Emergency locator transmitter
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An Emergency Locator Transmitter (ELT) is a radio-frequency transmitter that generates a signal to assist in search and rescue (SAR) for missing aircraft. In theory, an ELT is activated when subjected to the forces of a crash impact, or manually by survivors. Nearby aircraft or ground stations will then receive the anonymous emergency signal, broadcast on the standard 121.5 MHz distress frequency, and begin to search for the source. There are hundreds of thousands of such units in service today. Most general aviation aircraft in the U.S. are required to carry an ELT, while most commercial airliners are not.
More recently, a satellite system was developed to improve detection and location of ELT signals. Unfortunately, the more sensitive detection included many more unwanted signals. Most signals are accidental false alarms, or radio frequency interference (RFI). Even the actual emergencies are difficult to locate precisely (within a radius of 15-25 nautical miles). The ratio of false alarms to emergencies runs about 500 to 1. This has placed a huge burden upon the SAR infrastructure, with so many frustrating false starts.
A newer and more powerful version of satellite ELT has been activated that uses a digital signal encoded with an idenfication number and transmitted on 406.0 MHz. Only encoded signals are processed, removing the RFI alert problems. The new system permits nearly instantaneous detection, and the ability to pinpoint beacons within 2-5 nm. Because each alert is known to be from an ELT, SAR resources can be activated immediately and directed toward the general signal area. Where users have complied with the mandatory registration of their unique identifiers, providing emergency contact information, many false alarms can be quickly resolved with a corroborating telephone or radio call. With an additional Global positioning system (GPS) feature, digitally transmitting its location, the search radius can be reduced to less than 100 m (300 ft). The newer ELTs also transmit on the older frequencies, to better assist SAR units having existing radio detection equipment.
The international group responsible for monitoring and locating ELTs via satellite has announced a 9-year phase-out of the older ELT program, to be completed in early 2009. Aviators and mariners using the older technologies are being urged to upgrade, although the newer units can cost 2 or 3 times what the older units did (and more with GPS features). Widespread adoption is expected to save over 130 lives per year and millions of dollars of wasted SAR resources.
See also
External links
International Satellite System for Search and Rescue (http://www.cospas-sarsat.org/)