THE ROLE OF RADAR IN ATC

Identification of aircraft using primary or secondary radar is made by using one or more specified methods until the controller is satisfied beyond doubt that the response on the radar does indeed represent a particular aircraft. Thereafter the controller must ensure that radar identification is maintained at all times until the aircraft is transferred to another controller.

Ideally, pilots want an unrestricted climb to the most economic cruising level and then to maintain this level for as long as possible before starting a continuous descent right down to the runway.

The approach controller will be concerned with using the radar to expedite departing aircraft with an initial climb and separating them from the arrivals. At the same time the controller will be establishing an arrival sequence, to achieve maximum runway utilisation, with a variety of aircraft types having different speeds on the approach.

Radar is used primarily to expedite the flow of air traffic whilst maintaining safety.

Radar separation between identified aircraft is 5NM compared to 10 minutes (or up to 80NM) when using non-radar separation on the same track at the same level. The controller is now handling a great many more aircraft in the airspace and more aircraft are able to fly at their preferred flight levels.

Whilst under radar control and particularly when an aircraft is on a radar vector, the controller effectively takes over the navigation of the aircraft and will be responsible for preventing the aircraft from crashing into the ground.

The en-route, or area controller, will be endeavoring to climb departing aircraft to their requested cruising level and descend arriving aircraft from their high level cruise to a lower level for the approach to an aerodrome.

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NAVIAGTION SYSTEM – INS

Navigation across oceans and in areas where few ground-based navigation aids exist is normally achieved by using an Inertial Navigation System (INS), In use an INS is given the aircraft's start position, it then senses all movements the aircraft makes and continuously updates the position of the aircraft. Normally triplicated systems are used to eliminate the possibility of error and ground based radio aids are used, where available, to update the positional accuracy which has a tendency to drift by a mile or two an hour.

NAVIAGTION SYSTEM – NDB

Non Directional Beacons (NDB) transmit signals (200-1720KHz) in all directions and are point source navigational aids. Their range is normally around 40 miles. They are generally used is a locator beacon to provide airfields with holding facilities and a non-precision approach. With higher power their range can be around 150Nm when they can act as an en route navigational aid. The use in the en route role is only common in remote areas and areas of low traffic density.

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NAVIGATION SYSTEM – LANDING AIDS

The localizer (VHF 108-112MHz, range 25NM) provides guidance in azimuth and the glidepath (UHF 328 335 MHz, range 10NM) provides guidance in elevation.

In use the pilot establishes the aircraft on the final approach track (defined by the localiser) at distance of around 12 Nm and a height of 3000 ft. At this point the aircraft is below the approach slope defined by the Glidepath. When the pilot intercepts the Glidepath descent is started and the localiser and Glidepath are followed down to the pilots approach minima.

The pilot determines the distance from touchdown by fixed distance marker beacons (75MHz) located on the extended centre-line about 3.5NM(outer marker)and 3500ft (middle marker)from the runway threshold. Each beacon radiates a characteristicand easily identifiable signal. These provide fixed points where distance to go can be checked against the aircraft height, thus confirming theaccuracy of the altimeter setting.

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RADIO NAVIGATION AIDS - VOR/DME

Apart from taking their bearings from the stars, how do pilots navigate? The answer, of course, is in the use of radio navigation aids. There are a variety of different types of radio navigation aids but here we shall discuss VOR and DME only.

VOR and DME are often located at the same site. They operate on VHF and UHF respectively and, as a consequence, are not affected by static or other interferences. The maximum range of VOR is about 200 nautical miles. By flying VOR the pilot ensures he is flying directly to the station. Also by measuring his radials from more than one VOR station, a pilot can check his position.

The function of DME, which is short for Distance Measuring Equipment, is, as its title describes, to measure distance.

The DME measures, electronically, the time it takes for a signal, transmitted from an aircraft interrogator, to reach the ground base station transponder, and return. This elapsed time is converted to miles and appears on a digital indicator on the flight deck. The indicator actually seems to rapidly count the number of miles between the aircraft and the station giving the pilot a continuous digital reading of how far he is from, or to, a station.

With the many VOR/DME stations along his route, a pilot can make good his desired track; is constantly aware of his distance to or from a DME station; or, by using two VOR radials, establish his exact position.

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PRIMARY RADAR

Radio Detection and Ranging (RADAR) is based on the principle thatelectromagnetic energy propagated from a transmitter at the speed of light (1NM takes 6.2msec) can be directed onto an object e.g. an aircraft, and thatthe distance of the object from the transmitter can be measuredby calculating thetime a pulse of radio energytook to travel to the object and be reflected back again. By using an antenna whichrotates through360°, thenarrow beam of returning energyis "collected" and the direction fromwhich the reflected beam arrives can thus be determined.

The signal is decoded and amplifiedbefore being displayed as a "blip" on theradar controller's screen.

Radar signals follow the 'line of sight' principle. Due to its position relative to the radar antenna andallowing for the curvatureof the Earth, an aircraft can be below radar cover or out of range.

Detection of aircraft is also dependent upon the size, height and angle (relative to the antenna) of the aircraft.

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Date: 2015-12-11; view: 832