NOAA upgrades rescue system
Digital 406 MHz frequency helps satellite network locate boats, planes and individuals
- By William Jackson
- Feb 09, 2009
A satellite network that has helped rescue more than 6,000 people in the United States since 1982 has moved to a new digital system that gives responders even more reliable information about the locations of planes, boats and individuals in distress.
As of Feb. 1, the National Oceanic and Atmospheric Administration no longer processes 121.5 MHz emergency beacon signals on the weather satellites that are part of the Search and Rescue Satellite-Aided Tracking (SARSAT) system. Under an international agreement, SARSAT now receives only 406 MHz signals from Earth-based beacons.
The new digital signals provide more information and can determine location to within 5 kilometers, compared to 18 kilometers for the old, analog signals. The digital signals are transmitted at 5 watts, compared to a fraction of a watt for the analog signal, and there is less noise and interference in the 406 MHz band, reducing the number of false alarms.
“It is vital that anyone with an old 121.5 MHz beacon make the switch to 406 MHz immediately so their distress signals can be heard,” said Chris O’Connors, NOAA's SARSAT program manager. “Plans for this changeover started in 2000, and we want everyone who relies on these devices to have the proper equipment.”
All aircraft, large boats and some individuals who travel in remote areas use the beacons.
The change probably will not pose a problem for most users on land and water, said Shawn Maddock, NOAA’s SARSAT support officer. Personal locator transmitters have operated in the 406 MHz band since they were authorized in 2003, and manufacturers have phased out the old frequencies on the Emergency Position Indicating Radio Beacons used on boats. Now only models using the newer frequency can be sold, at the direction of the Coast Guard, which enforces beacon requirements for boats.
However, planes could be another matter. About 270,000 aircraft in the United States are registered as using emergency locator transmitters, but because no agency enforces aircraft beacon requirements, only about 30,000 use the 406 MHz equipment.
SARSAT is a joint effort with Russia, Europe and India. In the United States, the system piggybacks on two Geostationary Operational Environmental Satellites operated by NOAA: GOES East, which is stationed above the equator at 75 degrees west longitude, and GOES West, which sits at 135 degrees west longitude and the equator. SARSAT also relies on five Polar Operational Environmental Satellites, which circle the Earth in polar orbits. The geostationary satellites give continuous coverage of most of the Western Hemisphere. The polar satellites give global coverage but only intermittently in any given area.
When activated, beacons ping the satellites every 50 seconds. The polar satellites use Doppler frequency shift to establish the location of an emergency signal. However, because the geostationary satellites are not in motion, they cannot establish the location of a signal. But they can receive signals immediately from almost anywhere in the United States. If a geostationary satellite is the first to receive a signal, Maddock said, NOAA can retrieve the beacon owner’s registration information from its database and alert local authorities.
A polar satellite capable of determining the location of the signal would pick it up within 48 minutes, so rescuers would know where to target their efforts.
In the United States, six NOAA ground stations — in Maryland, Florida, California, Alaska, Hawaii and Guam — receive distress signals, which are relayed to NOAA’s Mission Control Center in Suitland, Md. If the beacon is registered to a foreign user, the signal is also sent to that country’s mission control center. In the United States, NOAA’s mission control alerts the Air Force for land rescues and the Coast Guard for water rescues.
William Jackson is a Maryland-based freelance writer.