Secondary Surveillance Radar (SSR) is used by Air Traffic Control (ATC). The main difference with Primary radar is the ability to detect aircraft. SSR can only detect aircraft that are equipped with a radar transponder that gives encoded information such as identity code, aircraft altitude and further information depending on the preselected mode of the transponder.

The target aircraft replies to signals from the interrogator, which is usually the ATC, by transmitting a coded reply signal containing the requested information.