COSPAS/SARSAT satellite transponders are payloads on (e.g.) weather satellites owned by the USA, Russia, France and Canada. The principle of position estimation remains the same as with 121,5 MHz beacons, but COSPAS/SARSAT 406-MHz EPIRBs carry very stable oscillators so-called OCXOs (oven controlled crystal oscillators) or TCXO (temperature compensated crystal oscillators). They improve the Doppler measurent. Depending on the satellite geometry and the ship's drift the position error goes down to 1 to 50 NM.
With COSPAS/SARSAT 406-MHz EPIRBs the
amount of false alarms could be reduced dramatically but is still quite
high. The major disadvantege of LEO satellites is the dependency of
satellites in the low earth orbit. The disabled vessel has to wait for the satellite to pass by.
Depending on the lattitude this may take hours. And a delay of 15
minutes to 4 hours may apply.
COSPAS/SARSAT EPIRBs like the navtec global-C plus EPIRB can be fitted with GPS-receivers. This improves the position estimation to an accurate position indication. GPS fitted COSPAS/SARSAT EPIRBs like the navtec global-C plus EPIRB can operate via geostationary satellites (GEOSAR) when available and when the geometry allows for reaching the satellite.