Emergency Locator Transmitter

In the field of Search and Rescue (SAR), distress radiobeacons, also collectively known as distress beacons, emergency beacons, or simply, beacons, are tracking transmitters which aid in the detection and location of boats, aircraft, and/or persons in distress. In the proper sense, the term refers specifically to the 3 types of radiobeacons (listed below) that interface with Cospas-Sarsat, the international satellite system for Search and Rescue. When activated, such beacons send out a distress signal that, when detected by non‑geostationary satellites, can be located by triangulation. In the case of 406 MHz beacons which transmit digital signals, the beacons can be uniquely identified almost instantly (via  GEOSAR), and furthermore, a GPS position can be encoded into the signal (thus providing both instantaneous identification & position.) Often using the initial position provided via the satellite system, the distress signals from the beacons can be homed by SAR aircraft and ground search parties who can in turn come to the aid of the concerned boat, aircraft, and/or persons.
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A transmitter of an aircraft or survival craft actuated manually or automatically that is used as an alerting and locating aid for survival purposes. [NTIA] [RR]

A radio transmitter attached to the aircraft structure which operates from its own power source on 121.5 MHz and 243.0 MHz. It aids in locating downed aircraft by radiating a downward sweeping audio tone, 2-4 times per second. It is designed to function without human action after an accident. (FAA4) (FAA8)

A radio transmitter attached to the aircraft structure which operates from its own power source on 121.5 MHz and 243.0 MHz. It aids in locating downed aircraft by radiating a downward sweeping audio tone, 2-4 times per second. It is designed to function without human action after an accident.
(Refer to 14 CFR Part 91.)
(Refer to AIM.)