Radar Aids and Navigations

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Defining Navigation

Navigation is the determination of the Position and Velocity of a Moving Vehicle on land, at sea, in the air, or space.

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What are Radio Navigational Aids ?Specialised Electronic and Communication Systems used for navigational purposes.

Long-range systems employ LF and HF bands.

Short-range systems employ VHF and UHF bands.

Microwave frequencies are used for long-range and short-range systems.

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Basic Principles of Radio Navigational Aides

Measuring the distance to a known location by measuring the round trip time of the Electromagnetic Waves.

Utilizing the directional radiation pattern of antennas to find the direction to a known location.

Antenna radiation patterns are of a prime interest in the field of radio navigation aids.

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Triangulation Systems

A

B

X

A and B are landmarks of known coordinates

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Line of Position (LOP)

90o90o

180o

135o

225o

270o

315o

45o

315o

270o

135o 45o

180o

225o

A B

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Bearings Measurement

A

B

X

A

A B

B

North

Heading

base-line

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Radio Direction FindingRDF equipment are used on board of aircraft, ships, and other moving platforms.

RDF systems employ a radio receiver with a rotatable Loop antenna.

Loop antennas have a special radiation pattern which is suited to the RDF function.

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Loop Antenna

one or more loops of wire

connection terminals

baseThe loop can be circular or square.

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Loop Antenna Radiation Pattern

Loop

Top View of Loop Antenna Radiation Pattern

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Loop Antenna Orientation

Directionof

Propagation

TxTx

Top View of Loop Antenna

vertically polarisedradio field

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Maximum Induced Voltage

Directionof

Propagation

TxTx



Magnetic component

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Directionof

Propagation

TxTx



Magnetic component

Minimum Induced Voltage

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Simple Direction Finder

Rx

Field Strengthmeter

RotatingLoop

Antenna

Bearing is found by searching for the minimum signal. Why ?

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Ambiguity of Simple RDF

Top View of Loop Antenna Radiation Pattern

Tx ? Tx ?

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Addition of Sense Antenna

Omnidirectional Antenna(Sense Antenna)

Loop Antenna

Voltage sum of Loop and Sense antennas

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Cardioid Antenna Pattern

Loop Antenna

v is proportional to 1 +cos

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Automatic Direction Finder (ADF)

Amp /2 Rx

SenseAntenna

Loop

IF AmpDetector

IF AmpDetector

Indicator

Motor

SynchroGen.

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Radio BeaconsA Radio Beacon (RB) is a transmitter used with RDF (300 kHz - 3 MHz).

Technically, RDF can be used to take bearing of any transmitter operating at the appropriate radio frequency.

Non-directional Beacons (NDB) are used specifically with RDF.

NDB transmit CW interrupted at regular interval by identification Morse code.

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Simple Radio Range System

/2Tx

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Radio Range Antenna Pattern

North

South

EastWest

A

N N

A

NENW

SESW

Morse Code:A :N :

CWCW

CW CW

CW = A + N

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VHF Omnidirectional Range (VOR)VHF Omnidirectional Range (VOR) provides Azimuth reading at the aircraft.

No direction finding antenna required at the aircraft position.

VOR Beacon, transmits continuous signals on one of 20 assigned channels in the 108 - 118 MHz band.

Limited range to line-of-sight, about 100 km.

Several VORs are needed to cover long distance over land.

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Principle of OperationAntenna with Cardioid pattern, rotating electrically at the rate of 30 Hz (Goniometer).

Bearing information is transmitted in the phase of the signal.

A separate 30 Hz reference signal is transmitted by an Omnidirectional antenna.

Bearings are calculated from the phase information received at the aircraft.

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Cardioid Antenna Pattern0o

360o

90o270o

180o

45o

135o

315o

225o

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VOR Antenna Pattern

North

X

30 revolutionsper second

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North

X

30 revolutionsper second

The signal at x is proportional to 1 + cos The signal at X is effectively AM modulated by 30 Hz

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Bearing Calculation

The amplitude of the signal at x is proportional to 1 + cos where

is the bearing of x relative to North.

The Cardioid pattern is rotated anticlockwise at the rate of 30 revolution s per second.

The amplitude of the signal at x is then proportional to:

1 + cos (230 t +

The phase difference

= cos (230 t +

cos (230 t

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Distance Measuring Equipment

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IntroductionDistance Measuring Equipment (DME) is used to measure the distance between an aircraft and a fixed point.

Generally the fixed point is a VOR or a destination point as an airport.

The useful range of DME systems is about 180 km, corresponding to a roundtrip delay of 1200 m sec.

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System ArchitectureAirborne equipment (Interrogator):

Transmitter

ReceiverGround equipment (Transponder) located at the fixed point.

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Basic Principle

TxRx

Transponder

1025 1025 -- 1150 MHz1150 MHz(126 channels)(126 channels)

962 962 -- 1024 MHz or1024 MHz or1151 1151 -- 1213 MHz1213 MHz

ReplyReply

InterrogationInterrogation

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OperationThe transmitter sends pseudo-random coded “interrogation pulses”

The transponder responds by sending “reply pulses” using the same pseudo-random pattern.

The onboard receiver intercepts the reply pulses that conforms to its pseudo-random pattern.

The time difference between sending the pulses, transponder processing delay, and receiving the pulses is used in calculating the distance.

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Transponder Access MethodThe system employs a random access method.

Up to 50 aircrafts may use the same transponder.

The interrogation pulses are pseudo-random coded.

The receiver only accepts replies that conform to the same pseudo-random pattern.

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Instrument Landing System

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IntroductionInstrument Landing Systems (ILS) are designed to guide an aircraft in its final approach and landing.

