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A transponder is a radio beacon fixed to an object meant to be tracked and identified. A active transponder listens for incoming radio signals waiting for an activation code. When a code is recognized the transponder replies presenting its identification.

When using the term transponder, the speaker usually means the unit that provides unique identification for a registered spaceship.

A transponder can operate in different modes and react to different kinds of incoming signals. This is used as a plot element several times.

  • A transponder will be designed to be "tamper-proof" making it hard or impossible for the crew to manipulate the transponders ID or behavior. In civil craft this serves to prevent faking identities. In military craft it is essential to prevent sabotage or interference after an enemy boarding a ship.
  • An active ship transponder will be in public mode, replying to all request with its public ID. In this mode a "ping" will be sent by the searcher and all transponders in range will reply.
  • The transponder may be addressed by its ID, ignoring all "pings" with non matching IDs. This is called "tracking mode" or "locking on to a transponder".
  • In inactive mode the a transponder will only respond to a ping using its secret ID. This mode would be common in military vehicles only. Friendly craft can still send commands and/or force the transponder to reveal its position.
Note that the explanation above is simplified and skips cryptographic aspects or stealth modes of operations as neither are relevant in the plot.


Humans started using transponders 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 constant 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 a sector.[2]


Use in spacecraft around 24th centuryEdit

During the 24th 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 technology is able to pinpoint an object's 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, the technology can't be used to track any specific object, so they usually only provide information for those who have fired the torpedoes. 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 used active stealth technology to hide them. When their propulsion systems were turned off, it was very difficult to track them. Over 100 were able to fly past Earth-Mars Coalition Navy (EMCN) 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 too close, Martians used their PDCs and destroyed the unidentified vessel, increasing the tensions between Mars and the outer planets.