The Airports Authority of India

8/7/2019 The Airports Authority of India

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The Airports Authority of India (AAI) was formed on 1st April 1995 by merging the International AirportsAuthority of India and the National Airports Authority with a view to accelerate the integrateddevelopment, expansion and modernization of the operational, terminal and cargo facilities at the airportsin the country conforming to international standards.

AAI TODAY:

Airports Authority of India (AAI) manages 125 Airports, which include 11 International Airports, 08Customs Airports, 81 Domestic Airports and 25 Civil Enclaves at Defence Airfields. AAI also provides AirTraffic Management Services (ATMS) over entire Indian Air Space and adjoining oceanic areas withground installations at all Airports and 25 other locations to ensure safety of Aircraft operations.

The Airports at Ahmedabad, Amritsar, Calicut, Guwahati, Jaipur, Trivandrum, Kolkata & Chennai aretoday International Airports open to operations even by Foreign International Airlines. Besides, theInternational flights, National Flag Carriers operate from Coimbatore, Tiruchirappalli, Varanasi, and GayaAirports too. Tourist Charters now touch Agra, Coimbatore, Jaipur, Lucknow, Patna Airports etc.

AAI has entered into Joint Venture at Mumbai, Delhi, Hyderabad, Bangalore and Nagpur Airports toupgrade these Airports and emulate the world standards.

All major air-routes over Indian landmass are Radar covered (29 Radar installations at 11 locations) alongwith VOR/DVOR coverage (89 installations) co-located with Distance Measuring Equipment (90installations). 52 runways provided with ILS installations with Night Landing Facilities at most of theseAirports and Automatic Message Switching System at 15 Airports.

AAI's successful implementation of Automatic Dependence Surveillance System (ADDS), usingindigenous technology, at Calcutta and Chennai Air Traffic Control Centres, gave India the distinction ofbeing the first country to use this advanced technology in the South East Asian region enabling effectiveAir Traffic Control over oceanic areas using satellite mode of communication. Use of remote controlledVHF coverage, along with satellite communication links, has given added strength to our ATMS. Linkingof 80 locations by V-Sat installations shall vastly enhance Air Traffic Management and in turn safety of

aircraft operations besides enabling administrative and operational control over our extensive Airportnetwork. Performance Based Navigation (PBN) procedures have been implemented at Mumbai, Delhiand Ahmedabad Airports. It is likely to be implemented at other Airports in phased manner.

AAI has undertaken GAGAN project in technological collaboration with Indian Space and ResearchOrganisation (ISRO). In GAGAN project, navigation will be through satellite based system. Navigationsignals received from GPS will be augmented to achieve the navigational requirement of aircrafts. 1stPhase of technology demonstration system has already been successfully completed in February 2008.Development team has been geared up to upgrade the system in operational phase.

AAI has also planned to provide Ground Based Augmentation System (GBAS) at Delhi and MumbaiAirports. GBAS equipment will be capable of providing Category-II (curved approach) landing signals tothe aircrafts. The proposed system will replace the existing instrument landing system in the long run,

which is required at each end of the runway.

The Advanced Surface Movement Guidance and Control System (ASMGCS), at Delhi, has been installedby AAI. It has upgraded operation to runway 28 from CAT-IIIA level to CAT-IIIB level. CAT-IIIA systempermits landing of aircrafts up to visibility of 200mtrs. However, CAT-IIIB will permit safe landing at theAirports when the visibility is below 200mtrs but is above 50mtrs.

AAI's endeavour, in enhanced focus on 'customer's expectations', has evinced enthusiastic response toindependent agency, which has organised customer satisfaction surveys at 30 busy Airports. These


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surveys have enabled us to undertake improvements on aspects recommended by the Airport users. Thereceptacles for our 'Business Reply Letters' at Airports have gained popularity; these responses enableus to understand the changing aspirations of Airport users. During the first year of the millennium, AAIendeavours to make its operations more transparent and the availability of instantaneous information tocustomers by deploying state-of-art Information Technology.

The specific training, focus on improving employee response and professional skill up-gradation, hasbeen manifested. AAI's four training establishments viz. Civil Aviation Training College (CATC) -Allahabad, National Institute of Aviation Management and Research (NIAMR) - Delhi and Fire TrainingCentres (FTCs) at Delhi & Kolkata are expected to be busier than ever before.

AAI has also undertaken initiatives to upgrade training facilities at CATC Allahabad and HyderabadAirport. Aerodrome Visual Simulator (AVS) has been provided at CATC recently and non-radarprocedural ATC simulator equipment is being supplied to CATC Allahabad and Hyderabad Airport.

Organisation

Airports Authority of India (AAI) was constituted by an Act of Parliament and came into being on 1st April1995 by merging erstwhile National Airports Authority and International Airports Authority of India. Themerger brought into existence a single Organization entrusted with the responsibility of creating,upgrading, maintaining and managing civil aviation infrastructure both on the ground and air space in thecountry.

AAI manages 127 airports, which include 14 international airport, 81 domestic airports and 27 civilenclaves at Defence airfields. AAI provides air navigation services over 2.8 million square nautical milesof air space. During the year 2008- 09, AAI handled aircraft movement of 1306532 Nos. [International270345 & Domestic 1036187], Passengers handled 44262137 Nos. [International 1047614 & Domestic33785990] and the cargo handled 499418 tonnes [International 318242 & Domestic 181176].

1. Passenger Facilities

The main functions of AAI inter-alia include construction, modification & management of passengerterminals, development & management of cargo terminals, development & maintenance of aproninfrastructure including runways, parallel taxiways, apron etc., Provision of Communication, Navigationand Surveillance which includes provision of DVOR / DME, ILS, ATC radars, visual aids etc., provision ofair traffic services, provision of passenger facilities and related amenities at its terminals thereby ensuringsafe and secure operations of aircraft, passenger and cargo in the country.

2. Air Navigation Services

In tune with global approach to modernization of Air Navigation infrastructure for seamless navigationacross state and regional boundaries, AAI has been going ahead with its plans for transition to satellitebased Communication, Navigation, Surveillance and Air Traffic Management. A number of co-operationagreements and memoranda of co-operation have been signed with US Federal Aviation Administration,US Trade & Development Agency, European Union, Air Services Australia and the French GovernmentCo-operative Projects and Studies initiated to gain from their experience. Through these activities moreand more executives of AAI are being exposed to the latest technology, modern practices & proceduresbeing adopted to improve the overall performance of Airports and Air Navigation Services.

Induction of latest state-of-the-art equipment, both as replacement and old equipments and also as newfacilities to improve standards of safety of airports in the air is a continuous process. Adoptions of newand improved procedure go hand in hand with induction of new equipment. Some of the major initiativesin this direction are introduction of Reduced Vertical Separation Minima (RVSM) in India air space toincrease airspace capacity and reduce congestion in the air; implementation of GPS And Geo AugmentedNavigation (GAGAN) jointly with ISRO which when operationlized would be one of the four such systems


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in the world.

3. Security

The continuing security environment has brought into focus the need for strengthening security of vitalinstallations. There was thus an urgent need to revamp the security at airports not only to thwart anymisadventure but also to restore confidence of traveling public in the security if air travel as a whole,

which was shaken after 9/11 tragedy. With this in view, a number of steps were taken includingdeployment of CISF for airport security, CCTV surveillance system at sensitive airports, latest and state-of-the-art x-ray baggage inspection systems, premier security & surveillance systems and smart cards foraccess control to vital installations at airports are also being considered to supplement the efforts ofsecurity personnel at sensitive airports.

4. Aerodrome Facilities

In Airports Authority of India, the basic approach to planning of airport facilities has been adopted tocreate capacity ahead of demand in our efforts. Towards implementation of this strategy, a number ofprojects for extension and strengthening of runway, taxi track and aprons at different airports has beentaken up. Extension of runway to 7500 ft. to support operation for Airbus-320/ Boeing 737-800 category ofaircrafts at all airports.

5. HRD Training

A large pool of trained and highly skilled manpower is one of the major assets of Airports Authority ofIndia. Development and Technological enhancements and consequent refinement of operating standardsand procedures, new standards of safety and security and improvements in management techniques callfor continuing training to update the knowledge and skill of officers and staff. For this purpose AAI has anumber of training establishments, viz. NIAMAR in Delhi, CATC in Allahabad, Fire Training Centres atDelhi & Kolkata for in-house training of its engineers, Air Traffic Controllers, Rescue & Fire Fightingpersonnel etc. NIAMAR & CATC are members of ICAO TRAINER programme under which they shareStandard Training Packages (STP) from a central pool for imparting training on various subjects. BothCATC & NIAMAR have also contributed a number of STPs to the Central pool under ICAO TRAINERprogramme. Foreign students have also been participating in the training programme being conducted bythese institutions.

