Emergency Locator Transmitters (ELT) are radio frequency transmitters whose functions and frequencies have been defined by ICAO and which transmit distinctive signals on designated frequencies in emergency situations.
Persons, vessels, or aircraftin distress or otherwise in distress are equipped with a navigation device and can be located and identified by the broadcast signals transmitted by the distress transmitters. The transmission of the locating signals can be triggered automatically or manually in an emergency. Emergency transmitters are very light, in addition to the RF transmitter they have a flashing light visible from far away and a battery with a service life of several years. The transmitted locating signal has an accuracy between 10 m and 100 m. In the past, emergency transmitters transmitted on 121 MHz, 243 MHz or 406 MHz, since several years only on the international emergency radio frequency of 406 MHz.
For hikers in mountain distress, there are small emergency transmitters, the Personal Locator Beacon (PLB), instead of the Emergency Locator Transmitter (ELT).