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 provides multitude of additional information - the ATC controllers are therefore able to see the squawk code (transponder code), flight level (provided by the transponder in mode C), groundspeed calculated by the SSR ground facility based on the change of bearing of the aircraft and a callsign (provided by the transponder in mode S or assigned by the ground facility to a specific squawk code).