EPIRB stands for Emergency Position Indicating Radio Beacon. An EPIRB is meant to help rescuers locate you in an emergency situation, and these radios have saved many lives since their creation in the 1970s. Boaters are the main users of EPIRBs.

A modern EPIRB is a sophisticated device that contains:

• A 5-watt radio transmitter operating at 406 MHz
• A 0.25-watt radio transmitter operating at 121.5 MHz
• A GPS receiver

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.

Below is a picture of a typical EPIRB.

Manual and Automatic Deployment and Activation

 For an EPIRB to begin transmitting a signal (or "activate") it first needs to come out of its bracket (or "deploy"). EPIRBs can be activated manually - when a button on the unit is pushed, or automatically - when water comes into contact with the unit's "sea-switch". Deployment can happen either manually - where someone has to physically take it out of its bracket - or automatically - where water pressure will cause a Hydrostatic Release Unit to release the EPIRB from its bracket. If it does not come out of the bracket it will not activate. There is a magnet in the bracket which operates a reed safety switch in the EPIRB. This is to prevent accidental activation when the unit gets wet from rain or shipped seas. The Category I - type is recommended by IMO because a float-free bracket will deploy automatically once the vessel sinks and the EPIRB will then be activated automatically by immersion in water. All modern EPIRBS provide both methods of activation. Depending on the circumstances, they are capable of being activated either manually (crewman flicks a switch) or automatically (the "sea-switch" is activated when the unit is immersed in water).

For further EPIRB information go to the USCG Navigation Center.