An emergency locator transmitter (ELT) is an independent battery-powered transmitter that is activated by the excessive G-forces exerted during an aircraft crash. They have the ability to transmit a digital signal every 50 seconds on a frequency of 406.025 MHz at 5 watts for a minimum of 24 hours. This signal can be received anywhere in the world by satellites in the COSPAS-SARSAT satellite system.
There are two types of satellites, those of which are called low earth orbiting (LEOSATs) and geostationary satellites (GEOSATs). The transmitted signal is partially processed and stored in these satellites before being relayed to ground stations referred to as local user terminals (LUTs). At this stage, the LUTs further decipher the signal, and the appropriate search and rescue operations are notified via mission control centers (MCCs).
Since their widespread implementation on commercial aircraft in the 1970s, ELTs have become commonplace in the aviation realm. Today, ELTs must be installed in aircraft according to FAR 91.207. For the most part, this encompasses most general aviation aircraft that do not operate under Parts 135 or 121. Furthermore, ELTs must be annually inspected for proper installation, battery corrosion, operation of the controls and crash sensor, and the presence of an ample signal at the antenna.
Beyond such inspections, built-in test equipment enables testing without the transmission of an emergency signal. The next part of the inspection is visual, and it must be recorded in maintenance records and on the outside of the ELT. Nonetheless, technicians are still warned to not accidentally activate the ELT and transmit an emergency distress signal.
ELTs are usually installed near the tail of an aircraft while the built-in G-force sensor is aligned with the longitudinal axis of the aircraft. ELTs that are equipped with an automatic G-force activation sensor mounted in the aircraft are easily removable. They contain a portable antenna, allowing crash victims to leave the site and carry the operating ELT with them. Meanwhile, a flight deck mounted panel alerts the pilots when the ELT is activated, and allows the ELT to be armed, tested, and manually activated if necessary.
Modern ELTS also have the ability to transmit a signal on the 121.5 MHz frequency. This is an analog transmission that can be utilized for homing. Before 2009, 121.5 MHz was the worldwide emergency frequency monitored by the CORPAS-SARSAT satellites. Since then, it has been replaced by the 406 MHz standard. As the use of 406 MHz ELTs has not been made compulsory by the FAA, the 121.5 MHz frequency is still an active emergency frequency that is monitored by over-flying aircraft and control towers.
Most aircraft technicians are required to carry out annual inspections of 121.5 MHz ELTs and vet them as thoroughly as 406 MHz ELTs. As older ELTs lack built-in test circuitry, an operational test for such versions may include activating the signal. This can be achieved by removing the antenna and installing a dummy load.
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