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 and pilots are the main users of EPIRBs.
A modern EPIRB is a sophisticated device that contains:
Once activated, both of the radios start transmitting. 24,000 or so miles 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. 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, 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.
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