Subspace transponder

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

Each 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 starship USS Essex. 

While searching for the mission recorder of the Klingon attack cruiser  near Deep Space 9, Chief Miles O'Brien detected the weak subspace transponder signal from the recorder. 

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.