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MEOSAR to the rescue

By Jim King

MEOSAR, the next generation satellite system for search and rescue, is now being developed, as the venerable Cospas-Sarsat LEOSAR system approaches a quarter century of service.

The Cospas-Sarsat system, in operation since 1982, was initially based on a constellation of four low-Earth orbiting (LEO) satellites carrying 121.5 & 406 MHz payloads, and was later enhanced in the 1990s when 406 MHz payloads were added to a few geostationary (GEO) satellites. Overall Cospas-Sarsat works very well, but both these systems have inherent limitations, including the time delay for a LEO satellite to pass near a distress location and the limited coverage of GEO satellites in polar regions and mountainous areas, where a direct line of sight to a GEO satellite might never be possible.

Figure 1 A MEO footprint is much larger than a LEO footprint and moves much slower

These time and coverage limitations will be overcome by the future MEOSAR system, comprising many satellites in medium-Earth orbit (MEO), at about 20,000 km, that will relay 406 MHz beacon signals to a new type of ground station called a MEOLUT. This system will quickly compute beacon locations by ranging or triangulating signals received via multiple satellites, using techniques similar to satellite navigation, but in reverse, since the user activates a transmitter rather than a receiver.

A MEO satellite has a footprint much larger than a LEO, and almost as large as a GEO, that slowly moves around the world, providing long periods of coverage, even in polar regions.

Figure 2Multiple MEO satellites relay beacon signals to a MEOLUT

Multiple MEO satellites in the sky will provide continuous coverage everywhere in the world, with various look angles to the satellites, so blockage by local terrain would have little impact.

Plans are now being made to have 406 MHz payloads on future global navigation satellites (GNSS), such as USA's GPS, Russia's GLONASS and Europe's new Galileo system. Each constellation could each have about 20 to 30 satellites, and the Galileo system might also provide a return link to the distress beacon to acknowledge receipt of the distress alert.

EMS Satcom, in partnership with the Canadian Space Agency (CSA) and Communications Research Centre Canada (CRC), developed a prototype MEOLUT that is now being used to conduct MEOSAR proof-of-concept tests using experimental payloads on some GPS satellites. This ground station has 3 antennas, like the one shown below, that track 3 satellites simultaneously, allowing 406 MHz beacons to be detected and located quickly.

Similar testing is also being done in USA, using a prototype ground station at NASA, and additional ground stations are planned in other countries in the future. Preliminary trials with both these ground stations are already demonstrating the great potential of the MEOSAR system.

During the coming years that it will take to deploy a MEOSAR constellation, there will not always be enough satellites to locate beacons by triangulation, but individual satellites would still relay distress alerts, just like GEOSAR satellites do, and have a slowly moving footprint. Hence, in the early days, MEOLUTs with just one tracking antenna could receive and decode distress alerts, but not necessarily locate beacons, and additional antennas could be added later when more MEOSAR satellites are in orbit.

Adding the MEOSAR enhancement will be like moving from a slow, dial-up Internet connection to a high-speed, always-on connection, and will offer many benefits, including:

• continuous, global coverage
  
  
• more reliable reception of beacon signals by multiple signal paths
  
  
• near-instantaneous detection and location of beacons
  
  
• ability to track moving beacons on a life raft or on an aircraft even before it crashes

The MEOSAR system will ensure that both search and rescue forces and 406 MHz beacon users worldwide will have the optimum distress alerting and locating service for many years to come.

Jim King is a Director at Industry Canada's Communications Research Centre (CRC) in Ottawa. He works on various Canadian and international satellite programs and has over 25 years experience on Cospas-Sarsat, including 10 years at the Cospas-Sarsat Secretariat at Inmarsat in London, England. He participated in the development, launch and testing of the first Sarsat LEO satellite in the early 1980s, the first GEO satellite and now works on the new MEOSAR system. Mr. King has a Master's degree in Electrical Engineering, specializing in satellite communications, is a Canadian delegate at Cospas-Sarsat and the European Space Agency, and has written many papers on satellite systems.

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