Air traffic controllers are able to keep planes separated using only radio and procedures. Typically, all aircraft flying to a particular airport will wait in a holding pattern, and one will be authorized to start the approach every 3 minutes. This is not optimal, but without a detailed knowledge of the situation, it’s the only possible way.
Radar is used to gain this extra information, making higher capacity possible. The first kind of radar, known as Primary, is based on waves reflection. The radar sends a pulse in a certain direction, and measure the time until it receives the echo. Knowing the propagation speed, it’s easy to determine the distance of the target. As the wave is sent in a narrow beam, the azimuth is well known.
As no special equipment is required on board the plane, this is technique is known as non-cooperative. On the other hand, it do not detect only aircraft, but all what can reflect a wave… Other limitations are the absence of altitude detection, and the large power required, as the wave has to travel back and forth. Primary radars use typical parabolic-ish antennas which make easy to identify them.
The second antenna, looking like a grid on top of the primary antenna is for secondary radar.
The Secondary radar is not based on reflection, but relies on a response message sent by an on-board equipment, the transponder. Each time the transponder is interrogated by a radar, it sends a code the pilot did set, and possibly the aircraft altitude. This makes possible for controllers to match echoes with planes, which was not possible with primary radar, except by requiring aircraft to fly some manoueuvers.
As all transponders in the radar beam send an answer, it’s not always possible for the receiver to identify them, leading to so-called garbling situation. This is particularly true in holding stacks. Another limitation is the very limited number of possible codes – only 4096. Obviously, this kind of radar works only if the transponder is answering, and it’s possible to cheat by delaying the answer.
The new generation of secondary radar – known as Mode-S, standing for selective – uses a new kind of transponder. The code is still present, but each transponder also has a specific address, making possible for the radar to interrogate only one transponder in the beam. This reduces garbling problems, and provide some help to solve the code shortage problem. Mode-S is also able to send more information, like the aircraft tail-number or commercial call-sign, and data computed on-board as well as Flight Management System settings.
The radar image made available to controllers normally relies on several radars. All the echoes are processed by a tracker, resulting in a better quality, smooth, and accurate picture. With appropriate radar coverage, it is legal to have aircraft flying with 2.5 to 5.0 miles between each other. Detection technology could allow for much shorter separations, but this distances are also needed to allow for some reaction time in case of problem.
There are other kind of radars used in ATC, like ground radars, and precision approach radars (PARs). PARs are mainly used on military airports, to literally “talk” airplanes down. This is unfortunately usually not accessible to civilian aircrafts, except for training purposes… mostly in the UK.
Radar technology has been around for a while now, and new solutions start to be used, including Automatic Dependent Surveillance (ADS) and Wide Area Multi-lateration (WAM), and I’ll soon post about them. If you have any question about surveillance technology, feel free, I have some knowledge about that…
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