6. IT Implementation

Information Technology holds the key to operational and managerial efficiency, transparency andemployee productivity. AAI initiated a programme to indoctrinate IT culture among its employees and thisis most powerful tool to enhance efficiency in the organization. AAI website with domain namewww.airportsindia.org.in or www.aai.aero is a popular website giving a host of information about theorganization besides domestic and international flight schedules and such other information of interest tothe public in general and passengers in particular.

Functions of AAI

Design, Development, Operation and Maintenance of international and domestic airports and civil enclaves.Control and Management of the Indian airspace extending beyond the territorial limits of the country, asaccepted by ICAO.

Construction, Modification and Management of passenger terminals.Development and Management of cargo terminals at international and domestic airports.Provision of passenger facilities and information system at the passenger terminals at airports.Expansion and strengthening of operation area, viz. Runways, Aprons, Taxiway etc.Provision of visual aids.Provision of Communication and Navigation aids, viz. ILS, DVOR, DME, Radar etc.


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Radaris an object detection system that uses electromagnetic waves to identify the range, altitude,direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weatherformations, and terrain. The term RADARwas coined in 1940 by the U.S. Navy as an acronym forRAdioDetection AndRanging.[1][2][3][4] The term has since entered the English language as a standard word,radar, losing the capitalization. Radar was originally called RDF (Range and Direction Finding) in theUnited Kingdom, using the same acronym as Radio Direction Finding to preserve the secrecy of itsranging capability[5].

A radar system has a transmitter that emits radio waves. When they come into contact with an object theyare scattered in all directions. The signal is thus partly reflected back and it has a slight change ofwavelength (and thus frequency) if the target is moving. The receiver is usually, but not always, in thesame location as the transmitter. Although the signal returned is usually very weak, the signal can beamplified through use of electronic techniques in the receiver and in the antenna configuration. Thisenables radar to detect objects at ranges where other emissions, such as sound orvisible light, would betoo weak to detect. Radar uses include meteorological detection ofprecipitation, measuring oceansurface waves, air traffic control, police detection ofspeedingtraffic, determining the speed of baseballsand by the military.

Several inventors, scientists, and engineers contributed to the development of radar. The first to use radio

waves to detect "the presence of distant metallic objects" was Christian Hlsmeyer, who in 1904demonstrated the feasibility of detecting the presence of a ship in dense fog, but not its distance. [6][7] Hereceived Reichspatent Nr. 165546[8] for his pre-radar device in April 1904, and later patent 169154 [9] for arelated amendment for ranging. He also received a patent [10] in England for his telemobiloscope onSeptember 23, 1904.[6][11]

In August 1917 Nikola Tesla first established principles regarding frequency and power level for the firstprimitive radar units.[12] He stated, "[...] by their [standing electromagnetic waves] use we may produce atwill, from a sending station, an electrical effect in any particularregion of the globe; [with which] we maydetermine the relative position or course of a moving object, such as a vessel at sea, the distancetraversed by the same, or its speed."

Before the Second World Wardevelopments by the British, the Germans, the French, the Soviets and the

Americans led to the modern version of radar. In 1934 the French mile Girardeau stated he was buildinga radar system "conceived according to the principles stated by Tesla" and obtained a patent (FrenchPatent n 788795 in 1934) for a working dual radar system, a part of which was installed on theNormandie liner in 1935.[13][14][15] The same year, American Dr. Robert M. Page tested the first monopulseradar[16] and the Soviet military engineer P.K.Oschepkov, in collaboration with Leningrad ElectrophysicalInstitute, produced an experimental apparatus RAPID capable of detecting an aircraft within 3 km of areceiver.[17] Hungarian Zoltn Bay produced a working model by 1936 at the Tungsram laboratory in thesame vein.

However, it was the British who were the first to fully exploit it as a defence against aircraft attack. Thiswas spurred on by fears that the Germans were developing death rays. Following a study of thepossibility of propagating electromagnetic energy and the likely effect, the British scientists asked by theAir Ministry to investigate, concluded that a death ray was impractical but detection of aircraft appeared

feasible.

[18]

Robert Watson-Watt demonstrated to his superiors the capabilities of a working prototype andpatented the device in 1935 (British Patent GB593017) [15][19][20] It served as the basis for the Chain Homenetwork of radars to defend Great Britain.

The war precipitated research to find better resolution, more portability and more features for radar. Thepost-war years have seen the use of radar in fields as diverse as air traffic control, weather monitoring,astrometry and road speed control.

[edit] Applications of Radar


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The information provided by radar includes the bearing and range (and therefore position) of the objectfrom the radar scanner. It is thus used in many different fields where the need for such positioning iscrucial. The first use of radar was for military purposes; to locate air, ground and sea targets. This hasevolved in the civilian field into applications for aircraft, ships and roads.

In aviation, aircraft are equipped with radar devices that warn of obstacles in or approaching their path

and give accurate altitude readings. They can land in fog at airports equipped with radar-assisted ground-controlled approach (GCA) systems, in which the plane's flight is observed on radar screens whileoperators radio landing directions to the pilot.

Marine radars are used to measure the bearing and distance of ships to prevent collision with other ships,to navigate and to fix their position at sea when within range of shore or other fixed references such asislands, buoys, and lightships. In port or in harbour, Vessel traffic service radar systems are used tomonitor and regulate ship movements in busy waters. Police forces use radar guns to monitor vehiclespeeds on the roads.

Radar has invaded many other fields. Meteorologists use radar to monitorprecipitation. It has become theprimary tool for short-term weather forecasting and to watch forsevere weathersuch as thunderstorms,tornadoes, winter storms precipitation types, etc... Geologists use specialised ground-penetrating radars

to map the composition of the Earth crust. The list is getting longer all the time.

[edit] Principles

The radar dish, or antenna, transmits pulses of radio waves or microwaves which bounce off any object intheir path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually locatedat the same site as the transmitter. The time it takes for the reflected waves to return to the dish enablesa computer to calculate how far away the object is, its radial velocity and other characteristics.

[edit] Reflection

Brightness can indicate reflectivity as in this 1960 weather radar image (ofHurricane Abby). The radar's

frequency, pulse form, polarization, signal processing, and antenna determine what it can observe.

Electromagnetic waves reflect (scatter) from any large change in the dielectric constant ordiamagneticconstants. This means that a solid object in airor a vacuum, or other significant change in atomic densitybetween the object and what is surrounding it, will usually scatter radar (radio) waves. This is particularly


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true forelectrically conductive materials, such as metal and carbon fiber, making radar particularly wellsuited to the detection ofaircraft and ships. Radar absorbing material, containing resistive and sometimesmagnetic substances, is used on military vehicles to reduce radar reflection. This is the radio equivalentof painting something a dark color so that it cannot be seen through normal means.

Radar waves scatter in a variety of ways depending on the size (wavelength) of the radio wave and the

shape of the target. If the wavelength is much shorter than the target's size, the wave will bounce off in away similar to the way light is reflected by a mirror. If the wavelength is much longer than the size of thetarget, the target is polarized (positive and negative charges are separated), like a dipole antenna. This isdescribed by Rayleigh scattering, an effect that creates the Earth's blue sky and red sunsets. When thetwo length scales are comparable, there may be resonances. Early radars used very long wavelengthsthat were larger than the targets and received a vague signal, whereas some modern systems useshorterwavelengths (a few centimeters or shorter) that can image objects as small as a loaf of bread.

Short radio waves reflect from curves and corners, in a way similar to glint from a rounded piece of glass.The most reflective targets for short wavelengths have 90 angles between the reflective surfaces. Astructure consisting of three flat surfaces meeting at a single corner, like the corner on a box, will alwaysreflect waves entering its opening directly back at the source. These so-called corner reflectors arecommonly used as radar reflectors to make otherwise difficult-to-detect objects easier to detect, and are

often found on boats in order to improve their detection in a rescue situation and to reduce collisions.

For similar reasons, objects attempting to avoid detection will angle their surfaces in a way to eliminateinside corners and avoid surfaces and edges perpendicular to likely detection directions, which leads to"odd" looking stealth aircraft. These precautions do not completely eliminate reflection because ofdiffraction, especially at longer wavelengths. Half wavelength long wires or strips of conducting material,such as chaff, are very reflective but do not direct the scattered energy back toward the source. Theextent to which an object reflects or scatters radio waves is called its radar cross section.

[edit] Radar equation

The powerPr returning to the receiving antenna is given by the radar equation:

where

y Pt = transmitter powery Gt = gain of the transmitting antennay Ar = effective aperture (area) of the receiving antennay = radar cross section, or scattering coefficient, of the targety F= pattern propagation factory Rt = distance from the transmitter to the targety Rr = distance from the target to the receiver.

