It is called SARSAT, or Search and Rescue Satellite Aided Tracking. The SARSAT system uses NOAA satellites to detect and locate pilots, mariners, hikers, etc, in trouble. The NOAA satellites relay weak signals from an emergency beacon to the U.S. Mission Control Center in Suitland, Maryland. (It is in between Ronald Reagan Washington National and Andrews Air Force Base.)
There are three types of emergency beacons to hail SARSAT. They are Emergency Position Indicating Radio Beacon (EPIRBs) for ships, Emergency Locator Transmitters (ELTs) for airplanes, and Personal Locator Beacons (PLBs) for terrestrial applications. They have a 406 MHz transmit signal to the satellite and a low-power homing signal at 121.5 MHz that assists rescue teams in the last 2-3 miles. As an example, the ACR Aqualink 406 PLB with GPS and a 5 year battery sells for $381.40 on Amazon. Mmm. I love technology! U.S. citizens are requested to register their locators in a data base that permits faster response. At the end of 2011, there were 329,000 registrations.
Total rescues in the United States for 2011 were 122 people at sea, 14 aviation rescues, and 71 PLB rescues. Although they don't provide numbers, false alerts make up the majority of all alerts received by the rescue center. Most of these, as you might guess, are from the personal locator beacons.
The Search and Rescue Satellites are: the Low-Earth Orbiting Search and Rescue (LEOSAR) Satellites and the Geostationary Orbiting Search and Rescue (GEOSAR) Satellites. SARSAT is a single instrument package aboard a large NOAA satellite. The LEOSARs are only 528 miles high and complete an orbit every 100 minutes. The GEOSARs lack the accuracy of location, but provide an early alert that can have rescue teams moving before a LEOSAR can provide an accurate location fix. Using Doppler processing, the LEOSAR can locate a beacon within a mile even if the beacon does not have GPS or if its GPS view is blocked.