COSPAS-SARSAT is the International Satellite System For Search and Rescue. Its space segment consists of two complementary constellations, of multiple satellites, which support worldwide search and rescue operations. One constellation is in low earth orbit, while the other is in geostationary orbit.
As of 2009, all satellites listen on the single UHF international rescue frequency of 406 MHz.
The actual satellites that detect signals are operated by different countries; the rescue signal receiver, processor, and downlink are usually secondary payloads on a satellite that performs other services.
COSPAS-SARSAT's secretariat is based in Canada, although there are Local User Terminals all over the world. The headquarters location was agreed among Canada, the Republic of France, the Russian Federation and the United States of America. To coordinate worldwide technical requirements, COSPAT-SARSAT cooperates with the International Telecommunications Union, the International Maritime Organization, and the International Civil Aviation Organization.
At present, there are 5 satellites in each constellation.
Geosynchronous satellites "stare" at the earth's surface and can almost immediately detect an emergency position indicating radio beacon (EPIRB) transmission, which may contain a position determined by a GPS receiver in the beacon. They are the early warning part of the system. Among their limitations is that they have relatively poor coverage of the high latitudes (i.e., Arctic or Antarctic)
GEOSAT orbiters include:
- Europe: Second-generation meteorological satellite (EUMETSAT); French operated
- India: Indian Geostationary Satellite (INSAT)
- Umited States: Geostationary operational environmental satellite (GOES)
If the beacon has no navigational receiver, or the receiver is malfunctioning, the LEO satellites can determine the beacon's surface coordinates by the Doppler effect of the radio signal received by satellites in known orbital positions.
Since the LEOSAT constellation spiral in a "ball of twine" orbit around the earth, they do provide polar coverage. Since they move in relation to the beacon, if an obstruction blocks the beacon signal to GEO, one of the LEO satellites will usually move into a position in which it has line of sight to the beacon.
- Russia: Space system for the search of vessels in distress (COSPAT); aboard Sterkh LEO satellites
- United States: National Oceanic and Atmospheric Administration LEO satellites
Obviously, the EPIRBs are an essential part of the system. Once a satellite detects a signal, it sends it to an appropriate Local User Terminal under its orbit. There are 45 LUT for LEOSAR and 19 for GEOSAR; since geostationary satellites are in a fixed position relative to the Earth, not as many stations are needed.
There are 29 Mission Control Centres (MCCs) that receive LUT messages, correlate observations, and then dispatch appropriate search and rescue organizations.