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SARSAT Future Enhancements

Medium Earth Orbit Search and Rescue (MEOSAR) System

Cospas-Sarsat is an international satellite system for search and rescue (SAR) distress alerting that was established in 1979 by Canada, France, the USA and the former USSR.  Since its inception the Cospas-Sarsat Programme has continually expanded and, as of 2010, 43 countries and organizations share in the management of the System.  At the end of 2009, more than 28,000 people had been rescued through the use of the System.  The System originally comprised satellites in low-altitude Earth orbit (LEO).  The LEO satellites and associated ground receiving stations (referred to as the LEOSAR system) receive signals from distress beacons operating at 406 MHz.  The LEOSAR system calculates the location of distress beacons using the Doppler effect on the received beacon signals.  Because of LEOSAR satellite orbit patterns, there can be delays between beacon activation and the generation of an alert message.

In 1998, following several years of testing, the Cospas-Sarsat Council decided to augment the LEOSAR system by formally incorporating SAR instruments on geostationary satellites for detecting 406 MHz beacons (referred to as the GEOSAR system).  Geostationary satellite footprints are fixed with respect to the Earth’s surface; therefore, each satellite provides continuous coverage over the geographic region defined by its footprint.  This reduces the detection delays associated with the LEOSAR system.  Because of their altitude each GEOSAR satellite provides coverage of a very large area (about one third the surface of the Earth excluding the Polar Regions).  However, because of these attributes (i.e. stationary with respect to the Earth and high altitude):

• GEOSAR systems provide location information only if this information is available from an external source (i.e. global navigation receiver in the beacon) and transmitted in the 406 MHz beacon message;
• obstructions 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 budget is less robust than the LEOSAR system because of the greater distances involved (decreasing the probability that 406 MHz beacon messages are properly detected by the GEOSAR system).

In 2000, the USA, the European Commission (EC) and the Russian Federation began consultations with Cospas-Sarsat regarding the feasibility of installing 406 MHz SAR instruments on their respective medium-altitude Earth orbit navigation satellite systems (hereafter referred to as MEOSAR constellations) and incorporating a 406 MHz MEOSAR capability into the Cospas-Sarsat System.  The USA MEOSAR system is called the SAR/GPS, the European system is called SAR/Galileo, and the Russian system is referred to as SAR/GLONASS.

Initial investigations identified many possible SAR alerting benefits that might be realized from a MEOSAR system, including:

• near instantaneous global coverage with accurate independent location capability,
• robust beacon to satellite communication links, high levels of satellite redundancy and availability,
• resilience against beacon to satellite obstructions, and
• the possible provision for additional (enhanced) SAR services, such as a ground to beacon return link.

In light of this potential, the Cospas-Sarsat Council decided to prepare for the introduction of a MEOSAR capability into the Cospas-Sarsat System and to encourage coordination among the space segment providers for System compatibility and interoperability with the proposed MEO satellite systems.

The primary missions for the three MEOSAR constellations, i.e. the Global Positioning System (GPS), Galileo and GLONASS, generally referred to as global navigation satellite systems (GNSS), are positioning, navigation, and timing.  As a secondary mission, the SAR payloads have been designed within the constraints imposed by the primary mission payloads.

The three MEOSAR satellite constellations will use transparent repeater instruments to relay 406 MHz beacon signals, without onboard processing, data storage, or demodulation/remodulation.  MEOSAR satellite providers will make their satellite downlinks available internationally for processing by MEOLUTs operated by MEOSAR ground segment participants.

The MEOSAR System Concept