The Primary Surveillance Radar (PSR) works on the echo principle that uses strong transmitted signals at a known time. These signals reflect from objects and the echoes are detected.
The Secondary Surveillance Radar (SSR) works on the principle of the "interrogation and response" technique. It uses a ground-installed transmitter and receiver that is the interrogator component of the system and the aircraft is also equipped with a transmitter and a receiver. There is therefore a two-way communication, which is the key difference from PSR. Since the aircraft transponder is capable of sending data via the response signal, Secondary Surveillance Radar (SSR) information transmission may provide flight parameters to the controller such as actual speed or altitude of the aircraft. Primary Radar is not able to obtain this information accurately.
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