A subspace rescue beacon is a portable piece of Starfleet equipment that is used to send out a distress call. It is standard equipment used on various Starfleet shuttlecrafts. In 2282 when the shuttlecraft Kepler crashed on a Gorn training planetoid, the survivors attempted to set up the subspace rescue beacon to notify the USS Reliant. (ST - Alien Spotlight comic: "Alien Spotlight: The Gorn")