Radar Basics

What is an ASR?

Air Surveillance Radar

An Airport Surveillance Radar (ASR) or Terminal Area Radar (TAR) is an ATC radar system used at airports. It is a midrange primary radar used to detect and display the presence and position of aircraft in the terminal area, the airspace around airports. It usually operates in the frequency range from 2 700 to 2 900 MHz (E-Band), since this frequency range provides low attenuation due to absorption in heavy rain regions. In addition, this frequency range is still high enough to be able to use highly directional antennas with relatively small dimensions and lower weight.

Redundancy

Due to the importance of the function, a high redundancy of all components is required, so that the probability of failure remains very low. In addition, an automatic reconfiguration is often organized, in which the operational readiness of the assemblies is permanently checked. In the event of an error, modules held in reserve are automatically switched into the signal processing path. Another possibility is to use a number of identical modules in the transmitter so that the radar can continue to be used if one of the modules fails (Soft Error Management). This transmitter is fault-tolerant and can withstand several failures of these modules without significant range loss (see The Radar Equation in Practice).

These transmitter modules can also be changed during operation (Hot Plugging). For this reason, an Air Surveillance Radar uses a passive antenna in most cases. With a rotating active antenna, this change could only be made when the antenna rotation is switched off. With a passive antenna, all these modules are accessible, although the radar continues to operate at slightly reduced power. The disadvantage of the passive antenna, however, is that even the very weak echo signals are subject to these high conduction losses caused by the long waveguide connections from the transmitter/receiver to the antenna.

Airports with a very high volume of air traffic such as the Munich airfield “Franz Josef Strauß” (ICAO code: EDDM) even have two independent airport surveillance radars. On the one hand also for reasons of redundancy, on the other hand however also because of the mutual covering of the cone of silence, which would mean otherwise a gap in the radar coverage exactly over the radar site.

Technical data

There are binding recommendations from the International Civil Aviation Organisation (ICAO) and the European Organisation for the Safety of Air Navigation EUROCONTROL for the technical characteristics of an air surveillance radar. The required radar coverage of an ASR conforms to the limits of an airfield's jurisdiction as defined by ECACEuropean Civil Aviation Conference (ECAC). The effective maximum range of an ASR for aircraft flying at an altitude of 3 000 feet (≙ about 1 000 m) should, therefore, be more than 40 (≙ about 75 km), up to 60 nautical miles (≙ about 110 km). In the height, on the other hand, only up to 10 000 feet (≙ about 3 000 m) are required. A range beyond that, e.g. a distance of 80 nautical miles, is welcome but not necessary. On the contrary, the time budget of an impulse radar then requires compromises with the rotational speed. The slower rotation worsens the data renewal rate. This time budget also influences the magnitude of the pulse repetition frequency (PRF) to be used, which subsequently reduces the number of hits per scan. The resulting losses in the probability of detection must be compensated by other measures, such as considerably greater reserves in the transmission power. In order to justify this effort, there must be considerable reasons why the range of 80 NM should be achieved by the primary radar.

The rotation speed of the ASR antenna is optimized at 12 to 15 revolutions per minute. This achieves a data renewal rate of 4 to 5 seconds. Since the air traffic controller has to give the pilot a course correction at least every 5 seconds during an approach to the airfield guided by the radar, this time interval is guaranteed by the rotation of the antenna. An ASR usually uses a parabolic reflector antenna with a cosecant-squared pattern. In many radar sets, two horn radiators are used to separate the high-beam and low-beam, which improves the performance capability of moving target indication (MTI) or moving target detection (MTD).

Flight Altitude Information

An Air Surveillance Radar is mostly only a 2D-Radar for reasons of effort. However, an ASR is always coupled with a secondary surveillance radar (SSR) whose identification for the targets is displayed on the ASR display units. The SSR operates synchronously with the primary radar. It also provides a flight altitude, which is determined barometrically on board the aircraft. Both information is linked with each other in the plot combiner of the radar data processor and is displayed alphanumerically on the screen in a data block next to the target sign. In most cases, the display units of the ASR have a switchable scale with twice the range of the primary radar and can therefore also display the information of the secondary radar for longer distances (for example up to 120 NM).

An elevation angle measurement by the primary radar and a calculation of a flight altitude based on this is also possible, for example, with the radar shown in Figure 1. This radar uses a system of three horn radiators, each with an independent receiving channel. Each elevation angle results in characteristic power distribution of the echo signals in these receiving channels, which are suitable for a rough calculation of an altitude.

Electronic maps

ASR can display electronic map material on the radar screen in most cases. This includes tactical lines and boundaries of responsibility, positions of obstacles, no-fly zones, radio beacons or prominent terrain points. In modern radar systems, these maps are stored as software in the computer. With older analog radar sets, this map material had to be stored in a complicated procedure in an external video mapper on a photographically produced film plate, which was scanned with a light beam moving synchronously to the screen deflection.