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Three of these satellites are spares, unused until needed. The rest vir-
tually guarantee that at least four satellites are in view nearly any-
where 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.
A minimum of three satellites are required to determine a 2D 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, the unit doesn’t have a compass or any other
navigation aid built inside. It relies solely on the signals from the sat-
ellites to calculate a position. Speed, direction of travel, and distance
are all calculated from position information. Therefore, in order it to
determine direction of travel, you must be moving and the faster, the
better. This is not to say that it won’t work at walking or trolling
speeds — it will. There will be more "wandering" of the data shown on
the display.
GPS alone is accurate for route navigation, but the U.S. Federal Avia-
tion Administration has special aircraft navigation needs that go be-
yond basic GPS. So, the FAA has developed a program to boost GPS
performance with its Wide Area Augmentation System, or WAAS. The
FAA commissioned the system on July 11, 2003.
WAAS is designed to increase GPS accuracy to within 7.6 meters verti-
cally and horizontally, but it consistently delivers accuracies within 1-2
meters horizontal and 2-3 meters vertical, according to the FAA. It does