An Emergency Position Indicating Radio Beacon (EPIRB)) is used to alert search and rescue services in the event of an emergency. They do this by transmitting a coded message on the 406 MHz distress frequency. This message is relayed via satellite and earth stations to the nearest rescue co-ordination center. 406 MHz EPIRBs work with the Cospas-Sarsat polar orbiting satellite system which provides true global coverage. The system has an alert delay of typically 45 minutes dependent on when the satellites come into view on the horizon.
The satellite can determine the position of the EPIRB to within 5km (3 miles). 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.
The GPS enabled EPIRBs have built-in transmitters that will typically alert the rescue services within 3 minutes. These models are capable of providing positional accuracy of +/- 62 Metres and position updates every 20 minutes, given a clear view skyward.
EPIRBs also have a secondary distress transmitter. This transmits on 121.5 MHz and is used for “homing” purposes. When the rescue services get close, this allows them to direction find on the signal.
Since its inception in 1982 the Cospas-Sarsat System has provided distress alert information which has assisted in the rescue of over 27,000 persons in over 7,200 distress situations. The Cospas-Sarsat programme assists search and rescue (SAR) activities on a worldwide basis by providing accurate, timely and reliable distress alert and location data to the International community on a non-discriminatory basis.
How does an EPIRB work?
Once activated, both of the radios start transmitting. Approximately 24,000 miles (39,000 km) up in space, a GOES weather satellite in a geosynchronous orbit can detect the 406-MHz signal. Embedded in the signal is a unique serial number, and, if the unit is equipped with a GPS receiver, the exact location of the radio is conveyed in the signal as well. If the EPIRB is properly registered, the serial number lets the Coast Guard know who owns the EPIRB. Rescuers in planes or boats can home in on the EPIRB using either the 406-MHz or 121.5-MHz signal.
Older EPIRBs did not contain the GPS receiver, so the GOES satellite received only a serial number. To locate the EPIRB, another set of satellites (like the TIROS-N satellite) orbiting the planet in a low polar orbit could pick up the signal as it passed overhead. This would give a rough fix on the location, but it took several hours for a satellite to come into range.