A transponder was a device on board a starship, starbase, or other spacecraft, which broadcast a signal identifying that craft by name, design, and allegiance. This allowed for easy recognition of craft especially at distances where visual inspection or accurate sensor readings were not possible.
Transponders could be deactivated or have their details forged, however, so the existence or absence of a transponder signal was not proof-positive of a craft's identity or existence. For this reason transponder signals could include some kind of code proving to those "in-the-know" that the signal was genuine.
In 2369, Chief Miles O'Brien and Lieutenant Jadzia Dax searched for the crash landed runabout USS Yangtzee Kiang and were even unable to find the transponder signal of the runabout. (DS9: "Battle Lines")
- Emergency transponder (TNG: "A Matter Of Honor")
- Subspace marker (TNG: "Unification I")
- Homing transponder (DS9: "Covenant")
- Subcutaneous transponder (TOS: "Patterns of Force")