm, (c) 40 nm.60. MADGE mode Cl derives elevation informationby using (a) radio altitude and slant range,(b) interferometry, (c) a directional beam narrow inelevation.

:!,

251

Index

Acquisition, 108ADSEL, 22IAircraft installation delay, l9l, 197Airways marker, 72Altitude hales. 178Angle of cut, 8lAntenna effect, 5 IAntenna tuning unit, 33Antennas. 3Area navigation

display and control, 207generalized system, 202RNAV computer,206standardization, 2 I 3VOR/DME-based principles, 204

ARINC, 19,230Associated identity, I l1ATC 600A (rFR), 120, 136ATC transponder

block diagram operation, 128characteristics, I 35coding, 123controls and operation, 127encoding alt imeter, 132false targets, 125instal lat ion, 126interrogation, l2lprinciples, 121ramp testing, 135rcply,122s L S , 1 2 5 , 1 2 8 , 1 3 3

Attitude director indicator,'l 4, 217Audio integrating system, 37Audio selection panel, 38Auto standby, 106Autoland, 198Automatic data input/output, 210Automatic direction finder

block diagram operation, 47calibration and testing, 55characteristics, 55

' controls and operation,54instal lat ion.52principles, 45system errors, 49

Automatic overload controt. t28Azinruth marks, 154

Balancing half cycle, 15 IB a l u n , 6 3Base line, 8 l. 95Basic rate, l0lBearing resolut ion, 143Beat frequency oscillator, 48

252

BendixBX-2000, 206c N - 2 0 1 1 , 2 lNP-2041A, 206RDR lE ,164,166RDR 1100, r48RDR 1200,146

Boeing747, t4 ,377 6 7 , 2 1 7

British Aerospace advanced flight deck, 219

Cabin interphone, 37, 4OCentilane, 80Clock offset, 82Coastal refract ion,50Cockpit voice recorder, 38,42Coding, 8Coding delay, l0lCollins

Alt 50, 200EFIS-700, 217w x R 7 0 0 , 2 1 6

Collision av oidance, 229Colocated beacons, ll0Conductivi ty map, 85Conto.ur, 143, 154Cosec'beam, 160Cossor 555, 67, 78Course deviation indicator, 63nrosstalk, r.3, 33

i , b i , 2 2 1Data l ink, 219Decca At \C 81 ,102Decca Doppler 70 serie., 183Decca navigator

ambigu i t ies , 97 ,99antenna, 99chain, 95, 96installation, 99Mk 1S/Danac,99-M k 1 9 , 9 9position fixing,96signals, 96

Decision height. 69, 196Decometer ,95 ,96Deviation indicator, 70Direct view storage tube, 144DITS, 230Diurnal effect, 85DME

analogue, I 14' block diagram operation, ll2

channel arrangements, I l0characteristics. I I 7controls and operation, I l2dig i ta l , I l4ground speed, 109ident i f icat ion, 1 06instal lat ion, I I Iinterrogation, 1 09principles, 105ramp test ing, 119rep l y , 109t ime to stat ion, 109

Doppler navigation systemantenna mechanization. I 74beam geometry, I 75characteristics, I 85controls and operation, 184Doppler ef fect , 173Doppler shi f t , 173, 187, 190Doppler spectrum, 175f ixed antenna system, 181instal lat ion, 182movlng antenna system, 179navlgation calculations, I 79overwater errors, 178test ing, 185transmission, 176

Dri f t angle, 174Drift indication (weathcr radar). 160

Echo protect ion, 108, 125Electromagnetic propagation, 4Electromagnet ic radiat ion, 2Electromagnetic spectrum, 4Encoding altimeter, 132, 210

Fan marker, 72Field alignment error, 52Flat p late antenna, 145Flight interphone, 37, 38Frame pulses, 122Freeze, 148Frequency pair ing, 71, I l0F ru i t i ng ,125

Carbling, 125Geomagnetic field, 85Ground conductivity, 85Ground crew cal l system, 38, 42Group count down, 128

Head up display (HUD). 219Hc4}rt ring, 145ILF- ;cmms

rn:enna, 30t{c<i diagram operation, 32*r:rlieristics, 34ccqu=:lr and operation, 3l-m[ls'a. l!re-'-.=- -:iirr- :*- f9

- : - 35tt- :[IO,rl l*.uil*: ::Cr:). 148llcltd

-5lI{ f series. 193

Horizontal situation indicator, 63, 217Hyperbolic navigation principles, 80

ldentification, friend or foc, l2 IIFR

ATC 600A, 120, 136NAV 40IL , 67NAV 4O2AP, 78RD 300, 167

ILsantennas, 76block diagram operation, 72categories, 69characteristics, 77controls and operation, 76coverage,70,72difference in depth of modulation, 70f iequency pair ing,7l'glideslope,

70identification, 70installation, 74loading compensation, 75Iocal izer , 69matker,72principles, 69ramp test ing. 78

Index ing ,102 , 104Instrument flight rules, 202Intensi ty modulat ion, 140Interference, I 3lnterferometer , 223, 226Interrogator , 105, 121 , 227Isodop, I 75

