ehusmann wrote:Radar doesn't receive transmissions from aircraft, but emits signals and receives the bounce of an object. The emitted data is only used to identify the aircraft with the dot on the screen. But the dot is still visible if the aircraft stops emitting signals. However, there isn't radar everywhere....
Erwin
Erwin, what you described is called Primary surveillance Radar used by military operators. SSR (secondary surveillance radar) whichis used by civilian ATC is in fact only receiving transmission data from aircraft (Mode S etc) and does not work with sending a signal and receiving the bounce of an object. So when the transmitters are shut off, SSR does not "see" an aircraft, also no dot will be visible.
Above the ocean, where there is no radar, surveillance is being done with ADs-C, in which position reports are triggered from the aircraft.
If, however, the target is an aircraft and you are an air-traffic controller, you will want to know a whole lot more. Enter secondary radar or, to give it its full title, Secondary Surveillance Radar (SSR).
Secondary radar
The big difference with SSR [compared to PSR] is that it doesn't rely on reflections. Aircraft are equipped with a transponder. This transponder transmits a 'reply' when it receives a radar 'interrogation' signal. The interrogation signal is completely separate from any primary signal. As the reply is not just a reflection much less power is needed, typically around 1kW for interrogation pulses, slightly less for replies. Range and direction can be determined from the SSR signal in much the same way as with primary radar, measuring the time between sending the interrogation and receiving the reply, making allowance for the turn-round delay in the transponder. The advantage of SSR is that all sorts of information can be encoded into the Transponder's reply.