406 MHz GEOSAR System
The current generation of Cospas-Sarsat type approved beacons which operate at 406 MHz can also be detected by monitoring search and rescue instruments on board geostationary satellites. The GEOSAR system consists of 406 MHz repeaters carried on board various geostationary satellites and the associated ground facilities called GEOLUTs which process the satellite signal. Geostationary satellites orbit the Earth at an altitude of 36,000 km, with an orbit period of 24 hours, thus appearing fixed relative to the Earth at approximately 0 degrees latitude (i.e. over the equator). A single geostationary satellite provides GEOSAR uplink coverage of about one third of the globe, except for polar regions. Therefore, three geostationary satellites equally spaced in longitude can provide continuous coverage of all areas of the globe between approximately 70 degrees North and 70 degrees South latitude.
Since GEOSAR satellites remain fixed relative to the Earth, there is no Doppler effect on the received frequency and, therefore, the Doppler positioning technique cannot be used to locate distress beacons. To provide rescuers with beacon position information, such information must be either:
acquired by the beacon though an internal or an external navigation receiver and encoded in the beacon message, or derived from the LEOSAR system.
Cospas-Sarsat has demonstrated that the GEOSAR and LEOSAR system search and rescue capabilities are complementary. For example the GEOSAR system can provide almost immediate alerting in the footprint of the GEOSAR satellite, whereas the LEOSAR system: provides excellent coverage of the polar regions (which are beyond the coverage of geostationary satellites); can calculate the location of distress events using Doppler processing techniques; and because the satellite is continuously moving with respect to the beacon, the LEOSAR system is less susceptible to obstructions which may block a beacon signal in a given direction.