Seconary Surveillance Radar (SSR) is a radar system used in air traffic control (ATC), which not only detects and measures the position of aircraft but also requests additional information from the aircraft itself such as its identity and altitude. Unlike primary radar systems, which measure only the range and bearing of targets by detecting reflected radio signals, rather like seeing an object in a beam of light, SSR relies on its targets being equipped with a radar transponder, which replies to each interrogation signal by transmitting its own response containing encoded data. SSR is based on the military Identification Friend or Foe (IFF) technology originally developed during World War II, and the two systems are still compatible today.