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How Does An Emergency Locator Transmitter (ELT) Work?

Emergency Locator Transmitters (ELTs) are radio beacons that are supplied on board aircraft and utilized in the event of a crash or other unprecedented incident. There are two main types of ELTs in service, those of which are the 406 MHz ELT and the 121.5 MHz ELT. To better understand the function of these ELT types, this blog will outline the purpose and function of ELTs.

What Are the Differences Between 406 MHz ELTs and 121.5 MHz ELTs?

406 MHz ELTs transmit a 406 MHz digital distress signal that indicates the type of emergency at hand. This signal serves as a packet of information containing the country and identification code of the beacon and other data to assist search and rescue operations. A lower-powered homing signal on 121.5 MHz is also transmitted to guide search and rescue teams to the aircraft once they arrive in the general area of the accident. 121.5 MHz ELTs, on the other hand, transmit an analog signal on 121.5 MHz containing only an audio alert which is intended to serve as both a distress signal and a homing signal.

What Does An ELT System Consist of?

The ELT is designed to transmit a digital distress signal to satellites that are part of the COSPAS-SARSAT system. The COSPAS-SARSAT system is an international, humanitarian satellite-based search and rescue system and service that can detect and locate transmissions from emergency beacons on ships, aircraft, or people. These satellites transmit the captured signal to a particular reception station on the ground. The signal is transmitted on the 406.028 MHz and is utilized to locate and identify the ELT.

The ELT transmits 121.5 MHz and 243 MHz signals to facilitate the final approach of the scene where the incident occurred. ELTs operate manually or automatically by means of an acceleration. Keep in mind that any encoding protocol defined by COSPAS-SARSAT can be used alongside the ELT including country code assignment. The ELT system transmits on three frequencies, those of which include 121.5 MHz (Civil) and 243 MHz (Military) homing signals and 406 MHz to the COSPAS-SARSAT satellite system. Within the ELT housing, a battery pack supplies power to operate the system.

The satellite antenna transmits the 406 MHz distress signal to a Local User Terminal (LUT) when a LUT is in range. The LUT receiving range has a radius of about 2500 km, and when the LUT is not in the receiving range, the satellite system stores the distress signal until transmission is possible. The LUT automatically processes the distress signal to identify and show the position of the aircraft to a radius of about 1.8 km. Then, the processed data is transmitted to a Mission Control Center (MCC). Once the MCC receives the signal, it sends the data to an applicable Rescue Coordination Center (RCC), where Search And Rescue (SAR) operations can begin. Meanwhile, 121.5 MHz and 243 MHz signals are utilized to locate aircraft in the final phase of SAR operation.


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