a radio, radar, or sonar transceiver that automatically transmits a signal upon reception of a designated incoming signal.
Also, transpondor.

1940–45; trans(mitter) + (res)ponder Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
transponder or transpondor (trænˈspɒndə)
1.  a type of radio or radar transmitter-receiver that transmits signals automatically when it receives predetermined signals
2.  the receiver and transmitter in a communications or broadcast satellite, relaying received signals back to earth
[C20: from transmitter + responder]
transpondor or transpondor
[C20: from transmitter + responder]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1945, from trans(mit) + (res)pond + -er
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
transponder   (trān-spŏn'dər)  Pronunciation Key 
A radio or radar transmitter and receiver that responds to an incoming signal either by broadcasting its own predetermined signal (as in aircraft identification systems) or by relaying the incoming signal at a different frequency (as in satellite communications).
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Unfortunately the plane's transponder communicates with secondary radar, not
  the primary receiver.
The discrepancy arose because of where the transponder boxes were placed in the
He is also developing a tiny acoustic transponder that could be implanted in
  the jelly and used to track it with an unmanned sub.
And transponder signals can be detected by anyone with an appropriate receiver.
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