In the common case where the transmitter and the receiver are at the same location, Rt = Rrand the term

Rt Rr can be replaced by R4

, where R is the range. This yields:

This shows that the received power declines as the fourth power of the range, which means that thereflected power from distant targets is very, very small.

The equation above with F= 1 is a simplification forvacuum without interference. The propagation factoraccounts for the effects ofmultipath and shadowing and depends on the details of the environment. In areal-world situation, pathloss effects should also be considered.


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[edit] Polarization

In the transmitted radar signal, the electric field is perpendicular to the direction of propagation, and thisdirection of the electric field is the polarization of the wave. Radars use horizontal, vertical, linear andcircular polarization to detect different types of reflections. For example, circular polarization is used tominimize the interference caused by rain. Linear polarization returns usually indicate metal surfaces.

Random polarization returns usually indicate a fractal surface, such as rocks or soil, and are used bynavigation radars.

[edit] Interference

Radar systems must overcome unwanted signals in order to focus only on the actual targets of interest.These unwanted signals may originate from internal and external sources, both passive and active. Theability of the radar system to overcome these unwanted signals defines its signal-to-noise ratio (SNR).SNR is defined as the ratio of a signal power to the noise power within the desired signal.

In less technical terms, SNR compares the level of a desired signal (such as targets) to the level ofbackground noise. The higher a system's SNR, the better it is in isolating actual targets from the

surrounding noise signals.

[edit] Noise

Signal noise is an internal source of random variations in the signal, which is generated by all electroniccomponents. Noise typically appears as random variations superimposed on the desired echo signalreceived in the radar receiver. The lower the power of the desired signal, the more difficult it is to discernit from the noise (similar to trying to hear a whisper while standing near a busy road). Noise figure is ameasure of the noise produced by a receiver compared to an ideal receiver, and this needs to beminimized.

Noise is also generated by external sources, most importantly the natural thermal radiation of thebackground scene surrounding the target of interest. In modern radar systems, due to the highperformance of their receivers, the internal noise is typically about equal to or lower than the externalscene noise. An exception is if the radar is aimed upwards at clear sky, where the scene is so "cold" thatit generates very little thermal noise.

There will be also flicker noise due to electrons transit, but depending on 1/f, will be much lower thanthermal noise when the frequency is high. Hence, in pulse radar, the system will be always heterodyne.See intermediate frequency.

[edit] Clutter

Clutter refers to radio frequency (RF) echoes returned from targets which are uninteresting to the radaroperators. Such targets include natural objects such as ground, sea, precipitation (such as rain, snow or

hail), sand storms, animals (especially birds), atmospheric turbulence, and other atmospheric effects,such as ionosphere reflections, meteortrails, and three body scatter spike. Clutter may also be returnedfrom man-made objects such as buildings and, intentionally, by radar countermeasures such as chaff.

Some clutter may also be caused by a long radarwaveguide between the radar transceiver and theantenna. In a typical plan position indicator(PPI) radar with a rotating antenna, this will usually be seenas a "sun" or "sunburst" in the centre of the display as the receiver responds to echoes from dust particlesand misguided RF in the waveguide. Adjusting the timing between when the transmitter sends a pulseand when the receiver stage is enabled will generally reduce the sunburst without affecting the accuracy


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of the range, since most sunburst is caused by a diffused transmit pulse reflected before it leaves theantenna.

While some clutter sources may be undesirable for some radar applications (such as storm clouds for air-defence radars), they may be desirable for others (meteorological radars in this example). Clutter isconsidered a passive interference source, since it only appears in response to radar signals sent by the

radar.

There are several methods of detecting and neutralizing clutter. Many of these methods rely on the factthat clutter tends to appear static between radar scans. Therefore, when comparing subsequent scansechoes, desirable targets will appear to move and all stationary echoes can be eliminated. Sea clutter canbe reduced by using horizontal polarization, while rain is reduced with circular polarization (note thatmeteorological radars wish for the opposite effect, therefore using linear polarization the better to detectprecipitation). Other methods attempt to increase the signal-to-clutter ratio.

Constant False Alarm Rate (CFAR, a form ofAutomatic Gain Control, or AGC) is a method relying on thefact that clutter returns far outnumber echoes from targets of interest. The receiver's gain is automaticallyadjusted to maintain a constant level of overall visible clutter. While this does not help detect targetsmasked by stronger surrounding clutter, it does help to distinguish strong target sources. In the past,

radar AGC was electronically controlled and affected the gain of the entire radar receiver. As radarsevolved, AGC became computer-software controlled, and affected the gain with greater granularity, inspecific detection cells.

Radar multipathechoes from a target cause ghosts to appear.

Clutter may also originate from multipath echoes from valid targets due to ground reflection, atmosphericducting orionospheric reflection/refraction. This clutter type is especially bothersome, since it appears tomove and behave like other normal (point) targets of interest, thereby creating a ghost. In a typicalscenario, an aircraft echo is multipath-reflected from the ground below, appearing to the receiver as anidentical target below the correct one. The radar may try to unify the targets, reporting the target at anincorrect height, or - worse - eliminating it on the basis of jitteror a physical impossibility. These problemscan be overcome by incorporating a ground map of the radar's surroundings and eliminating all echoeswhich appear to originate below ground or above a certain height. In newer Air Traffic Control (ATC) radarequipment, algorithms are used to identify the false targets by comparing the current pulse returns, tothose adjacent, as well as calculating return improbabilities due to calculated height, distance, and radartiming.

[edit] Jamming

Radar jamming refers to radio frequency signals originating from sources outside the radar, transmitting inthe radar's frequency and thereby masking targets of interest. Jamming may be intentional, as with anelectronic warfare (EW) tactic, or unintentional, as with friendly forces operating equipment that transmitsusing the same frequency range. Jamming is considered an active interference source, since it is initiatedby elements outside the radar and in general unrelated to the radar signals.

Jamming is problematic to radar since the jamming signal only needs to travel one-way (from the jammerto the radar receiver) whereas the radar echoes travel two-ways (radar-target-radar) and are thereforesignificantly reduced in power by the time they return to the radar receiver. Jammers therefore can bemuch less powerful than their jammed radars and still effectively mask targets along the line of sight fromthe jammer to the radar (Mainlobe Jamming). Jammers have an added effect of affecting radars alongother lines of sight, due to the radar receiver's sidelobes (Sidelobe Jamming).


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The distance resolution and the characteristics of the received signal as compared to noise dependsheavily on the shape of the pulse. The pulse is often modulated to achieve better performance using atechnique known as pulse compression.

Distance may also be measured as a function of time. The radar mile is the amount of time it takes for aradar pulse to travel one nautical mile, reflect off a target, and return to the radar antenna. Since a

nautical mile is defined as exactly1,852 meters, then dividing this distance by the speed of light (exactly299,792,458 meters per second), and then multiplying the result by 2 (round trip = twice the distance),yields a result of approximately 12.36 microseconds in duration.

[edit] Frequency modulation

Another form of distance measuring radar is based on frequency modulation. Frequency comparisonbetween two signals is considerably more accurate, even with older electronics, than timing the signal. Bychanging the frequency of the returned signal and comparing that with the original, the difference can beeasily measured.

This technique can be used in continuous wave radar, and is often found in aircraft radar altimeters. Inthese systems a "carrier" radar signal is frequency modulated in a predictable way, typically varying up

and down with a sine wave or sawtooth pattern at audio frequencies. The signal is then sent out from oneantenna and received on another, typically located on the bottom of the aircraft, and the signal can becontinuously compared using a simple beat frequencymodulator that produces an audio frequency tonefrom the returned signal and a portion of the transmitted signal.

Since the signal frequency is changing, by the time the signal returns to the aircraft the broadcast hasshifted to some other frequency. The amount of that shift is greater over longer times, so greaterfrequency differences mean a longer distance, the exact amount being the "ramp speed" selected by theelectronics. The amount of shift is therefore directly related to the distance traveled, and can be displayedon an instrument. This signal processing is similar to that used in speed detecting Dopplerradar.Example systems using this approach are AZUSA, MISTRAM, and UDOP.

A further advantage is that the radar can operate effectively at relatively low frequencies, comparable tothat used by UHF television. This was important in the early development of this type when highfrequency signal generation was difficult or expensive.

A new terrestrial radar uses low-power FM signals that cover a larger frequency range. The multiplereflections are analyzed mathematically for pattern changes with multiple passes creating a computerizedsynthetic image. Doppler effects are not utilized which allows slow moving objects to be detected as wellas largely eliminating "noise" from the surfaces of bodies of water. Used primarily for detection ofintruders approaching in small boats or intruders crawling on the ground toward an objective.

