Learning Objective 062.03.04.01.01: State that the ATC system is based on the replies provided by the airborne transponders in response to interrogations from the ATC secondary radar.

Unlike a Primary Radar System, which sends out pulses and then waits for any of those pulses to be reflected off an item such as an aircraft, a Secondary Surveillance Radar (SSR) sends out pulses, which are interrogations of aircraft SSR transponders. The interrogation is sent out of frequency 1030 MHz, and the aircraft transponder then emits its own string of reply pulses on 1090 MHz, a different frequency.

This method of radar usage has many benefits, as the signals only need to be a quarter as powerful to travel half the distance, primary radar signals have to travel to and object and then return to the station, whereas with SSR, the reply signals are freshly produced by the aircraft transponder, so cover the air to ground journey, reducing the required power of the Radar.

SSR also has the benefits of being able to transmit a series of encoded pulses, giving a 4 digit transponder squawk code of an aircraft in mode A operation, and a pressure altitude readout when also using mode C. More data can be sent out by mode S also, and this is all encoded into strings of reply pulses emitted by the aircraft transponder.