Subspace transponder

From Memory Alpha, the free Star Trek reference.

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

Each starship sends 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")

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 acts more like a homing transponder.

Subspace communication
amplifier • antenna • bandwidth • beacon • hyper- • interference • interplexing beacon • log • message • neural transceiver • oscillation • relay • relay network • relay station • transceiver • transmitter • transponder • wide-band