Three distinct subsystems are used:

Localiser,

Glide Slope, and

Markers.

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Localiser Plan

LocaliserTx90 Hz

150 Hz

5oRunway

(top view)

DIRECTION OF APPROACH

(108.1 - 111.9 MHz)

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LocaliserAids the pilot in lining up his/her aircraft in the proper azimuth approach to the runway.

Consists of a group of transmitters and antennas positioned at the far end of the runway.

The antenna radiation pattern has a 5o beawidth, centred along the runway.

The VHF frequencies used for the localise are in the range 108.1 to 111.9 MHz.

The useful range of the system is about 40 km.

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Glide Slope Plan

LocaliserTx90 Hz

150 Hz

5oRunway

(top view)

Tx

Tx

1o90 Hz

150 Hz

2.5o - 2.75o

DIRECTION OF APPROACH (side view)

(329.3 - 335.0 MHz)

(108.1 - 111.9 MHz)

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Glide SlopeAids the pilot in making his/her approach at the proper elevation angle to the runway.

Consists of a group of transmitters and antennas positioned beside the runway.

The antenna radiation pattern has a 1o beawidth, and elevated about 2.5o to 2.75o in the direction of approach.

The VHF frequencies used for the glide slope are in the range 329.3 to 335.0 MHz.

The useful range of the system is about 40 km.

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Frequency Arrangement

90 Hz

150 Hz

90 Hz 150 Hz

1o

5o

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Markers Plan

Runway

(top view)

MarkerTx

MarkerTx

400 Hz 1300 Hz

6 - 10 km

1 km


Carrier 75 MHz

75 MHz narrowvertical radio beam

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MarkersMarkers are transmitters that radiate continuous narrow vertical radio beams.

The carrier frequency is 75 MHz modulated by special tones.

The first transmitter is modulated by a 400 Hz tone, located at 6 - 10 km from the end of the runway.

The second transmitter is modulated by a 1300 Hz tone, located at 1 km from the end of the runway.

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Microwave Landing System

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IntroductionMicrowave Landing System (MLS) was designed to handle the increase in air traffic volume and to satisfy the demand for all-weather landing facilities.

Employs microwave frequencies (5 - 5.25 GHz band) rather than VHF.

MLS provides better accuracy, ease of application, and automation. enables landing down to zero ground visibility.

ICAO-approved replacement for the ILS system.

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ArchitectureMLS consists of three subsystems:

Localiser,

Glide Slope, and

Flare.The Localiser and Glide Slope subsystems serve the same purpose as in ILS.

The Flare provides information on the actual height of the aircraft above the plane of the runway.

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Aircraft Height Measurement

FlareTx

d

Runway(end view)

h

h = d tan

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Height CalculationThe angle q made by the aircraft and the runway at the point where the flare transmitter is situated is measured.

The distance d between the flare transmitter and the runway is known.

The height is calculated using the equation:

h = d tan

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Time Reference Scanning Beam

LocaliserTx

Runway

(top view)


A

B

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OperationThe system is based on Time-Reference Scanning Beams (TRSB), referenced to the runway, that enables an aircraft to determine precise azimuth and elevation angles.

The angular position is made by measurement of the time intervals between the TO and FRO azimuth antenna beam scan (typically +/- 60o) and UP and DOWN scan (typically 0o - 30o) of the elevation antenna pattern.

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Calculation

Tt1

The angles (or

are calculated using the equation:

t is the time difference between pulses and T is the scanning period from A to B and back to A

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Hyperbolic Navigation Systems

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IntroductionHyperbolic systems are based on measuring the difference in distance between a craft and two known sites.

Fixed distance difference between the craft and the two sites lie on a hyperbolic locus that passes between the sites.

Definition of terms:

base-line

base-line extension

line of position (LOP)

centre-line (CL)

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Basic Concept

A B

X

CL

LOP

base-linebase-lineextension

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Hyperbolic Navigation

A BD

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360o

d =

Decca ConceptDecca was the first of several hyperbolic electronic navigation systems developed during and after World War II.

It uses radio waves as a means of measuring the difference in distance between sites.

Master/slave radio transmitters arrangement.

Calculating the difference in distance based on phase difference measurement.

The range difference is given by:

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Decca Lines

A B

0o360o

or0o

720o

or0o

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A Master and two Slaves

A B1

B2

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Decca SystemTo overcome ambiguities and increase accuracy.

Three bas-lines system.

A set of four transmitters, a master and three slaves, referred to as a “chain”.

The base lines identified as Red, Green, and Purple.

The system reading is displayed on Red, Green, and Purple “Decometers” respectively.

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Decca Station Topology

purple

green

redA

B3

B2

B1

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Basic System

Tx340 kHz

340 kHz340 kHz

Tx340 kHz

Master Slave

Red340 340

340

Q340 340

Onboard receivingequipment

In phase

Red Decometer

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Possible Working System

Tx340 kHz

X 3X 4

Tx340 kHz

Master Slave

Red85 113.33

Q85 113.33


In phase

Red Decometer

X 1/3X 1/4

340 340

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Practical 85 kHz Scheme

X 3X 4

Tx85 kHz

Master Slave

85113.33

Q85 113.33


In phase

Red Decometer

X 1/6

340 340

113.33

X 8

14.166

85

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Decca Communication Receiver

5f

6f

9f

8f

5f 30f

30f

24f

18f

Purple

Green

RedRed Slave

Purple Slave

Master

Green Slave

Decometers

MultipliersIn Out

8f 24f

9f 18f

24f

18f

30f

6f

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Thank you for your attention

Radar Aids and Navigations

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