[edit] Speed measurement

Speed is the change in distance to an object with respect to time. Thus the existing system for measuring

distance, combined with a memory capacity to see where the target last was, is enough to measurespeed. At one time the memory consisted of a user making grease-pencil marks on the radar screen, andthen calculating the speed using a slide rule. Modern radar systems perform the equivalent operationfaster and more accurately using computers.

However, if the transmitter's output is coherent (phase synchronized), there is another effect that can beused to make almost instant speed measurements (no memory is required), known as the Doppler effect.Most modern radar systems use this principle in the pulse-doppler radarsystem. Return signals fromtargets are shifted away from this base frequency via the Doppler effect enabling the calculation of the


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[edit] Antenna design

Radio signals broadcast from a single antenna will spread out in all directions, and likewise a singleantenna will receive signals equally from all directions. This leaves the radar with the problem of decidingwhere the target object is located.

Early systems tended to use omni-directional broadcast antennas, with directional receiver antennaswhich were pointed in various directions. For instance the first system to be deployed, Chain Home, usedtwo straight antennas at right angles for reception, each on a different display. The maximum returnwould be detected with an antenna at right angles to the target, and a minimum with the antenna pointeddirectly at it (end on). The operator could determine the direction to a target by rotating the antenna soone display showed a maximum while the other shows a minimum.

One serious limitation with this type of solution is that the broadcast is sent out in all directions, so theamount of energy in the direction being examined is a small part of that transmitted. To get a reasonableamount of power on the "target", the transmitting aerial should also be directional.

[edit] Parabolic reflector

More modern systems use a steerable parabolic "dish" to create a tight broadcast beam, typically usingthe same dish as the receiver. Such systems often combine two radar frequencies in the same antenna inorder to allow automatic steering, orradar lock.

Parabolic reflectors can be either symmetric parabolas or spoiled parabolas:

y Symmetric parabolic antennas produce a narrow "pencil" beam in both the X and Y dimensionsand consequently have a higher gain. The NEXRADPulse-Dopplerweather radar uses a

symmetric antenna to perform detailed volumetric scans of the atmosphere.

Surveillance radar antenna

y Spoiled parabolic antennas produce a narrow beam in one dimension and a relatively widebeam in the other. This feature is useful if target detection over a wide range of angles is more

important than target location in three dimensions. Most 2D surveillance radars use a spoiled

parabolic antenna with a narrow azimuthal beamwidth and wide vertical beamwidth. This beam

configuration allows the radar operator to detect an aircraft at a specific azimuth but at an

indeterminate height. Conversely, so-called "nodder" height finding radars use a dish with a

narrow vertical beamwidth and wide azimuthal beamwidth to detect an aircraft at a specific

height but with low azimuthal precision.


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[edit] Types of scan

y Primary Scan: A scanning technique where the main antenna aerial is moved to produce ascanning beam, examples include circular scan, sector scan etc

y Secondary Scan: A scanning technique where the antenna feed is moved to produce a scanningbeam, examples include conical scan, unidirectional sector scan, lobe switching etc.

y Palmer Scan: A scanning technique that produces a scanning beam by moving the main antennaand its feed. A Palmer Scan is a combination of a Primary Scan and a Secondary Scan.

[edit] Slotted waveguide

Slotted waveguide antenna)

Main article: Slotted waveguide

Applied similarly to the parabolic reflector, the slotted waveguide is moved mechanically to scan and isparticularly suitable for non-tracking surface scan systems, where the vertical pattern may remainconstant. Owing to its lower cost and less wind exposure, shipboard, airport surface, and harboursurveillance radars now use this in preference to the parabolic antenna.

[edit] Phased array


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Phased array: Not all radar antennas must rotate to scan the sky.

Main article: Phased array

Another method of steering is used in a phased array radar. This uses an array of similar aerials suitably

spaced, the phase of the signal to each individual aerial being controlled so that the signal is reinforced inthe desired direction and cancels in other directions. If the individual aerials are in one plane and thesignal is fed to each aerial in phase with all others then the signal will reinforce in a directionperpendicular to that plane. By altering the relative phase of the signal fed to each aerial the direction ofthe beam can be moved because the direction of constructive interference will move. Because phasedarray radars require no physical movement the beam can scan at thousands of degrees per second, fastenough to irradiate and track many individual targets, and still run a wide-ranging search periodically. Bysimply turning some of the antennas on or off, the beam can be spread for searching, narrowed fortracking, or even split into two or more virtual radars. However, the beam cannot be effectively steered atsmall angles to the plane of the array, so for full coverage multiple arrays are required, typically disposedon the faces of a triangular pyramid (see picture).

Phased array radars have been in use since the earliest years of radar use in World War II, but limitationsof the electronics led to fairly poor accuracy. Phased array radars were originally used formissiledefense. They are the heart of the ship-borne Aegis combat system, and the Patriot Missile System, andare increasingly used in other areas because the lack of moving parts makes them more reliable, andsometimes permits a much larger effective antenna, useful in fighter aircraft applications that offer onlyconfined space for mechanical scanning.

As the price of electronics has fallen, phased array radars have become more and more common. Almostall modern military radar systems are based on phased arrays, where the small additional cost is far offsetby the improved reliability of a system with no moving parts. Traditional moving-antenna designs are stillwidely used in roles where cost is a significant factor such as air traffic surveillance, weather radars andsimilar systems.

Phased array radars are also valued for use in aircraft, since they can track multiple targets. The first

aircraft to use a phased array radar is the B-1B Lancer. The first aircraft fighter to use phased array radarwas the Mikoyan MiG-31. The MiG-31M's SBI-16 Zaslon phased array radar is considered to be theworld's most powerful fighter radar[2]. Phased-array interferometry or, aperture synthesis techniques,using an array of separate dishes that are phased into a single effective aperture, are not typically usedfor radar applications, although they are widely used in radio astronomy. Because of the Thinned arraycurse, such arrays of multiple apertures, when used in transmitters, result in narrow beams at theexpense of reducing the total power transmitted to the target. In principle, such techniques used couldincrease the spatial resolution, but the lower power means that this is generally not effective. Aperturesynthesis by post-processing of motion data from a single moving source, on the other hand, is widelyused in space and airborne radar systems (see Synthetic aperture radar).


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[edit] Frequency bands

The traditional band names originated as code-names during World War II and are still in military andaviation use throughout the world in the 21st century. They have been adopted in the United States bythe IEEE, and internationally by the ITU. Most countries have additional regulations to control which partsof each band are available for civilian or military use.

Other users of the radio spectrum, such as the broadcasting and electronic countermeasures (ECM)industries, have replaced the traditional military designations with their own systems.

[edit] Radar modulators

Modulators act to provide the short pulses of power to the magnetron, a special type of vacuum tube thatconverts DC (usually pulsed) into microwaves. This technology is known as Pulsed power. In this way,the transmitted pulse of RF radiation is kept to a defined, and usually, very short duration. Modulatorsconsist of a high voltage pulse generator formed from an HV supply, a pulse forming network, and a highvoltage switch such as a thyratron.

A klystron tube may also be used as a modulator because it is an amplifier, so it can be modulated by itslow power input signal.

[edit] Radar coolant

Coolanoland PAO (poly-alpha olefin) are the two main coolants used to cool airborne radar equipmenttoday.[citation needed]

The U.S. Navy has instituted a program named Pollution Prevention (P2) to reduce or eliminate thevolume and toxicity of waste, air emissions, and effluent discharges. Because of this Coolanol is usedless often today.

PAO is a synthetic lubricant composition is a blend of a polyol esteradmixed with effective amounts of anantioxidant, yellow metal pacifier and rust inhibitors. The polyol ester blend includes a major proportion ofpoly (neopentyl polyol) ester blend formed by reacting poly(pentaerythritol) partial esters with at least oneC7 to C12 carboxylic acid mixed with an ester formed by reacting a polyol having at least two hydroxylgroups and at least one C8-C10 carboxylic acid. Preferably, the acids are linear and avoid those whichcan cause odours during use. Effective additives include secondary arylamine antioxidants, triazolederivative yellow metal pacifier and an amino acid derivative and substituted primary and secondaryamine and/or diamine rust inhibitor.

A synthetic coolant/lubricant composition, comprising an ester mixture of 50 to 80 weight percent of poly(neopentyl polyol) ester formed by reacting a poly (neopentyl polyol) partial ester and at least one linearmonocarboxylic acid having from 6 to 12 carbon atoms, and 20 to 50 weight percent of a polyol esterformed by reacting a polyol having 5 to 8 carbon atoms and at least two hydroxyl groups with at least one

linear monocarboxylic acid having from 7 to 12 carbon atoms, the weight percents based on the totalweight of the composition.

