These are transmitters carried by and registered to individuals, marine vessels, and aircraft. In emergencies, they are activated manually or automatically to initiate a distress alert. They transmit a 406 MHz signal (a protected frequency just for this purpose) that carries unique ID information.
A transceiver on a geostationary (GEO) satellite detects the emergency alert. Unless the beacon transmitter has a GPS feature that also sends location information, the GEO satellite will not “know” the location. It immediately relays the distress alert to a ground station, where the beacon ID can be looked up in the registration database to possibly give an approximate location.
when Low-Earth Orbiting satellites receive distress alerts they can use a Doppler effect calculation to find out the approximate location of the distress. As soon as the LEO satellite is in view of a Local User Terminal, it relays this information.
These automated ground stations receive emergency beacon distress signals from the GEO and LEO satellites. These stations actually track the LEOs, so they are always in communication. The Local User Terminal relays the distress signal to the Mission Control Center that operates it.
This is where the SARSAT information is received from all the Local User Terminals, sorted, and distributed to the Rescue Coordination Center.
In the United States, these are operated by the U.S. Coast Guard and the U.S. Air Force. They receive distress signals from the U.S. Mission Control Center and are responsible for coordinating the rescue response. The Air Force coordinates all inland rescue activity in the continental U.S., which are usually carried out by the Civil Air Patrol, state police, or local rescue services. The Coast Guard coordinates and usually conducts maritime rescue missions.