406 MHz and 121.5 MHz
Modern beacons transmit at 406 MHz with a unique digital ID registered to the vessel or aircraft. Older beacons used 121.5 MHz — still monitored, but without identification or precise location. The 406 MHz signal is detected by geostationary satellites (instant, but no Doppler) and polar-orbiting satellites (delay, but precise Doppler location). Together they provide global coverage.
Doppler location from orbit
A Low Earth Orbit (LEO) satellite moving at 7 km/s hears the beacon frequency shifted by Doppler as it approaches and recedes. Plotting frequency vs. time gives a curve whose peak marks the point of closest approach. Two satellite passes give a fix within a few kilometres. LEOSAR accuracy is about 2–5 km — enough to vector a rescue aircraft.
MEOSAR: the next generation
Medium Earth Orbit SAR uses GPS/Galileo satellites as listeners. At 20,000 km altitude they see a much larger footprint, with more satellites in view simultaneously. The result is faster alerting — minutes instead of the up-to-90-minute LEO orbit period — and the potential for multiple-satellite triangulation giving sub-kilometre accuracy without Doppler.
From detection to rescue
The beacon signal is relayed to a Mission Control Centre, then to a Rescue Coordination Centre near the distress. They dispatch aircraft, ships or helicopters. The 406 MHz beacon with GPS position (encoded in the digital message) can reduce the search radius from kilometres to metres. In Alaska, the North Sea, the Southern Ocean — this system has saved tens of thousands of lives since 1982.