See also: Radar engineering details

[edit] Radar configurations and types

Main article: Radar configurations and types


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Radars configurations include Monopulse radar, Bistatic radar, Doppler radar, Continuous-wave radar,etc.. depending on the types of hardware and software used. It is used in aviation (Primary andsecondary radar), sea vessels, law enforcement, weather surveillance, ground mapping, geophysicalsurveys, and biological research.

Air traffic control (ATC) is a service provided by ground-based controllers who direct aircraft on the

ground and in the air. The primary purpose of ATC systems worldwide is to separ

ate aircraft to preventcollisions, to organize and expedite the flow of traffic, and to provide information and other support forpilots when able.[1] In some countries, ATC may also play a security or defense role (as in the UnitedStates), or be run entirely by the military (as in Brazil).

Preventing collisions is referred to as separation, which is a term used to prevent aircraft from coming tooclose to each other by use of lateral, vertical and longitudinal separation minima; many aircraft now havecollision avoidance systems installed to act as a backup to ATC observation and instructions. In additionto its primary function, the ATC can provide additional services such as providing information to pilots,weather and navigation information and NOTAMs (NOtices To AirMen).

In many countries, ATC services are provided throughout the majority of airspace, and its services areavailable to all users (private, military, and commercial). When controllers are responsible for separating

some or all aircraft, such airspace is called "controlled airspace" in contrast to "uncontrolled airspace"where aircraft may fly without the use of the air traffic control system. Depending on the type of flight andthe class of airspace, ATC may issue instructions that pilots are required to follow, or merely flightinformation (in some countries known as advisories) to assist pilots operating in the airspace. In all cases,however, the pilot in command has final responsibility for the safety of the flight, and may deviate fromATC instructions in an emergency.

In 1919, the International Commission for Air Navigation (ICAN) was created to develop General Rulesfor Air Traffic. Its rules and procedures were applied in most countries where aircraft operated. The UnitedStates did not sign the ICAN Convention, but later developed its own set of air traffic rules after passageof the Air Commerce Act of 1926. This legislation authorized the Department of Commerce to establish airtraffic rules for the navigation, protection, and identification of aircraft, including rules as to safe altitudesof flight and rules for the prevention of collisions between vessels and aircraft. The first rules were brief

and basic. For example, pilots were told not to begin their takeoff until there is no risk of collision withlanding aircraft and until preceding aircraft are clear of the field. As traffic increased, some airportoperators realized that such general rules were not enough to prevent collisions. They began to provide aform of air traffic control (ATC) based on visual signals. Early controllers, like Archie League (one of thefirst systems flagmen), stood on the field, waving flags to communicate with pilots.

As more aircraft were fitted for radio communication, radio-equipped airport traffic control towers began toreplace the flagmen. In 1930, the first radio-equipped control tower in the United States began operatingat the Cleveland Municipal Airport. By 1935, about 20 radio control towers were operating.

Increases in the number of flights created a need for ATC that was not just confined to airport areas butalso extended out along the airways. In 1935, the principal airlines using the Chicago, Cleveland, andNewark airports agreed to coordinate the handling of airline traffic between those cities. In December, the

first Airway Traffic Control Center opened at Newark, New Jersey. Additional centers at Chicago andCleveland followed in 1936.

The early controllers tracked the position of planes using maps and blackboards and little boat-shapedweights that came to be called shrimp boats. They had no direct radio link with aircraft but usedtelephones to stay in touch with airline dispatchers, airway radio operators, and airport traffic controllers.


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In July 1936, en route ATC became a federal responsibility and the first appropriation of $175,000 wasmade ($2,665,960 today). The Federal Government provided airway traffic control service, but localgovernment authorities where the towers were located continued to operate those facilities.

In 1941, Congress appropriated funds for the Civil Aeronautics Administration (CAA) to construct andoperate ATC towers, and soon the CAA began taking over operations at the first of these towers, with

their number growing to 115 by 1944. In the postwar era, ATC at most airports was eventually to becomea permanent federal responsibility. In response to wartime needs, the CAA also greatly expanded its enroute air traffic control system.

The postwar years saw the beginning of a revolutionary development in ATC, the introduction of radar, asystem that uses radio waves to detect distant objects. Originally developed by the British for militarydefense, this new technology allowed controllers to see the position of aircraft tracked on visual displays.In 1946, the CAA unveiled an experimental radar-equipped tower for control of civil flights. By 1952, theagency had begun its first routine use of radar for approach and departure control. Four years later, itplaced a large order for long-range radars for use in en route ATC.

In 1960, the FAA began successful testing of a system under which flights in certain positive control areaswere required to carry a radar beacon, called a transponder that identified the aircraft and helped to

improve radar performance. Pilots in this airspace were also required to fly on instruments regardless ofthe weather and to remain in contact with controllers. Under these conditions, controllers were able toreduce the separation between aircraft by as much as half the standard distance.

For many years, pilots had negotiated a complicated maze of airways. In September 1964, the FAAinstituted two layers of airways, one from 1,000 to 18,000 feet (305 to 5,486 meters) above ground leveland the second from 18,000 to 45,000 feet (13,716 m) above mean sea level. It also standardized aircraftinstrument settings and navigation checkpoints to reduce the controllers' workload.

From 1965 to 1975, the FAA developed complex computer systems that would replace the plasticmarkers for tracking aircraft thereby modernizing the National Airspace System. Controllers could nowview information sent by aircraft transponders to form alphanumeric symbols on a simulated threedimensional radar screen. The system allowed controllers to focus on providing separation by automating

complex tasks.

The FAA established a Central Flow Control Facility in April 1970, to prevent clusters of congestion fromdisrupting the nationwide air traffic flow. This type of ATC became increasingly sophisticated andimportant, and in 1994, the FAA opened a new Air Traffic Control System Command Center withadvanced equipment.

In January 1982, the FAA unveiled the National Airspace System (NAS) Plan. The plan called formodernized flight service stations, more advanced systems for ATC, and improvements in ground-to-airsurveillance and communication. Better computers and software were developed, air route traffic controlcenters were consolidated, and the number of flight service stations reduced. New Doppler Radars andbetter transponders complemented automatic, radio broadcasts of surface and flight conditions.

In July 1988, the FAA selected IBM to develop the new multi-billion-dollar Advanced Automation System(AAS) for the Nation's en route ATC centers. AAS would include controller workstations, called "sectorsuites," that would incorporate new display, communications and processing capabilities. The system hadupgraded hardware enabling increased automation of complex tasks.

In December 1993, the FAA reviewed its order for the planned AAS. IBM was far behind schedule andhad major cost overruns. In 1994 the FAA simplified its needs and picked new contractors. The revisedmodernization program continued under various project names. In 1999, controllers began their first useof an early version of the Standard Terminal Automation Replacement System, which included new


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displays and capabilities for approach control facilities. During the following year, FAA completeddeployment of the Display System Replacement, providing more efficient workstations for en routecontrollers.

In 1994, the concept of Free Flight was introduced. It might eventually allow pilots to use on boardinstruments and electronics to maintain a safe distance between planes and to reduce their reliance on

ground controllers. Full implementation of this concept would involve technology that made use of theGlobal Positioning System to help track the position of aircraft. In 1998, the FAA and industry beganapplying some of the early capabilities developed by the Free Flight program.

Current studies to upgrade ATC include the Communication, Navigation and Surveillance for Air TrafficManagement System that relies on the most advanced aircraft transponder, a global navigation satellitesystem, and ultra-precise radar. Tests are underway to design new cockpit displays that will allow pilots tobetter control their aircraft by combining as many as 32 types of information about traffic, weather, andhazards.

[edit] Airport control

Inside the So Paulo/Guarulhos International Airport's tower, Latin America's second busiest airport.

The primary method of controlling the immediate airport environment is visual observation from the airporttraffic control tower (ATCT). The ATCT is a tall, windowed structure located on the airport grounds.Aerodrome orTowercontrollers are responsible for the separation and efficient movement of aircraft andvehicles operating on the taxiways and runways of the airport itself, and aircraft in the air near the airport,generally 2 to 5 nautical miles (3.7 to 9.2 km) depending on the airport procedures.

Radardisplays are also available to controllers at some airports. Controllers may use a radar systemcalled Secondary Surveillance Radarfor airborne traffic approaching and departing. These displaysinclude a map of the area, the position of various aircraft, and data tags that include aircraft identification,speed, heading, and other information described in local procedures.

