Thursday, January 1, 2004
Emergency Locator Transmitters
How they work and why things are about to change
Search and rescue is serious and often expensive business, not to mention potentially hazardous to the searchers and rescuers. SAR teams often fly in the same conditions that contributed to an accident in the first place. For those reasons, it's vital that the AFRCC weed out phony alerts from real ones, a time-consuming process during which victims of an emergency could be in desperate need of help.
The age of Emergency Position Indicating Radio Beacons (EPIRBs) ushered in dramatic changes in SAR capabilities, all for the better. First came standard EPIRBs, transmitting with five watts of power rather than the half-watt of the old 121.5 ELTs. This increased range dramatically. EPIRBs broadcast on 121.5 plus 406 MHz, and their digital signals are available to a variety of satellites. Each EPIRB must be registered and any activation transmits contact information, so SAR authorities can track down the owner and determine if there's a real emergency.
The latest EPIRBs are even more sophisticated devices, designed to broadcast not only the user's identity and contact information, but fitted with integrated GPS sensors to allow transmitting the exact lat/long coordinates of a downed aircraft. Other EPIRBs interrogate the aircraft's onboard GPS equipment for position information.
As the name implies, Geostationary Operational Environmental Satellites (GOES) maintain position 22,000 miles above a fixed location and serve a variety of functions in telecommunications, from sending down live weather photos to expediting communications. The GOES universe also listens for EPIRB activations, flying so high that they can receive signals from a huge area, nearly a third of the earth's surface.
The bad news is that because GOES orbits at the same rate as the Earth below and therefore can't employ Doppler shift, the satellites can't fix position from a standard 406 EPIRB transmission. They can, however, alert an appropriate rescue communications center with encoded information from the EPIRB. If the beacon includes a GPS sensor, GOES can transmit position as well as ID code.
Search And Rescue Satellite Aided Tracking (SARSAT) satellites circle the Earth at 13,500 knots and can fix position of an EPIRB signal precisely. They fly polar orbits 528 miles up and view a moving, circular area of Earth with a diameter of 2,500 miles as they orbit. Similarly, the Russian COSPAS universe of satellites orbit at 620 miles with comparable coverage. Between the two systems, they cover every location on Earth every 30 to 45 minutes, so their response time for search and rescue applications is excellent.
The difference in position accuracy of the three types of beacons is dramatic. A standard 121.5 MHz ELT defines a search area to about a 12-nm radius, roughly 450 square nautical miles. A non-GPS-equipped EPIRB reduces the search area to a 2-nm radius, 12.5 square nm. Finally, the top-of-the-line GPS/EPIRB narrows the search to a 0.05-nm radius, equal to a 0.008-square-nm search area.
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