A
transponder (short-for
transmitter-res
ponder and sometimes abbreviated to XPDR, XPNDR, TPDR or TP) is an electronic device that produces a response when it receives a radio-frequency interrogation. Aircraft have transponders to assist in identifying them on air traffic control
radar; and
collision avoidance systems have been developed to use transponder transmissions as a means of detecting aircraft at risk of colliding with each other.