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A transponder (XPDR, XPNDR, TPDR or TP) is an automated and electronic monitoring device that receives, cross examines, amplifies and retransmits incoming signals. A transponder is also a transceiver that generates reply signals upon proper electronic examination.
The transponder term is a blend of the words transmitter and responder.
A transponder receives an interrogator signal and automatically transmits a radio wave signal at a predetermined frequency. Broadcast frequency signals, which are different than received signals, use a special frequency converter component for broadcasting.
Each communication satellite channel is a transponder and has separate transceivers or repeaters. Numerous audio and video channels may pass through a single transponder on a single wideband carrier with digital video data compression and multiplexing.
The transponder was first used on aircraft during World War II as part of the Identification Friend or Foe (IFF) system. Transponders are still used in military and commercial aviation.