EPIRB stands for Emergency Position Indicating Radio Beacon.  An EPIRB is meant to help rescuers locate you in an emergency situation.

When activated an EPIRB starts sending a unique serial number via a satellite radio.  The signal frequency used (406 Mhz) has been designated internationally for distress use only.  The signal allows the satellite to locate the EPIRB to less then 5 km accurate anywhere in the world.  However, this requires multiple satellite orbits and could take several hours.  Some EPIRB's are therefore equipped with a GPS and send their exact location with the emergency signal.

 

All EPIRB's need to be registered with the authorities so rescue services know full details about the vessel and owner immediately.