Radar was developed before and during World War II, where it played a key role in all aerial battles. The term RADAR was coined in 1941 by the United States Navy as an acronym for radio detection and ranging.
A radar system has a transmitter that emits electromagnetic radio signals in predetermined directions. When these come into contact with an object they are usually reflected back towards the receiver. A radar receiver is usually in the same location as the transmitter. By using using radiotelemetry techniques, the position of the radiated object can be determined and displayed. If the object is moving, there is a slight change in the frequency of the radio waves due to the Doppler effect.
Figure 11.16: Radar antenna and ATC display.
In aviation, two radar techniques are applied:
The original technique described above, that detects the objects due to its finite magnitude. This kind of radars are referred to as simply primary radars (PSR). In this case the aircraft is a passive object.
The secondary radar (SSR): in this case, the radar requires the aircraft to carry an on board equipment called transponder. The transponder is interrogated from earth, responding with coded values such as flight level, flight code, direction, or velocity. This version was standardized by ICAO in the 80s with the aim at supporting air traffic control and surveillance.
The presentation of data in the screen that use controllers is very different in both cases. In the primary radar, only points (called targets) are presented with no identification, nor any information. Fixed targets can be mountains or any other orographic accident, while mobile targets can be identified with aircraft. Thus, the PSR is more interpretative. In the case of the secondary radar, the targets that are presented in the screen have a identification code, and provide also data such as flight level or velocity. Obviously, this information is much more useful for a controller to fulfill the surveillance function since each aircraft has a unique transponder.
The information is supplied in three different scenarios with three different types of equipment: Long range secondary radar for En-route control; primary radar and short range secondary radar for approach; surface radar (primary) at the airport.