Founded by Canada, France, Russia and the US, Cospas-Sarsat began operations in 1982 with transponders on low-Earth orbiting satellites – known as ‘LEOSAR’. Using satellites including Europe's MetOp, their rapid orbital motion means that Doppler ranging can be performed to pinpoint the location of distress calls but it only a small area of Earth is covered at a time. It may take valuable time to line up with a ground station to relay a message – and it takes two satellite passes to pinpoint the distress call.
In the 1990s Cospas-Sarsat introduced ‘GEOSAR’ coverage using geostationary orbit. With these satellites, including Europe's MSG, remaining in a fixed point in the sky, distress calls are detected and relayed immediately, although Doppler-based ranging is not possible.