|Figure 1: COSPAS-SARSAT satellites in polar, low-Earth orbit (LEO)|
The original COSPAS-SARSAT system of the 1980s comprised a constellation of four satellites in polar, low-Earth orbit, dubbed the LEOSAR system (Figure 1), and provided services for 121.5, 243 and 406 MHz beacons. This system worked well, and is still in use today, but has inherent time delays, ranging from minutes to hours, in detecting and relaying distress signals because the low altitude satellites (at about 1000 km) view only a portion of the Earth at any instant as they circle the globe.
This LEO system could not be made much better for 121.5 / 243 MHz beacons, due to technical limitations of the beacons and those radio channels.
|Figure 2: LEO and Geostationary-Earth orbit (GEO) satellite constellations|
Enchancements were made to the 406 MHz system in the 1990s, when 406
MHz repeaters were added to new satellites in geostationary-Earth orbit
at 36,000 km (Figure 2). These satellites, known as GEOSARs, constantly
view a huge, fixed area of the Earth, thereby eliminating the time delay
to relay 406 MHz distress signals. This rapid relay of the distress signal
and identification of the user was a big improvement, but this system
was not able to automatically compute the location of the distress as
the LEO system could do. However, there is an option available in 406
MHz beacons to include the location as part of the distress signal, which
can easily be done if a navigation receiver, such as GPS, is connected
to, or built into the distress beacon. Such beacons are now becoming more
common as the additional cost of including GPS decreases.
This GEOSAR system still has some limitations since the beacon signal requires a direct line of sight to one of the satellites. There are some distress situations where this is impossible, such as in polar regions or when a plane crashes on the wrong side of a mountain or in a deep valley or when a maritime beacon is blocked by the ship superstructure.
To further improve the performance of the system, plans are now being made to fly 406 MHz payloads on future navigation satellites, such as the United States' GPS, Russia's Glonass and Europe's new Galileo system (Figure 3). These satellites, in medium-Earth orbit at about 20,000 km, will be known as MEOSARs. These constellations could each have about 20 to 30 satellites that are continually moving across the sky, thereby providing global coverage, including the poles, with multiple viewing angles to the satellites so no area would be blocked. This MEOSAR system could automatically detect and locate all active 406 MHz beacons in the world with no time delay and determine exactly when each beacon was turned on and off.
|Figure 3: Navigation satellites in medium-Earth orbit (MEO), such as GPS, Glonass and the new Galileo system.|
This MEOSAR system would provide the ultimate distress alerting and locating service for worldwide operations, and will be demonstrated in the next few years. If such demonstrations confirm the viability of this system, it would be implemented over the next 5 to 10 years. Since satellite reception of the old 121.5 MHz beacons is to be phased out starting in 2009, an enhanced 406 MHz satellite system would be in place to provide far superior service.
*COSPAS-SARSAT is the international satellite system launched by Canada, France, the United States and the former USSR in 1982 that will receive the signal of an emergency beacon and relay the beacon position to rescue authorities.
Jim King is the Director of Major Satellite Communications Programs at the Communications Research Centre Canada, a research laboratory of Industry Canada.
Emergency beacons have saved over 15,000 lives world-wide, and with the new Canadian Beacon Registry database the potential to save more lives will increase.
Did you know?
There are three types of emergency beacons:
Currently, Canada is the only country in the world to have an online beacon registration database and it is also one of only 17 countries to have a database with beacon owner information. The United States is working on a database of its own that should be operational in summer 2003, and work is also being done on an international beacon registry database that will be available to search and rescue agencies around the world.
When a beacon goes off in Canada - either because someone is in distress and set if off themselves, or because it was triggered in a crash - a signal is sent to one of the satellites orbiting the Earth and then the satellite redirects the signal to the Canadian Mission Control Centre in Trenton, Ontario. Once the signal and beacon code are received, the rescue personnel decode it to find its owner and emergency contact information. With the additional emergency contact information, rescue personnel are able to call the contact and see if an emergency situation is taking place before beginning a costly search effort.
|Satellites orbiting the earth receive the distress signal from an emergency beacon and then redirect it to the Canadian Mission Control Centre in Trenton, Ontario where personnel determine whether a rescue mission is necessary.|