A subspace beacon is an omnidirectional repeating signal transmitted in the subspace realm.

A standard Starfleet combadge can be modified to emit a subspace beacon.

In an alternate timeline, Kathryn Janeway activated the subspace beacon in her combadge when she and Tom Paris were thrown back in time on an unknown Class M planet. The senior staff of the USS Voyager used the beacon to locate and attempt to retrieve Janeway and Paris. (VOY: "Time and Again")

Federation escape pods are equipped with subspace beacons, allowing personnel to track evacuating crew. (VOY: "Year of Hell")

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