The areas of responsibility for ATCT controllers fall into three general operational disciplines; Local

Control or Air Control, Ground Control, and Flight Data/Clearance Delivery other categories, such asApron Control or Ground Movement Planner, may exist at extremely busy airports. While each ATCT mayhave unique airport-specific procedures, such as multiple teams of controllers ('crews') at major orcomplex airports with multiple runways, the following provides a general concept of the delegation ofresponsibilities within the ATCT environment.

[edit] Ground Control


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Ground Control (sometimes known as Ground Movement Control abbreviated to GMC or SurfaceMovement Control abbreviated to SMC) is responsible for the airport "movement" areas, as well as areasnot released to the airlines or other users. This generally includes all taxiways, inactive runways, holdingareas, and some transitional aprons or intersections where aircraft arrive, having vacated the runway ordeparture gate. Exact areas and control responsibilities are clearly defined in local documents andagreements at each airport. Any aircraft, vehicle, or person walking or working in these areas is requiredto have clearance from Ground Control. This is normally done via VHF/UHF radio, but there may bespecial cases where other processes are used. Most aircraft and airside vehicles have radios. Aircraft orvehicles without radios must respond to ATC instructions via aviation light signals or else be led byvehicles with radios. People working on the airport surface normally have a communications link throughwhich they can communicate with Ground Control, commonly either by handheld radio or even cellphone. Ground Control is vital to the smooth operation of the airport, because this position impacts thesequencing of departure aircraft, affecting the safety and efficiency of the airport's operation.

Some busier airports have Surface Movement Radar (SMR), such as, ASDE-3, AMASS orASDE-X,designed to display aircraft and vehicles on the ground. These are used by Ground Control as anadditional tool to control ground traffic, particularly at night or in poor visibility. There are a wide range ofcapabilities on these systems as they are being modernized. Older systems will display a map of theairport and the target. Newer systems include the capability to display higher quality mapping, radartarget, data blocks, and safety alerts, and to interface with other systems such as digital flight strips.

[edit] Local Control or Air Control

Local Control (known to pilots as "Tower" or "Tower Control") is responsible for the active runwaysurfaces. Local Control clears aircraft for takeoff or landing, ensuring that prescribed runway separationwill exist at all times. If Local Control detects any unsafe condition, a landing aircraft may be told to " go-around" and be re-sequenced into the landing pattern by the approach or terminal area controller.

Within the ATCT, a highly disciplined communications process between Local Control and GroundControl is an absolute necessity. Ground Control must request and gain approval from Local Control tocross any active runway with any aircraft or vehicle. Likewise, Local Control must ensure that GroundControl is aware of any operations that will impact the taxiways, and work with the approach radar

controllers to create "holes" or "gaps" in the arrival traffic to allow taxiing traffic to cross runways and toallow departing aircraft to take off. Crew Resource Management (CRM) procedures are often used toensure this communication process is efficient and clear, although this is not as prevalent as CRM forpilots.

[edit] Flight Data / Clearance Delivery

Clearance Delivery is the position that issues route clearances to aircraft, typically before they commencetaxiing. These contain details of the route that the aircraft is expected to fly after departure. ClearanceDelivery or, at busy airports, the Traffic Management Coordinator (TMC) will, if necessary, coordinate withthe en route center and national command center or flow control to obtain releases for aircraft. Often,however, such releases are given automatically or are controlled by local agreements allowing "free-flow"

departures. When weather or extremely high demand for a certain airport or airspace becomes a factor,there may be ground "stops" (or "slot delays") or re-routes may be necessary to ensure the system doesnot get overloaded. The primary responsibility of Clearance Delivery is to ensure that the aircraft have theproper route and slot time. This information is also coordinated with the en route center and GroundControl in order to ensure that the aircraft reaches the runway in time to meet the slot time provided bythe command center. At some airports, Clearance Delivery also plans aircraft pushbacks and enginestarts, in which case it is known as the Ground Movement Planner (GMP): this position is particularlyimportant at heavily congested airports to prevent taxiway and apron gridlock.


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Flight Data (which is routinely combined with Clearance Delivery) is the position that is responsible forensuring that both controllers and pilots have the most current information: pertinent weather changes,outages, airport ground delays/ground stops, runway closures, etc. Flight Data may inform the pilotsusing a recorded continuous loop on a specific frequency known as the Automatic Terminal InformationService (ATIS).

[edit] Approach and terminal control

PotomacTRACON, Washington, D.C., United States.

Main article: Terminal Control Center

Many airports have a radar control facility that is associated with the airport. In most countries, this isreferred to as TerminalControl; in the U.S., it is often still referred to as a TRACON (Terminal RadarApproach Control.) While every airport varies, terminal controllers usually handle traffic in a 30 to 50nautical mile (56 to 93 km) radius from the airport. Where there are many busy airports in close proximity,one consolidated TRACON may service all the airports. The airspace boundaries and altitudes assignedto a TRACON, which vary widely from airport to airport, are based on factors such as traffic flows,neighboring airports and terrain. A large and complex example is the London Terminal Control Centre

which controls traffic for five main London airports up to 20,000 feet (6,100 m) and out to 100 nauticalmiles (190 km).

Terminal controllers are responsible for providing all ATC services within their airspace. Traffic flow isbroadly divided into departures, arrivals, and overflights. As aircraft move in and out of the terminalairspace, they are handed off to the next appropriate control facility (a control tower, an en-route controlfacility, or a bordering terminal or approach control). Terminal control is responsible for ensuring thataircraft are at an appropriate altitude when they are handed off, and that aircraft arrive at a suitable ratefor landing.

Not all airports have a radar approach or terminal control available. In this case, the en-route center or aneighboring terminal or approach control may co-ordinate directly with the tower on the airport and vectorinbound aircraft to a position from where they can land visually. At some of these airports, the tower may

provide a non-radarprocedural approach service to arriving aircraft handed over from a radar unit beforethey are visual to land. Some units also have a dedicated approach unit which can provide the proceduralapproach service either all the time or for any periods of radar outage for any reason.

[edit] En-route, center, or area control


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The training department at the Washington Air Route Traffic Control Center, Washington, D.C., United

States.

Main article: Area Control Center

ATC provides services to aircraft in flight between airports as well. Pilots fly under one of two sets of rulesfor separation: Visual Flight Rules (VFR) orInstrument Flight Rules (IFR). Air traffic controllers havedifferent responsibilities to aircraft operating under the different sets of rules. While IFR flights are under

positive control, in the US VFR pilots can request flight following, which provides traffic advisory serviceson a time permitting basis and may also provide assistance in avoiding areas of weather and flightrestrictions. In the UK, a pilot can request for "Deconfliction Service", which is similar to flight following.

En-route air traffic controllers issue clearances and instructions for airborne aircraft, and pilots arerequired to comply with these instructions. En-route controllers also provide air traffic control services tomany smaller airports around the country, including clearance off of the ground and clearance forapproach to an airport. Controllers adhere to a set of separation standards that define the minimumdistance allowed between aircraft. These distances vary depending on the equipment and proceduresused in providing ATC services.

[edit] General characteristics

En-route air traffic controllers work in facilities called Area Control Centers, each of which is commonlyreferred to as a "Center". The United States uses the equivalent term Air Route Traffic Control Center(ARTCC). Each center is responsible for many thousands of square miles of airspace (known as a FlightInformation Region) and for the airports within that airspace. Centers control IFR aircraft from the timethey depart from an airport or terminal area's airspace to the time they arrive at another airport or terminalarea's airspace. Centers may also "pick up" VFR aircraft that are already airborne and integrate them intothe IFR system. These aircraft must, however, remain VFR until the Center provides a clearance.

Center controllers are responsible for climbing the aircraft to their requested altitude while, at the sametime, ensuring that the aircraft is properly separated from all other aircraft in the immediate area.Additionally, the aircraft must be placed in a flow consistent with the aircraft's route of flight. This effort iscomplicated by crossing traffic, severe weather, special missions that require large airspace allocations,and traffic density. When the aircraft approaches its destination, the center is responsible for meetingaltitude restrictions by specific points, as well as providing many destination airports with a traffic flow,which prohibits all of the arrivals being "bunched together". These "flow restrictions" often begin in themiddle of the route, as controllers will position aircraft landing in the same destination so that when theaircraft are close to their destination they are sequenced.

As an aircraft reaches the boundary of a Center's control area it is "handed off" or "handed over" to thenext Area Control Center. In some cases this "hand-off" process involves a transfer of identification anddetails between controllers so that air traffic control services can be provided in a seamless manner; inother cases local agreements may allow "silent handovers" such that the receiving center does not


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require any co-ordination if traffic is presented in an agreed manner. After the hand-off, the aircraft isgiven a frequency change and begins talking to the next controller. This process continues until theaircraft is handed off to a terminal controller ("approach").

