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forces of the U.S. and its allies. Civilian use was also available at first,
but it was less accurate because the military scrambled the signal some-
what, using a process called Selective Availability (SA).
GPS proved so useful for civilian navigation that the federal govern-
ment discontinued SA on May 2, 2000, after the military developed
other methods to deny GPS service to enemy forces. Reliable accuracy
for civilian users jumped from 100 meters (330 feet) under SA to the
present level of 10 to 20 meters (about 30 to 60 feet.)
Twenty-four satellites orbit 10,900 nautical miles above the Earth, pass-
ing overhead twice daily. A series of ground stations (with precisely sur-
veyed locations) controls the satellites and monitors their exact locations
in the sky. Each satellite broadcasts a low-power signal that identifies
the satellite and its position above the earth. Three of these satellites are
spares, unused until needed. The rest virtually guarantee that at least
four satellites are in view nearly anywhere on Earth at all times.
A minimum of three satellites are required to determine a 2D fix.
The system requires signal reception from three satellites in order to
determine a position. This is called a 2D fix. It takes four satellites to
determine both position and elevation (your height above sea level —
also called altitude). This is called a 3D fix.
Remember, the unit must have a clear view of the satellites in order to
receive their signals. Unlike radio or television signals, GPS works at
very high frequencies. These signals can be easily blocked by trees,
buildings, an automobile roof, even your body.
Like most GPS receivers, this unit doesn’t have a compass or any other
navigation aid built inside. It relies solely on the signals from the satel-
lites to calculate a position. Speed, direction of travel, and distance are
all calculated from position information. Therefore, in order for the
GlobalMap 5300c to determine direction of travel, you must be moving
and the faster, the better.
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