User's Manual

Page 10
How does it work?
Once you activate the PLB, the PLB transmits a continuous series of
radio signals on 406 MHz. Providing the PLB’s batteries are in good
condition, the signal should continue for a minimum of 24 hours. These
signals can be detected by aircraft within range and by orbiting and
geostationary satellites. The Commerce Department’s National Oceanic
and Atmospheric Administration (NOAA) operates a series of satellites in
low-earth and geostationary orbits to detect and locate aviators, mariners
and land-based users in distress. The satellites, along with a network of
ground stations and the U.S. Mission Control Center (MCC) in Suitland,
MD, are part of the International Cospas-Sarsat Program whose mission
is to relay distress signals to the international search and rescue
community.
The PLB transmits radio signals to a constellation of Cospas-Sarsat
satellites. Ground stations track these satellites and process the distress
signals to obtain a location (by using Doppler location techniques) of the
distress. The processed information is then forwarded to an MCC where
it is combined with other location and registration information and passed
to the Air Force Rescue Coordination Center (AFRCC) who alert the
appropriate SAR authorities. Successive satellite passes refine and
confirm this information. Alternatively, some beacons operating at the 406
MHz frequency can use the GPS system to obtain a very accurate
position to within 200 feet. This position is then transmitted as part of the
distress signal.
GPS systems do require a clear view of the sky in order to work
effectively. Positions where the sky area is obscured significantly, such
as under a tree canopy or in a steep-sided canyon, may result in the GPS
system failing to acquire its position correctly. Under such conditions the
GPS system “fails safe”; it does not transmit an inaccurate position, but
instead sends information to indicate that no position is available.
Position information is then dependent on the orbiting satellite system,
with its 3 miles accuracy. However, the homing signal from the 121.5
MHz transmitter is detectable from the air and in some cases from the
ground and may be of great value in refining the position of the beacon.
The Cospas-Sarsat program is operated in the United States by NOAA,
the U.S. Coast Guard, U.S. Air Force and NASA. Since the inception of
the system in 1982, over 14,000 lives have been saved world-wide and
nearly 4,500 persons have been rescued just in the United States.
The network of orbiting satellites can detect signals over the entire
surface of the earth; in addition, there are four geostationary satellites that
appear fixed in position and which detect signals over enormous areas
(approximately one-third of the surface of the earth, but excluding Polar
regions).