Transponder
Device that emits an identifying signal in response to a received signal / From Wikipedia, the free encyclopedia
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In telecommunications, a transponder is a device that, upon receiving a signal, emits a different signal in response.[1] The term is a blend of transmitter and responder.[2][3]
![Thumb image](http://upload.wikimedia.org/wikipedia/commons/thumb/f/f8/407_Transponder.jpg/120px-407_Transponder.jpg)
In air navigation or radio frequency identification, a flight transponder is an automated transceiver in an aircraft that emits a coded identifying signal in response to an interrogating received signal.
In a communications satellite, a satellite transponder receives signals over a range of uplink frequencies, usually from a satellite ground station; the transponder amplifies them, 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.