Description

A transponder (XPDR) is a receiver/transmitter which will generate a reply signal upon proper interrogation; the interrogation and reply being on different frequencies. (ICAO)

Transponders first came into use to enable military authorities to identify friendly aircraft, which transmitted a coded signal when interrogated by military radar. This was known as IFF (Identification Friend or Foe).

Subsequently, transponders have come into widespread use in civil as well as military aviation. It is now standard practice to allocate a specific transponder code to each aircraft flying in controlled airspace so that the ATCO can readily identify a specific aircraft on a crowded radar screen, using SSR (Secondary Surveillance Radar).