Transponder Overview

• A transponder ('transmitter-responder') receives a signal from a ground station (an 'interrogation') and automatically transmits a reply.
• Transponders were developed for the military when radar could locate airplanes but couldn't tell friendlies from enemy — the reply provides the airplane's ID, altitude, and other data.
• Originally called 'IFF' (Identification, Friend Or Foe) — still used mostly by the military.
• In civil aviation, it operates in a system called SSR (Secondary Surveillance Radar) — 'secondary' because primary radar simply reflects a signal off the airplane's metal surface ('skin return').
• In the airline world, the transponder is labeled 'ATC,' referring to Air Traffic Control.