406 MHz EPIRBs and PLBs (Personal Locator Beacons) work with the Cospas-Sarsat polar orbiting satellite system, giving true global coverage. The satellite can determine the position of your beacon to within 5km (3 miles). The alert is then relayed via an earth station to the nearest Rescue Coordination Centre (RCC). As the satellites are in a polar orbit they offer true global coverage – with a 406 MHz beacon you can summon help wherever you are on the planet, no matter how remote. The coded message identifies the exact vessel to which the EPIRB is registered. This information allows the rescue services to eliminate false alerts and launch an appropriate rescue.
GPS-enabled beacons have a built-in transmitter which will typically alert the rescue services within 3 minutes and to a positional accuracy of +/- 50 metres (updated every 20 minutes) given a clear view skywards. McMurdo beacons also have a secondary distress transmitter. This transmits on 121.5 MHz and is used for "homing" purposes.