Emergency Locator Transmitter (ELT) Function & Uses

Emergency Locator Transmitter (ELT) image

Emergency Locator Transmitter (ELT)

An Emergency Locator Transmitter (ELT) is installed on an aircraft for transmitting the distressed signal automatically in case of a crash. It is a self-contained, battery-powered transmitter that is activated by severe G-forces during a crash. The ELTs can be activated manually by the Flight Crew/Maintenance personal or automatically by a G-switch installed in the unit.

A position may be transmitted by some 406 MHz ELTs as part of the distress alarm. This position could be a one-time input from the aircraft's navigation system, or it could be updated on a regular basis by a GPS processor inside the ELT. Because it is unknown when the ELT's location was last placed into the message, the one-time position input may not represent the most precise position of the ELT.

How ELT Works

ELTs transmit a digital signal at a frequency of 406.025 MHz at 5 watts at every 50 second interval for at least 24 hours. Distress signals from anywhere on the earth are received by satellites in the COSPAS-SARSAT satellite system. Low earth orbiting (LEOSATs) and geostationary satellites (GEOSATs) are two types of spacecraft that have different but complementary capabilities. The received signal is partially processed and stored by satellites unit before being relayed to local user terminals installed on the ground. At the LUTs, a signal is decoded further, and necessary search and rescue operations are notified via mission control centres (MCCs) set up for this purpose.

Some other types of emergency locator beacons are maritime vessel emergency locator beacons, which are known as EPIRBs, and personal locator beacons (PLBs), which work on the same techniques as aviation ELTs.

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ELT Antenna

Aircraft Installation of ELT

ELTs are typically installed as far aft as possible in an aircraft's fuselage, just before the empennage. The aircraft's longitudinal axis is aligned with the built-in G-force sensor. ELTs on helicopters can be placed anywhere on the aircraft. They have activation devices that can be used in several directions. For correct installation, inspection, and maintenance of all ELTs, follow the ELT and airframe manufacturer's recommendations. The crash site's location precision is reduced to within 100 metres as a result of this.

According to regulatory requirements, ELTs must be installed in aircraft. Within 12 months of the preceding inspection, ELTs must be inspected for appropriate installation, battery corrosion, control and crash sensor operation, and the presence of an adequate signal at the antenna. Testing can be done without sending an emergency signal thanks to built-in test equipment. The rest of the inspection is done visually. It is advised that technicians do not use the ELT to transmit an emergency distress signal. The inspection must be documented in maintenance records, along with the battery's new expiration date. This must be written on the outside of the ELT as well.

406 MHz ELT Programming

A unique ELT identification number, often known as a hex code, is transmitted by every ELT with a 406 MHz transmitter. This hex code provides information about the aircraft's country of registration as well as the ELT, or aircraft identification. When this hex code is registered, it allows the relevant government authorities in the aircraft's registered country to assist in an emergency.

After being transmitted to the satellite, a hex code is broadcast to a rescue coordination centre. The hex code will be decoded, the registration will be cross-referenced, and the search and rescue process will begin. By registering your beacon, you can reduce the number of false alarms.

The digital signal also contains unique registration data. It contains information about the aircraft, the owner, and contact information, among other things. When a signal is received, it is used to instantly investigate the alert's legitimacy to ensure that it is a legitimate emergency message, preventing rescue resources from being dispatched unnecessarily.

ELT Activation & Cockpit Switch

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ELT Cockpit Switch

ELTs with automated G-force activation can be easily removed from aircraft. They frequently include a portable antenna so that crash victims can leave the scene and take the ELT with them. If the ELT is activated, the pilot must be notified via a flight deck panel.  The Switch installed in the cockpit also allows the ELT to be armed, tested, and manually activated if needed.

Modern ELTs may also transmit a signal on 121.5 MHz frequency. This is an analogue broadcast with homing capabilities only. Prior to 2009, the CORPAS-SARSAT satellites monitored 121.5 MHz as a global emergency frequency. The 406 MHz standard, on the other hand, has replaced it.

ELT Testing/Activation

Older ELTs, on the other hand, frequently lack the built-in test circuitry seen in newer ELTs certified to TSO C-126. As a result, a true operational test may include signal activation. This can be accomplished by removing the antenna and replacing it with a dummy load.

It is advised that any Testing/activation of an ELT equipment is required to only be done between the top of each hour and 5 minutes after the hour which enables to find out actual or false signals. The activation period must be no more than three audible sweeps long. Prior to testing, make contact with the nearest control tower or flight service station.

It's worth noting that older 121.5 MHz analogue signal ELTs frequently broadcast an emergency signal at 243.0 MHz. This has long been the military emergency frequency. The use of old ELTs is being phased out in favour of digital ELT signals and satellite monitoring. The 406 MHz has best capabilities in coverage, precise location, identification of false alerts, and shortened the response times. These days 406 MHz ELTs are currently the service standard worldwide.

The older generation 121.5 MHz ELTs have maximum percentage of false alarm rate (97%), activate properly in only 12 percentage of crashes, and provide no identification data of aircraft.

The Modern ELTs capable of 406 MHz have dramatically reduce the false alert impact on SAR resources, have higher accident survivability success rate, and also decrease the time required to reach accident victims site by an average of 6 hours.

Monitoring Station of ELT

The United States portion of the COSPAS-SARSAT system is maintained and operated by NOAA. The COSPAS-SARSAT system is maintained and monitored by INMCC in India, SAR/Galileo in Europe & many more in other countries.


The International Cospas-Sarsat Program stopped monitoring 121.5 MHz by satellites on February 1st, 2009, due to the clear advantages of 406 MHz beacons and the major shortcomings of the older 121.5 MHz beacons. Both the NOAA and the FAA strongly recommend all pilots to consider switching to 406. The Modern ELTs capable of 406 MHz have dramatically reduce the false alert impact on SAR resources, have higher accident survivability success rate, and also decrease the time required to reach accident victims site by an average of 6 hours.

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