Emergency Locator Transmitter
An Emergency Locator Transmitter (ELT) is an emergency beacon used in aircrafts to alert rescue authorites and to indicate the location and the identity of an aircraft in distress. Most civil aircrafts are required to carry them.
Modern ELTs operates on 406 MHz. The signal of the ELT is detected by the COSPAS-SARSAT satellite network anywhere in the world. The network passes the alert to the nearest rescue authority.
The COSPAS-SARSAT satellite network can detect the location of the ELT with an accuracy of 2 to 5 km. There are also 406 MHz ELTs available with an integrated GPS navigation receiver. These ELTs will transmit an accurate location.
The ELT also transmits a low-power "homing" signal on 121.5 MHz. This allows rescue forces to home in on a beacon as soon as they are in the vicinity.
The ELT transmits the identity of the aircraft. This identity is a 15 character hexadecimal code including the ICAO 24 bit code of the aircraft. It is important to register the ELT with the associated identity at the local authority.
ELTS were the first emergency beacons developed. The first ELTs operated on the 121.5 MHz. They could only be detected by overflying commercial or military aircrafts. Satellites were designed to detect these EPIRBs as well, but detection by satellites is limited on this frequency. COSPAS-SARSAT announced that it will cease detection on 121.5 MHz by February 1st, 2009.
Military aircraft use a ELT that operates on 243 MHz. These are also phased out.