Janus configuration, 175, 187J i t t e r , 106

KhgKCU s65A, 210KDE 566, 210Kr 20416, ' t sK M A 2 0 , 7 5KN 72. 73 , 75K N 7 4 , 2 0 5KN 75, 75K N R 6 6 5 , 2 l lK N S 8 0 , 1 2 , l l lK P I 5 3 3 , l l lKPr 552, 64K X 1 7 5 8 , 7 5K Y 1 9 6 , 2 r , 2 3

Lane ambiguities, 86, 97Lane count, 80Lane slip, 80Lane width, 80Laning, 80Line of position, 79Litton LTN 211,87Loading compensation (lLS), 75Lobe switching. 179Loop alignment error, 52Loop antenna, 3,45Loop swing, 55

' rli

Loran Cblock diagram operation, I 03

"&t&

253

Loran C Gont'd)cha in ,101installation, 102principles, 102signals, 101

MADGEblock diagram opention, 227controls and instrument ation, 227parameters, 229principles, 225

Mapping, 150Marconi

AD 560, 179AD 650,182, 185

Master, 80Maximum permissible exposive level, 164Microcomputer, 9, 26, 94, 208Microwave landing system' 224Middle marker, ?2Modal interference, 85Mode interlace, 122Modulat ion,5Mountain effect, 50MP mode,97Multiplexing, T

NAV 401L oFR) ,67NAV 402AP (tFR),78Night etfect, 50Noise figure, 170Nonspheroidal effect, 85Notching,99

Omegabroadcast pattem, 84characteristics, 95controls and oPeration, 90hardware,93installation, 87interface. 89position fixing, E6ramp testing,95signal propagation, 84skin mapping,8Tsoftware,90stations, 83

Omni-bearing selector, 63Open centre, 15lOutbound search, 108Outer marker, 72Overwater calibration shift error, l?8

Passenger address, 38, 40Passenger entertainment, 38, 4lPercentage reply, 108Performance index, 171Phantom beacon, 58, 203Phase offset, 82Pictorial navigation indicator, 63, I I IPtan position indicator, 140Planar array, 145Polar cap disturbance, 86Polar diagram, 3Precipitation static, l3Program discrete pins, 89

2g

Pulse compression, 14lPulse crowding,215Pulse width limiter, 129

Quadrantal error, 5 IQuadrantal error correction cuwe, 57Quadrantal error corrector, 5 2

Radar range equation, 169Radar systems tester, 165Radio

categorization, 4, I Ihistorical develoPment, Iprinciples, 2receivers and transmitters, 7

Radio altimeterblock diagram operation, 192characteristics, 200factors affecting performance, 19 Iindicator, 196installation, 196installation delay, l9l, 197interface, 198monitoring and self test, 195multiple installation, 199principleq,189ramp testing, 200sinusoidal frequency modulation, 201

Radio magnetic indicator, 53Radome, 147Range marks, 151, 154, 157Range resolution, l4lRate aiding, 86RCA

AVQ 85, 114Data Nav l I , 162Primus 20. 154Pr imus 30 , 155, 16 lPrimus 40, 151Primus 50, 16lPrimus 200, 146,163Weather Scout I, 147

RD 300 (IFR), 167Reciprocal search, 108Residual altitude, 190, 19l, l9EP$o2, Rho3 navigation, 79, 8lRho-theta navigation, 58, 105Rising runway,

'14,199

Ryan Stormscope, 139, 158

Satcom/satnav, 223Scanner stabilization, 143, L57Scott-T-transformer,.93Sea bias, 178Search,107Second trace echoes, 142Secondary surveillance radar, l2lSelcal, 35Sense antenna, 45Sensitivity time control, 143,194Service interphone, 38, 40-sia;i;fibe

;;tression,' I 25, r28, BtSignal activated search, 105Skin mapping, 87Slant range, 105,206Slave, 80

Solar effects,85Sperry FMCS, 217Spike eliminator, 129Spoking, 149Squelch, 22, 23Squ i t te r ,106Static interference, 50Static memory, 108Station interference, 50Station rate. 101Stormscope, 139, 168Suppression, L-band, l l1, 126Swept gain, 143

TACAN, IO5Terminal VOR.58TIC

T24A, t tgT268, T288, T298,78T278,67T30B, 67 , 78T338, T43B, 137T50A, 120

Time referenced scanning beam,224Track, 107Transponder, 105, 12l, 160TRT radio altimeters, 191

Velocity memory, 108Vertical effect, 5 IVHF comms

block diagram operation, 23characteristics, 28controls and operation, 22installation. 20principles, 20ramp testing, 29

Visual flight rutes, 69, 202VLF comms,87voR

automatic, manual, 61, 65block diagram operation, 55characteristics, 65controls and operation, 65conventional, 58doppler,6lidentification, 59installation, 63outpu ts ,63 , 66principles, 58ramp testing,5T

VSWR check, weather ndat,167

Weather radaranalogue, l50beacon mode, 160characteristics, 1 6 2condition and assembly, 164controls and operation, 147digital, rho-theta, 15 Idigital, t.v. (X-Y), 156installation. 146multifunction display, 161other applications, 160principles, 140ramp test, 165safety precautions, 154scanner, 145scanner stabilization, 143, l5'l

Wobbulation, f 78, l9l

Z-matker,72Zone, Decca,96

255