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Cospas-Sarsat System Description
Cospas-Sarsat History
International Parties
Emergency Beacons
Satellites
Cospas-Sarsat System Operation

 
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Satellites

The Cospas-Sarsat System uses two different types of satellites. The system was originally designed to function with Low Earth Orbiting (LEO) satellites which remain the heart of the system. However, 406 MHz repeaters located on Geostationary Earth Orbiting (GEO) satellites have been added to the system.

The LEO satellites in the Cospas-Sarsat System are provided by Russia and the United States with components contributed by Canada, France, Russia, and the United States. Cospas, (Russian words: Cosmicheskaya Sistyema Poiska Avariynich Sudov) which means: "Space System for the Search of Vessels in Distress", are actually electronics packages onboard Russian NADEZHDA navigation satellites. The Sarsat (Search and Rescue Satellite-Aided Tracking) packages are carried on the NOAA POES (TIROS) weather satellites. The LEO satellites are in polar orbits. Sarsat satellites, which orbit every 100 minutes, are inclined 99 degrees from the equator at an altitude of 528 miles (850 kilometers). Cospas satellites orbit every 105 minutes at an altitude of 620 miles (1000 kilometers) and an orbital inclination of 83 degrees. Each satellite, circling the earth around the poles, eventually views the entire Earth's surface. The "orbital plane", or path of the satellite, remains fixed, while the earth rotates underneath. At most, it takes only one half rotation of the Earth (i.e. 12 hours) for any location to pass under the orbital plane.

The 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 drawing at right shows the NOAA-M satellite, successfully launched on 24 June 2002, which carries the latest generation of Sarsat instruments.

The Search and Rescue packages onboard the LEO satellites use two different modes of operation:

Bent-Pipe

Repeater Mode

Satellite Launch

In the bent-pipe mode of operation, the satellite immediately retransmits received beacon signals to any Local User Terminal (LUT), which is located in the satellite's footprint at the time of transmission. Some areas of the Earth are not included in LUT coverage, particularly in mid-ocean regions of the southern hemisphere and southern Africa. Detailed information may be viewed in a map of global LUT coverage. The bent-pipe mode is provided on Sarsat satellites by a Canadian electronics package called the Search and Rescue Repeater or SARR.

Search and Rescue Repeater (SARR)

The repeater mode or store and forward mode is provided on Sarsat satellites by a French electronics package called the Search and Rescue Processor or SARP. The SARP is composed of two units: the RPU (receiver and power unit) and the SPU (signal processing unit). The satellite receives and records beacon transmission and repeatedly retransmits them, along with measured beacon frequency, to numerous LUTs as the satellite orbits the Earth. Store and forward mode provides true global coverage. This mode of operation is only possible with 406 MHz beacons.

SARP - RPU and SPU

The system operates with existing low-tech 121.5 or 243 MHz beacons and more modern 406 MHz beacons which transmit digital information. The location of the received beacon is determined by Doppler frequency analysis performed in the ground stations. The Doppler shift is caused by the relative motion of the satellite as it orbits with respect to the stationary beacon on the Earth's surface. This process can locate beacons with surprising accuracy, without the requirement for any location information to be included in the transmission. The quoted location accuracies are within 20 kilometers (12.4 miles) for 121.5 MHz beacons and within 5 kilometers (3.1 miles) for 406 MHz beacons. Most locations are determined significantly more accurately than these numbers. Final location is aided by a low-power 121.5 MHz homing signal included in most 406 MHz beacons. 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.

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