Mostcommon epirb operates on 406MHz using polar orbiting satelites. Its the simplest means of comunication, providing only one way of communication.
The EPIRB transmits a distress signal and a satallite system calculates distress position and relays coordinates to nearest rescue centre.
Most 406MHz EPIRB also transmits signal on 121.5MHz that allows search and rescue units to obtain a radio bearing of the EPIRB.
Epirb is attached to the vesselwith a hidrostatic release, that lets it float free in case the vessel sinks.
It is very important to register your EPIRB with the appropriate national authority, so that when you activate your epirb, your vessel details, such as name, size and type of vessel are known immediately to the RCCs.