Under the current system, which first became operational in the mid-1980s as part of the international COSPAS-SARSAT system, the so-called "repeaters" are placed on NOAA weather satellites operating in low-Earth (LEO) and geostationary orbits. Although it has proven its effectiveness, as evidenced by the number of persons rescued over the system's lifetime, the current capability does have limitations, Affens said.
The LEO spacecraft orbit the Earth 14 times a day and use the Doppler effect to help pinpoint the location of the signal. However, a satellite may not be in position to pick up a distress signal the moment a user activates the beacon.
NOAA's geosynchronous weather satellites, on the other hand, orbit above the Earth in a fixed location over the equator. Although they do provide continuous visibility of much of the Earth, they cannot independently locate a beacon unless it contains a navigation receiver that encodes and transmits its position. Emergency beacons are offered both with and without GPS location data. Furthermore, the beacon-to-satellite link can be obstructed by terrain.
DASS overcomes these limitations, said Mickey Fitzmaurice, space systems engineer for the NOAA Search and Rescue Satellite-Aided Tracking (SARSAT) program, the organization that operates the U.S. component of the COSPAS-SARSAT system now comprised of 40 nations. "With a mid-Earth orbit search and rescue capability provided by GPS, one emergency signal goes off, and six satellites will be in view," he said. "Almost instantly, I can begin processing the signal to determine its precise location. Right now, it can take an hour or more before we can even act on a signal," he said.