[edit] Radar coverage

Since centers control a large airspace area, they will typically use long range radar that has the capability,at higher altitudes, to see aircraft within 200 nautical miles (370 km) of the radar antenna. They may alsouse TRACON radar data to control when it provides a better "picture" of the traffic or when it can fill in aportion of the area not covered by the long range radar.

In the U.S. system, at higher altitudes, over 90% of the U.S. airspace is covered by radar and often bymultiple radar systems; however, coverage may be inconsistent at lower altitudes used by unpressurizedaircraft due to high terrain or distance from radar facilities. A center may require numerous radar systemsto cover the airspace assigned to them, and may also rely on pilot position reports from aircraft flyingbelow the floor of radar coverage. This results in a large amount of data being available to the controller.To address this, automation systems have been designed that consolidate the radar data for thecontroller. This consolidation includes eliminating duplicate radar returns, ensuring the best radar for eachgeographical area is providing the data, and displaying the data in an effective format.

Centers also exercise control over traffic travelling over the world's ocean areas. These areas are alsoFIRs. Because there are no radar systems available for oceanic control, oceanic controllers provide ATCservices using procedural control. These procedures use aircraft position reports, time, altitude, distance,and speed to ensure separation. Controllers record information on flight progress strips and in speciallydeveloped oceanic computer systems as aircraft report positions. This process requires that aircraft beseparated by greater distances, which reduces the overall capacity for any given route.

Some Air Navigation Service Providers (e.g. Airservices Australia, The Federal Aviation Administration,NAVCANADA, etc.) have implemented Automatic Dependent Surveillance - Broadcast (ADS-B) as part oftheir surveillance capability. This new technology reverses the radar concept. Instead of radar "finding" atarget by interrogating the transponder, the ADS-equipped aircraft sends a position report as determinedby the navigation equipment on board the aircraft. Normally, ADS operates in the "contract" mode wherethe aircraft reports a position, automatically or initiated by the pilot, based on a predetermined timeinterval. It is also possible for controllers to request more frequent reports to more quickly establishaircraft position for specific reasons. However, since the cost for each report is charged by the ADSservice providers to the company operating the aircraft, more frequent reports are not commonlyrequested except in emergency situations. ADS is significant because it can be used where it is notpossible to locate the infrastructure for a radar system (e.g. over water). Computerized radar displays arenow being designed to accept ADS inputs as part of the display. This technology is currently used inportions of the North Atlantic and the Pacific by a variety of states who share responsibility for the controlof this airspace.

Precision approach radars are commonly used by military controllers of airforces of several countries,to assist the Pilot in final phases of landing in places where Instrument Landing System and othersophisticated air borne equipements are unavaiable to assist the pilots in marginal ornear zero visibility

conditions. This procedure is also called Talkdowns.

[edit] Flight traffic mapping

The mapping of flights in real-time is based on the air traffic control system. In 1991, data on the locationof aircraft was made available by the Federal Aviation Administration to the airline industry. The NationalBusiness Aviation Association (NBAA), the General Aviation Manufacturers Association, the AircraftOwners & Pilots Association, the Helicopter Association International, and the National Air Transportation


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Association petitioned the FAA to make ASDI information available on a "need-to-know" basis.Subsequently, NBAA advocated the broad-scale dissemination of air traffic data. The Aircraft SituationalDisplay to Industry (ASDI) system now conveys up-to-date flight information to the airline industry and thepublic. Some companies that distribute ASDI information are FlightExplorer, FlightView, and FlyteComm.Each company maintains a website that provides free updated information to the public on flight status.Stand-alone programs are also available for displaying the geographic location of airborne IFR(Instrument Flight Rules) air traffic anywhere in the FAA air traffic system. Positions are reported for bothcommercial and general aviation traffic. The programs can overlay air traffic with a wide selection of mapssuch as, geo-political boundaries, air traffic control center boundaries, high altitude jet routes, satellitecloud and radar imagery.

[edit] Problems

[edit] Traffic

For more information see Air traffic flow management.

The day-to-day problems faced by the air traffic control system are primarily related to the volume of airtraffic demand placed on the system and weather. Several factors dictate the amount of traffic that canland at an airport in a given amount of time. Each landing aircraft must touch down, slow, and exit therunway before the next crosses the beginning of the runway. This process requires at least one and up tofour minutes for each aircraft. Allowing for departures between arrivals, each runway can thus handleabout 30 arrivals per hour. A large airport with two arrival runways can handle about 60 arrivals per hourin good weather. Problems begin when airlines schedule more arrivals into an airport than can bephysically handled, or when delays elsewhere cause groups of aircraft that would otherwise be separatedin time to arrive simultaneously. Aircraft must then be delayed in the air by holding over specifiedlocations until they may be safely sequenced to the runway. Up until the 1990s, holding, which hassignificant environmental and cost implications, was a routine occurrence at many airports. Advances incomputers now allow the sequencing of planes hours in advance. Thus, planes may be delayed beforethey even take off (by being given a "slot"), or may reduce speed in flight and proceed more slowly thussignificantly reducing the amount of holding.

[edit] Weather

Beyond runway capacity issues, weather is a major factor in traffic capacity. Rain orice and snow on therunway cause landing aircraft to take longer to slow and exit, thus reducing the safe arrival rate andrequiring more space between landing aircraft. Fog also requires a decrease in the landing rate. These, inturn, increase airborne delay for holding aircraft. If more aircraft are scheduled than can be safely andefficiently held in the air, a ground delay program may be established, delaying aircraft on the groundbefore departure due to conditions at the arrival airport.

In Area Control Centers, a major weather problem is thunderstorms, which present a variety of hazards toaircraft. Aircraft will deviate around storms, reducing the capacity of the en-route system by requiring

more space per aircraft, or causing congestion as many aircraft try to move through a single hole in a lineof thunderstorms. Occasionally weather considerations cause delays to aircraft prior to their departure asroutes are closed by thunderstorms.

Much money has been spent on creating software to streamline this process. However, at some ACCs,air traffic controllers still record data for each flight on strips of paper and personally coordinate theirpaths. In newer sites, these flight progress strips have been replaced by electronic data presented oncomputer screens. As new equipment is brought in, more and more sites are upgrading away from paperflight strips.


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[edit] Call signs

A prerequisite to safe air traffic separation is the assignment and use of distinctive call signs. These arepermanently allocated by ICAO (pronounced "ai-kay-oh") on request usually to scheduled flights andsome air forces formilitary flights. They are written callsigns with 3-letter combination like KLM, AAL,SWA , BAW , DLH followed by the flight number, like AAL872, BAW018. As such they appear on flightplans and ATC radar labels. There are also the audio orRadio-telephonycallsigns used on the radiocontact between pilots and Air Traffic Control not always identical with the written ones. For exampleBAW stands for British Airways but on the radio you will only hear the word Speedbirdinstead. By default,the callsign for any other flight is the registration number(tail number) of the aircraft, such as "N12345" or"C-GABC". The term tail number is because a registration number is usually painted somewhere on thetail of a plane, yet this is not a rule. Registration numbers may appear on the engines, anywhere on thefuselage, and often on the wings. The short Radio-telephonycallsigns for these tail numbers is the firstletter followed by the last two, like C-BC spoken as Charlie-Bravo-Charlie for C-GABC or the last 3 lettersonly like ABC spoken Alpha-Bravo-Charlie for C-GABC or the last 3 numbers like 345 spoken as three-four-five for N12345. In the United States the abbreviation of callsigns is required to be a prefix (such asaircraft type, aircraft manufacturer, or first letter of registration) followed by the last three characters of thecallsign. This abbreviation is only allowed after communications has been established in each sector.

The flight number part is decided by the aircraft operator. In this arrangement, an identical call sign mightwell be used for the same scheduled journey each day it is operated, even if the departure time varies alittle across different days of the week. The call sign of the return flight often differs only by the final digitfrom the outbound flight. Generally, airline flight numbers are even if eastbound, and odd if westbound. Inorder to reduce the possibility of two callsigns on one frequency at any time sounding too similar, anumber of airlines, particularly in Europe, have started using alphanumeric callsigns that are not based onflight numbers. For example DLH23LG, spoken as lufthansa-two-tree-lima-golf. Additionally it is the rightof the air traffic controller to change the 'audio' callsign for the period the flight is in his sector if there is arisk of confusion, usually choosing the tail number instead.

Before around 1980 International Air Transport Association (IATA) and ICAO were using the same 2-letter callsigns. Due to the larger number of new airlines after deregulation ICAO established the 3-lettercallsigns as mentioned above. The IATA callsigns are currently used in aerodromes on theannouncement tables but never used any longer in Air Traffic Control. For example, AA is the IATAcallsign forAmerican Airlines ATC equivalent AAL. Other examples include LY/ELY forEl Al, DL/DALforDelta Air Lines, LH/DLH for Lufthansa etc.

