In telecommunication, a transponder is one of two types of devices. In air navigation or radio frequency identification, a transponder is a device that emits an identifying signal in response to an interrogating received signal. In a communications satellite, a transponder gathers signals over a range of uplink frequencies and re-transmits them on a different set of downlink frequencies to receivers on Earth, often without changing the content of the received signal or signals.
The term is a portmanteau for Transmitter-responder. It is variously abbreviated as XPDR, XPNDR, TPDR or TP).
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