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the armed forces of the U.S. and its allies. Civilian use was also avail-
able at first, but it was less accurate because the military scrambled
the signal somewhat, using a process called Selective Availability (SA).
GPS proved so useful for civilian navigation the federal government
discontinued SA on May 2, 2000, after the military developed other
methods to deny GPS service to enemy forces. Reliable accuracy for ci-
vilian users jumped from 100 meters (330 feet) under SA to the present
level of 10 to 20 meters (about 30 to 60 feet).
A minimum of three satellites are required to determine a 2D fix.
Twenty-four satellites orbit 10,900 nautical miles above the Earth,
passing overhead twice daily. A series of ground stations (with pre-
cisely surveyed locations) controls the satellites and monitors their ex-
act 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.
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