[edit] Technology

Many technologies are used in air traffic control systems. Primary and secondary radarare used toenhance a controller's situation awareness within his assigned airspace all types of aircraft send backprimary echoes of varying sizes to controllers' screens as radar energy is bounced off their skins, andtransponder-equipped aircraft reply to secondary radar interrogations by giving an ID (Mode A), analtitude (Mode C) and/or a unique callsign (Mode S). Certain types of weather may also register on theradar screen.

These inputs, added to data from other radars, are correlated to build the air situation. Some basicprocessing occurs on the radar tracks, such as calculating ground speed and magnetic headings.

Usually, a Flight Data Processing System manages all the flight plan related data, incorporating - in a lowor high degree - the information of the track once the correlation between them (flight plan and track) isestablished. All this information is distributed to modern operational display systems, making it availableto controllers.


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The FAA has spent over USD$3 billion on software, but a fully-automated system is still over the horizon.In 2002 the UK brought a new area control centre into service at Swanwick, in Hampshire, relieving abusy suburban centre at West Drayton in Middlesex, north ofLondon Heathrow Airport. Software fromLockheed-Martin predominates at Swanwick. However, Swanwick was initially troubled by software andcommunications problems causing delays and occasional shutdowns.

Some tools are available in different domains to help the controller further:

y Flight Data Processing Systems: this is the system (usually one per Center) that processes all theinformation related to the Flight (the Flight Plan), typically in the time horizon from Gate to gate

(airport departure/arrival gates). It uses such processed information to invoke other Flight Plan

related tools (such as e.g. MTCD), and distributes such processed information to all the

stakeholders (Air Traffic Controllers, collateral Centers, Airports, etc).

y STCA (Short Term Conflict Alert) that checks possible conflicting trajectories in a time horizon ofabout 2 or 3 minutes (or even less in approach context - 35 seconds in the French Roissy & Orly

approach centres[3]

) and alerts the controller prior to the loss of separation. The algorithms

used may also provide in some systems a possible vectoring solution, that is, the manner in

which to turn, descend, or climb the aircraft in order to avoid infringing the minimum safety

distance or altitude clearance.

y Minimum Safe Altitude Warning (MSAW): a tool that alerts the controller if an aircraft appearsto be flying too low to the ground or will impact terrain based on its current altitude and

heading.

y System Coordination (SYSCO) to enable controller to negotiate the release of flights from onesector to another.

y Area Penetration Warning (APW) to inform a controller that a flight will penetrate a restrictedarea.

y Arrival and Departure Manager to help sequence the takeoff and landing of aircraft.o The Departure Manager (DMAN): A system aid for the ATC at airports, that calculates a

planned departure flow with the goal to maintain an optimal throughput at the runway,

reduce queuing at holding point and distribute the information to various stakeholdersat the airport (i.e. the airline, ground handling and Air Traffic Control (ATC)).

o The Arrival Manager (AMAN): A system aid for the ATC at airports, that calculates aplanned Arrival flow with the goal to maintain an optimal throughput at the runway,

reduce arrival queuing and distribute the information to various stakeholders.

o passive Final Approach Spacing Tool (pFAST), a CTAS tool, provides runway assignmentand sequence number advisories to terminal controllers to improve the arrival rate at

congested airports. pFAST was deployed and operational at five US TRACONs before

being cancelled. NASA research included an Active FAST capability that also provided

vector and speed advisories to implement the runway and sequence advisories.

y Converging Runway Display Aid (CRDA) enables Approach controllers to run two finalapproaches that intersect and make sure that go arounds are minimized

y Center TRACON Automation System (CTAS) is a suite of human centered decision support toolsdeveloped by NASA Ames Research Center. Several of the CTAS tools have been field tested and

transitioned to the FAA for operational evaluation and use. Some of the CTAS tools are: Traffic

Management Advisor (TMA), passive Final Approach Spacing Tool (pFAST), Collaborative Arrival

Planning (CAP), Direct-To (D2), En Route Descent Advisor (EDA) and Multi Center TMA.

y Traffic Management Advisor (TMA), a CTAS tool, is an en route decision support tool thatautomates time based metering solutions to provide an upper limit of aircraft to a TRACON from

the Center over a set period of time. Schedules are determined that will not exceed the


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specified arrival rate and controllers use the scheduled times to provide the appropriate delay

to arrivals while in the en route domain. This results in an overall reduction in en route delays

and also moves the delays to more efficient airspace (higher altitudes) than occur if holding near

the TRACON boundary is required to not overload the TRACON controllers. TMA is operational

at most en route air route traffic control centers (ARTCCs) and continues to be enhanced to

address more complex traffic situations (e.g. Adjacent Center Metering (ACM) and En Route

Departure Capability (EDC))

y MTCD & URETo In the US, User Request Evaluation Tool (URET) takes paper strips out of the equation

for En Route controllers at ARTCCs by providing a display that shows all aircraft that are

either in or currently routed into the sector.

o In Europe, several MTCD tools are available: iFACTS (NATS), ERATO (DSNA fr:DSNA),VAFORIT (DFS), New FDPS (MASUAC). The SESAR

[4] Programme should soon launch new

MTCD concepts.

URET and MTCD provide conflict advisories up to 30 minutes in advance and have a suite of

assistance tools that assist in evaluating resolution options and pilot requests.

y Mode S: provides a data downlink of flight parameters via Secondary Surveillance Radarsallowing radar processing systems and therefore controllers to see various data on a flight,

including airframe unique id (24-bits encoded), indicated airspeed and flight director selected

level, amongst others.

y CPDLC: Controller Pilot Data Link Communications allows digital messages to be sent betweencontrollers and pilots, avoiding the need to use radiotelephony. It is especially useful in areas

where difficult-to-use HF radiotelephony was previously used for communication with aircraft,

e.g. oceans. This is currently in use in various parts of the world including the Atlantic and Pacific

oceans.

y ADS-B: Automatic Dependent Surveillance Broadcast provides a data downlink of various

flight parameters to air traffic control systems via the Transponder (1090 MHz) and reception ofthose data by other aircraft in the vicinity. The most important is the aircraft's latitude,

longitude and level: such data can be utilized to create a radar-like display of aircraft for

controllers and thus allows a form of pseudo-radar control to be done in areas where the

installation of radar is either prohibitive on the grounds of low traffic levels, or technically not

feasible (e.g. oceans). This is currently in use in Australia, Canada and parts of the Pacific Ocean

and Alaska.

y The Electronic Flight Strip system (e-strip): A system of electronic flight strips replacing the oldpaper strips is being used by several Service Providers, such as NAV CANADA, MASUAC, DFS,

being produced by several industries, such as Indra Sistemas, Thales Group, Frequentis, Avibit,

SAAB etc. E-strips allows controllers to manage electronic flight data online without Paper Strips,

reducing the need for manual functions.y SkyRec: Hardware based video recording tool that records and replays all information captured

on ATCO screens. Used for legal recording (coupled with voice recording), training and post

event analysis.[5]

[edit] Major accidents

A list of recent accidents can be found in this list.


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Air traffic controllers are the people who operate the air traffic control systems to expedite and maintaina safe and orderly flow ofair traffic and help prevent mid-air collisions. They apply separation rules tokeep aircraft apart from each other in their area of responsibility and move all aircraft safely and efficientlythrough their assigned sector of airspace. Because controllers have an incredibly large responsibilitywhile on duty, the ATC profession is regarded around the world as one of the most difficult jobs today,and can be notoriously stressful depending on many variables (equipment, configurations, weather, trafficvolume, human factors, etc.).

Although the media frequently refers to them as air controllers, orflight controllers, most air trafficprofessionals use the term air traffic controllers. They are also called air traffic control officers(ATCOs), air traffic control specialists, or simply controllers.

Features of the job

[edit] Core skills of a controller

Air traffic controllers are generally individuals who are well organized, are quick with numericcomputational skills, and possess excellent short-term memory abilities. In addition, studies have shown

that air traffic controllers generally have a degree of situational awareness that is much higher than theaverage population. Excellent hearing and speaking skills are a requirement, and trainees undergo rigidphysical and psychological testing. In addition they are generally assertive but calm under pressure, andthey are able to follow and apply rules yet be flexible when necessary. Air traffic controllers must maintainsome of the strictest medical and mental requirements for professions; conditions such as diabetes,epilepsy, and heart disease typically disqualify people from the position. Conditions such as hypertensionare taken seriously and must be monitored with medical exam. Controllers must take precautions toremain healthy and avoid certain medications that are banned for them. Many drugs approved d

The Airports Authority of India

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