The COSPAS-SARSAT System, operational 24 hours a day/365 days a year, automatically detects and locates emergency radio beacons globally by satellites and alerts appropriate rescue authorities.

COSPAS-SARSAT is an international, humanitarian search and rescue system that uses satellites to detect and locate emergency beacons carried by ships, aircraft, or individuals. COSPAS-SARSAT has helped save over 10,600 lives (as of November 1999) worldwide since its inception in 1982. The system consists of a network of satellites, ground stations, mission control centers, and rescue coordination centers. When an emergency beacon is activated, the signal is received by a satellite and relayed to the nearest available ground station. 

A “virtual” constellation of low earth orbiting (LEO) satellites and geostationary (GEO) satellites, launched to serve other purposes, carry auxiliary COSPAS-SARSAT transponders capable of receiving and retransmitting the beacon signals to earth stations. Emergency locator beacons, called ELTs in aircraft, EPIRBs in maritime use, and PLBs for personal use, are activated in emergency situations. The beacons operate on three different frequencies: 121.5MHz, 243MHz, and 406MHz. Doppler processing can be used to estimate the location of the beacon signals from LEO satellites. Some new 406MHz beacons are capable of transmitting GPS location on the digital data stream.  Worldwide discussions are currently taking place regarding the discontinuation of COSPAS-SARSAT satellite coverage for 121.5 MHz and 243 MHz beacons.

The COSPAS-SARSAT system is designed to have at least two COSPAS and two SARSAT LEO satellites (4 total) operational at all times. The number of operational satellites and their status is variable based on failures in orbit. Presently, there are more operational satellites than the minimum required by the system specification. Periodically, new satellites are launched to replace those which have degraded or failed in orbit due to age. The NOAA satellites orbit at an altitude of 850 km (528 miles), with their orbits inclined 99° from the equator, completing an orbit every 100 minutes. The Nadezhda navigation satellite orbit the Earth every 105 minutes at an altitude of 1000 km (620 miles) with an orbital inclination of 83°. Because the satellites are in polar low Earth orbits, there is up to a 1.5 hour delay before a satellite passes over the beacon site and receives its transmission. The delay is longest at the Equator and shortest at the poles.