Every year, search and rescue crews from St. John's to Victoria to Nunavut are called into action at the news of a distress signal received from an emergency beacon. Over the past 27 years, the COSPAS-SARSAT (C-S) system has helped save over 27,000 people worldwide. But what few Canadians know is that our country is a world leader in the design and development of search and rescue satellite-aided tracking (SARSAT) technology.

While Canada's contribution to the international COSPAS-SARSAT system is coordinated by the National Search and Rescue Secretariat, much of the R&D is carried out by the Communications Research Centre (CRC). Work now in progress at the CRC is paving the way for a next-generation C-S system with capabilities far beyond what currently exist.

"In the C-S system," explains CRC's Richard Paiement, engineer and project leader in the Satellite Systems Research division, "satellites pick up signals from radio beacons that are triggered by an emergency." The satellites then relay the signals to a ground station. From there, the information is sent to a mission control centre, then on to the local rescue coordination centre.

There are two types of satellites in the current system. Four low Earth orbit (LEO) satellites form the backbone of the search and rescue system. Orbiting at an altitude of approximately 850 km, they receive and relay the emergency beacon's signal. Because these satellites move relative to the position of the beacon, ground station operators are able to use the shift in the received frequency (Doppler shift) that occurs as the satellite moves across the sky to calculate the location of the emergency beacon. But, says Paiement, because LEO satellites orbit so close to the Earth, they have a small footprint.

"They ‘see' only a small patch of the planet at any given moment, an area with a radius of about 3000 km."

While this may sound vast at first glance, if you're adrift on a disabled vessel somewhere between Vancouver and Hong Kong, this is a small window indeed. The problem is compounded, says Paiement, by the fact that LEO satellites move at approximately 26,750 km per hour, giving them about a 15-minute view of any particular area. So, if your beacon was triggered just after the 15 minutes when you were within the satellite's range, it could be hours until another satellite passes over your location. Several hours stranded in Arctic conditions or injured on the side of a mountain can be the difference between life and death.

To overcome constraints posed by LEO satellites, geostationary Earth orbit (GEO) satellites were added to the C-S arsenal in the 1990s. These high-altitude satellites orbit directly above the equator, moving in synchrony with the Earth. A GEO satellite positioned over Brazil, for example, will remain over Brazil as the Earth rotates through its day/night cycle. Because they orbit at about 36,000 km above the Earth, GEO satellites have a huge footprint. They can, says Paiement, instantaneously detect and relay a distress signal from an area covering slightly more than one-third the surface of the Earth, and because they move in synchrony with the planet, they are always "watching" a specific area.

They have, however, several downsides. GEOs are unable to "see" the poles, leaving much of the Arctic and Antarctic without service. Also, mountains or other obstacles may block the satellite's view of the beacon. Finally, because GEO satellites maintain a steady position over a portion of the Earth, they don't move relative to a signalling beacon. With no Doppler shift to provide additional information, GEOs can readily detect a distress signal but are unable to help in determining the beacon's location.

To overcome the combined limitations of LEO and GEO satellites, search and rescue organizations are now working to recruit MEO satellites into the search and rescue effort. MEO - medium Earth orbit - satellites orbit between the LEO and GEO satellites. If you use a GPS receiver in your car, you're already making use of MEO satellites in your everyday life. Unlike the LEO satellites, MEO satellites have a substantial footprint - a radius of over 14,000 km- and their 20,000 km altitude gives them a view time of a few hours from a given ground location, rather than the brief 15 minutes of a LEO satellite. And, says Paiement, a constellation of several MEO satellites brings a huge added bonus.

"When a distress beacon is triggered it repeats its signal every 50 seconds. With a constellation of MEO satellites, there is usually enough information in two to three bursts [repeats] to determine the beacon's coordinates. Often it can be done in a single burst. That means that within a few minutes we can locate the beacon's position."

Currently, says Paiement, Europe, Russia and the U.S. all have plans to launch constellations of navigation satellites in MEO with search and rescue transponders aboard to form the MEOSAR system, and by 2018 one or more of these constellations should be fully operational. But, says Paiement, while the use of MEO for search and rescue holds tremendous potential, there are significant issues to resolve before depending on them to save lives. For example, his division is working on the satellite tracking problem.

"Given current launch plans, there may be 12 or more MEO satellites ‘in view' of a single ground station, but that station may, for example, only have four antennas, so it can only track four of those satellites. And that constellation of satellites is changing all the time as these satellites speed across the sky. Ground station operators need to know which four satellites to track. In other words, which four satellites will give them the most precise location information if a distress beacon starts transmitting in their area of responsibility? We're developing an algorithm to make these kinds of decisions."

In addition to the work being done by Paiement and his colleagues, CRC has also set up an experimental tracking station able to collect distress and test signals from experimental MEOSAR and operational GEOSAR satellites. CRC engineers and scientists are using the test facility to devise, among other things, advanced signal processing techniques for improving both the detection and location of beacons, as well as develop tools to monitor satellite traffic and to locate and identify sources of interference.

With the help of work being carried out at CRC, by 2013, when the first operational MEOSAR satellites are launched, Canada will be ready to harness their lifesaving potential.

For more information contact Richard Paiement, engineer and project leader in the Satellite Systems Research division, at mailto:%20richard.paiement@crc.gc.ca or 613-998-2861.