A Surveillance radar system which uses transmitters/receivers (interrogators) & transponders, detects and measures the position of aircraft but also requests additional information from the aircraft itself such as its identity and altitude. 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. The transponder is a radio receiver and transmitter which receives on one frequency (1090 MHz) and transmits on another (1030 MHz). The target aircraft's transponder replies to signals from an interrogator (usually, but not necessarily, a ground station co- located with a primary radar) by transmitting a coded reply signal containing the requested information. An SSR continuously transmits interrogation pulses (selectively rather than continuously in Mode-4, Mode-5, and Mode-S) as its antenna rotates, or is electronically scanned in space. A transponder on an aircraft that is within line-of-sight range 'listens' for the SSR interrogation signal and sends back a reply that provides aircraft information.