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Transponder is a unit fixed on object meant to be tracked. When external unit designed to work with a specific transponder is used, its ID and position could be identified. In the universe of The Expanse, when using the term transponder, the speaker usually means the unit that provides unique identification for a registered spaceship.


Human race started using transponder when radio technology became more accessible. The word consists of words transmitter and responder. Transponder was a unit, that upon receiving a radio signal, reacted, sending an answer that contained its identification.[1]

Use in motorsportEdit

Transponders were used in racing. Thanks to constants increases in speed, it became still more and more difficult to keep track of all the competitors. The transponders allowed the placement of all participants to be updated instantly when they passed the end of an sector.[2]


Use in spacecraft around 23rd centuryEdit

During 23rd century, transponders are used for long range identification and location of spaceships. All spaceships are required to have them turned on at all times [3]. Military spacecraft often turn them off when they want to hide their location [4]. Since the transponder is able to pinpoint its location, it is also used when a tightbeam to a ship with unknown coordinates is required . In this case, tightbeam communication will calibrate based on provided transponder ID.[5]


All standard torpedoes are outfitted with transponders. It works the same way it does with ships. Without the torpedo ID, it can´t be used to track them, so they usually only provide information for those who have fired them. The opposing party turns to ladar readings to guide its PDCs when possible.[6]

Tracking invisible torpedoesEdit

When the personnel of Io base shot its torpedoes carrying Protomolecule monsters, they have used active stealth technology to hide them. When their propellers were turned off, it was very difficult to trap them and a over 100 was able to fly past Earth Mars Coalition NavyEMCN undetected and only were successfully destroyed when the transponder IDs were obtained.[7]

Destruction of XinglongEdit

Xinglong was a small prospector spaceship. Her owners were behind three payments to their bank, Consolidated Holdings and Investments, which has in response laid a lien on their ship. In order to keep their ship, the crew of Xinglong has disabled its transponder, so it couldn't be easily located. Unfortunately for the crew, they flew in the vicinity of Martian warship, Scipio Africanus. It is unclear whether the Scipio Africanus hailed Xinglong and the ship didn't respond or if they have opened fire without a warning, but when the prospecting ship got to close, Martians used their PDCs and destroyed the unidentified vessel, increasing the tensions between Mars and the outer planets.


  1. Schantz, Hans Gregory. "On the origins of rf-based location." Wireless Sensors and Sensor Networks (WiSNet), 2011 IEEE Topical Conference on. IEEE, 2011.
  2. Actionbike racing agency
  3. Novel Leviathan Wakes, Chapter 14
  4. Novel Leviathan Wakes, Chapter 15
  5. Novel Caliban's war, Chapter 37
  6. Novel Caliban's war, Chapter 52
  7. Novel Caliban's war, Chapter 48

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