What?
EPIRB stands for "
Emergency Position Indicating Radio Beacon". They were invented in the 1970s to help rescuers locate those in an emergency at sea. EPIRBs resemble large bright orange walkie talkies with two or more aerials. There are two basic types: Automatic & manual release.
Automatic release
Automatic release EPIRBs are activated by water,either by removing them by hand from their mounting bracket and placing them in water or allowing the rising water level to do this for you. As a result this variety is sometimes known as a
hydrostatic EPIRB.
Manual release
Manual release EPIRBs are usually activated by removing a waterproof cap,flicking a switch and then placing back on the protective cap.
Regardless of the method of release, EPIRBs work in the same manner. Contained in the EPIRB are two radio transmitters.
406 MHz
A 5 watt radio transmitter operating at 406 MHz sends a distress signal to a satelite in geosynchronous orbit such as
COSPAT or
SARSAT. This distress signal contains the vessels unique
MMSI. This signal gives a location to approximately 3 miles accuracy and also allows the coastguard to identify the vessel in distress.
121.5 MHz
Also on the EPIRB is a 0.25 watt radio transmitter operating at 121.5 MHz. This is an international emergency frequency and allows rescuers to locate the EPIRB to an accuracy of approximately 15 miles.
Newer, more advanced EPIRBs also contain a GPS receiver. This allows the EPIRBs exact signal to be transmitted on the 406 MHz radio transmitter with an accuracy of approximately several feet.
Sources:
Short Radio Course Notes
vhf.ie
howstuffworks.com