Secondary surveillance radar (SSR) is a radar

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 system used in air traffic control

Air traffic control

Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...

 (ATC), that not only detects and measures the position of aircraft i.e. range and bearing, but also requests additional information from the aircraft itself such as its identity and altitude. Unlike primary radar systems, that measure only the range and bearing of targets by detecting reflected radio signals, SSR relies on targets equipped with a radar transponder

Transponder (aviation)

A transponder is an electronic device that produces a response when it receives a radio-frequency interrogation...

, that replies to each interrogation signal by transmitting a response containing encoded data.