Twenty-four satellites orbit 10,900 nautical miles above the Earth, passing
overhead twice daily. A series of ground stations (with precisely surveyed
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 de-
termine a position. This is called a 2D fix. It takes four satellites to deter-
mine 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 re-
ceive their signals. Unlike radio or television signals, GPS works at very
high frequencies. These signals can be easily blocked by trees, buildings,
an autoHomeile roof, even your body.
Unlike most GPS receivers, the Atlas MNS has a digital compass built in-
side. This aids in navigation drastically as you will not need to be in motion
to determine direction of travel.
GPS alone is plenty accurate for route navigation, but the U.S. Federal
Aviation Administration has special aircraft navigation needs that go be-
yond basic GPS. So, the FAA has developed a program to boost GPS per-
formance 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 vertically
and horizontally, but it consistently delivers accuracies within 1-2 meters hori-
zontal and 2-3 meters vertical, according to the FAA. It does this by broadcast-
ing correction signals on GPS frequencies. Your unit automatically receives
both GPS and WAAS signals.
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