A subspace transponder

A subspace transponder was a subspace communications device aboard starships used to locate and identify objects in space, such as other starships or shuttlecraft.

Each Starfleet starship sent out a unique subspace transponder signal. By comparing the signal sent from a moon of Mab-Bu VI with stored transponders signals from other starships, Data was able to identify the distress signal as originating from the Daedalus-class starship USS Essex. (TNG: "Power Play")

While searching for the mission recorder of the Klingon Vor'cha-class attack cruiser IKS Toh'Kaht near Deep Space 9, Chief Miles O'Brien detected the weak subspace transponder signal from the recorder. (DS9: "Dramatis Personae")

Captain Kathryn Janeway gave the Vhnori Ptera a subspace transponder before beaming her into a subspace vacuole where she should give the transponder to Harry Kim. (VOY: "Emanations")

The Doctor implanted a transponder in Neelix and Chakotay's palms before they infiltrated Quarra. It allowed them to maintain an open communications link and transport them through the Quarren shield grid. (VOY: "Workforce")

The Doctor calls this a subspace transponder, but it also has many similarities to a homing transponder.
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