There are three types of satellite systems operating within the Cospas-Sarsat programme.
Initially, satellites used the 121.5 MHz frequency to locate distress calls. To enable faster response times, the new satellites launched from 2005 onwards were designed to use the 406 MHz frequency. Beacons still transmit signals on both frequencies, but the 121.5 MHz frequency is only used now to refine positions, for example when looking for a shipwreck.
Satellites in low Earth orbit (LEO), at an altitude of 900 to 1,000 km, are usually the first to respond when a locator beacon is activated. The first LEO satellites enabling Cospas-Sarsat to save its first lives were launched in 1982. They take about 100 minutes to circle the planet and form a system called LEOSAR. These satellites have been launched by METOP (France) and NOAA (National Oceanic & Atmospheric Administration, United States), and are also used for local weather forecasting. The LEOSAR system calculates the position of activated beacons using Doppler processing techniques: the shift in frequency of the signal received from a beacon is measured and the ground receiving station is then able to calculate the position of the person(s) to be rescued from the precise position of the satellite.
Geostationary (GEO) satellites orbit at around 36,000 km and therefore have a wider field of view, but their fixed equatorial position with respect to Earth means the Doppler effect can’t be used to obtain positional fixes. They therefore complement LEO readings by confirming estimated positions or by retrieving the position in the encoded message of beacons equipped with an internal or external GPS receiver. GEO satellites form the GEOSAR system and have been launched by NOAA, (GOES satellites), the Indian Space Research Organization (ISRO, INSAT satellite) and the European Space Agency (ESA, Meteosat Second Generation/MSG satellites).
Recently, satellites in medium Earth orbit (MEO) have started to be launched to an altitude of 20,000 km to progressively replace the current generation and optimize search-and-rescue operations. To operate properly, the MEOSAR system needs a large number of satellites in various orbits, a large number of receiving antennas on the ground and a network to enable a fast response in near-real time. There are 43 MEO satellites operating today, including the Galileo satellites for Europe, GPS for the United States and Glonass for Russia, and 70 are planned for after 2020. MEOSAR doubled the number of lives saved every day between 2016 and 2017.
France is the only nation operating active or hybrid antennas (supplied by CNES, through Thales) that can search and emit a conjunction of digital signals without any mechanical movement and therefore handle several distress signals at the same time.
Cospas-Sarsat is now looking at ways to give users the ability to confirm that their distress call has been received (currently a red button only indicates that it has been sent). Mission Control Centres (MCCs) will probably send signals at the same time as messages for rescue coordination centres.