EPIRB

EPIRB stands for Emergency Position Indicating Radio Beacon. An EPIRB is meant to help rescuers locate you in an emergency situation. A modern EPIRB is a sophisticated device that contains:

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

Once activated, the radios start transmitting. Approximately 24,000 miles (39,000 km) up in space, a 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. Airborne or surface rescue units can home in on the EPIRB using either the 406-MHz or 121.5-MHz signal.