In the mid- to late 1970s, several nations independently experimented with space-based assets for detection and location of emergency beacons. Their objective was to improve distress alerting capability beyond what current airborne and ground-based systems could provide.
As a result of cooperation in this area, a Memorandum of Understanding (MOU) among agencies of the former Soviet Union, the United States, Canada and France was signed in 1979 which allowed for the international development of a coordinated satellite system for SAR. The system was known as Cosmicheskaya Sistyema Avariynich Sudov (“space system for the search of vessels in distress”) – Search and Rescue Satellite-Aided Tracking, or Cospas-Sarsat.
Following the successful completion of a demonstration and evaluation phase, the Centre National d'Etudes Spatiales (CNES) of France, the Department of National Defence (DND) of Canada, the Ministry of Merchant Marine (MORFLOT) of the former USSR, and the National Oceanic and Atmospheric Administration (NOAA) of the USA signed a second MOU on 5 October 1984. The Cospas-Sarsat System was declared operational in 1985, and on 1 July 1988, the four States providing the space segment signed the International Cospas-Sarsat Programme Agreement (ICSPA) which ensures the continuity of the System and its availability to all States on a non-discriminatory basis. Under the provisions of the ICSPA, a number of other States non-party to the Agreement have since associated themselves with the Program and participate in the operation and management of Cospas-Sarsat.
Cospas-Sarsat was originally comprised of satellites in low-altitude Earth orbit (LEO). The LEO satellites and associated ground receiving stations, or Local User Terminals (LEOLUTs), are compatible with distress beacons operating at 121.5 MHz, 243 MHz or 406 MHz. The LEOSAR system calculates the location of distress beacons using the Doppler effect on the received beacon signals. However, because of LEOSAR satellite orbit patterns which result in non-continuous Earth coverage, delays are possible between beacon activation and the generation of an alert message.
In 1998, following several years of testing, Cospas-Sarsat decided to augment the LEOSAR system by formally incorporating SAR instruments for detecting 406 MHz beacons on geostationary (GEO) satellites. Each GEO satellite provides continuous coverage over the geographic region defined by its footprint and reduces the detection delays associated with the LEOSAR system. Because of their altitude each GEOSAR satellite provides coverage of about one third the surface of the Earth excluding the polar regions.
Because of their high altitude and fixed position with respect to the Earth, GEOSAR satellites have several limitations. They cannot independently locate a beacon, and can provide only location information if the beacon contains a navigation receiver and transmits its position. Terrain features blocking the beacon to satellite link cannot be overcome because the satellite is stationary with respect to the beacon, and the beacon to satellite to LUT communication link is less robust than a LEOSAR link because of the greater distances involved. They do, however, provide continuous visibility to much of the Earth and thus improve the overall Cospas-Sarsat constellation.
Cospas-Sarsat today is composed of 39 countries and organizations. More than 900,000 maritime, aviation, and land-based distress beacons use the system worldwide. Cospas-Sarsat is credited with assisting search and rescue forces in the saving of more than 18,500 lives since the first satellite launch in 1982.
For more information on Cospas-Sarsat, go to www.cospas-sarsat.org .
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