Transponder

An onboard radio transmitter-receiver that responds to radar interrogation signals from air traffic control (ATC) by sending back a coded signal with the aircraft's assigned squawk code and, if Mode C or Mode S equipped, pressure altitude. ATC uses this information to identify and track aircraft on radar. Common codes include 1200 (VFR), 7500 (hijack), 7600 (communication failure), and 7700 (emergency).