Emergency Locator Transmitters (ELTs) are radio beacons that are carried on board aircraft and triggered in the event of a crash or other unplanned downing.
The two types of ELT now in service are the 406 MHz ELT and the 121.5 MHz ELT.
406 MHz ELTs transmit a 406 MHz digital distress signal containing information on the type of emergency, the country and identification code of the beacon, and other data to assist search and rescue operations; and a lower-powered homing signal on 121.5 MHz to guide search and rescue teams to the aircraft once they arrive in the general area.
121.5 MHz ELTs transmit an analog signal on 121.5 MHz containing only an audio alert, intended to serve both as a distress signal and a homing signal.
ELT System
The ELT is designed to transmit a digital distress signal to satellites that are a part of the COSPAS-SARSAT SYSTEM. These satellites transmit the captured signal to the reception station on the ground. The signal is transmitted on the 406.028 MHz and is used to precisely locate and identify the ELT.
The ELT transmits 121.5 MHz and 243 MHz signals to facilitate the final approach of the distress scene (homing).
The ELT operates manually or automatically by means of an acceleration (G-Switch in accordance with EUROCAE ED-62 standard). Any encoding protocol defined by COSPAS/SARSAT can be used with ELT including country code assignment.
The ELT system transmits on 3 frequencies, 121.5 MHz (Civil) and 243 MHz (Military) homing signals and 406 MHz to the COSPAS-SARSAT satellite system. The battery pack, installed in the ELT housing, supplies the power to operate the system.
The satellite system transmits the 406 MHz distress signal to a Local User Terminal (LUT) when the LUT is in range. The LUT receiving range is a radius of approximately 2500 km (1367 NM). When the LUT is not in receiving range, the satellite system stores the distress signal until the transmission is possible. The LUT automatically processes the distress signal to identify and show the position of the aircraft to a radius of approximately 1.8 km (5900 ft.). The processed data is transmitted to a Mission Control Center (MCC). The MCC sends the data to an applicable Rescue Coordination Center (RCC), where Search And Rescue (SAR) operations are started. The 121.5 MHz and 243 MHz signals are used to find the aircraft in the final stage of SAR operation.
The ELT, installed in the aircraft, transmits distress signals. It can be operated automatically or manually. The battery pack supplies the power to operate the ELT. The Global Positioning System (GPS) receiver locates the position of the ELT for approximately 60 meters. The ELT transmits the position within minutes following the distress.
Operation Modes
Automatic Operation: Occurs when the ELT is connected to its system in the aircraft and the gravity switch detects an impact sufficient to start transmission.
Manual Operation: Occurs when the ELT is connected to its system in the aircraft and you do a self-test (BITE) when the ON-ARMED-TEST/RESET switch on the remote control panel is in the “ON” position.
Accidental Operation: Occurs when the gravity switch starts transmission without an emergency (e.g. hard landing). The unwanted transmission stops, when the ON-ARMED-TEST/RESET switch on the remote control panel is pushed to the TEST/RESET position and held for more than one second in this position.
ELTs are – Fixed, and Portable.
Emergency locator transmitter (ELT). A generic term describing the equipment that broadcast distinctive signals on designated frequencies and, depending on the application, may be automatically activated by impact or be manually activated. An ELT may be any of the following:
- Automatic fixed ELT – An automatically activated ELT which is permanently attached to an aircraft.
- Automatic portable ELT – An automatically activated ELT which is rigidly attached to an aircraft but readily removable from the aircraft.
- Survival ELT – An ELT which is removable from an aircraft, stowed so as to facilitate its ready use in an emergency, and manually activated by survivors.
