Lowrance electronic Lowrance LMS-522c iGPS - Manual (Page 19)

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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, the 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 it to de-
termine direction of travel, you must be moving and the faster, the bet-
ter. This is not to say that it won’t work at walking or trolling speeds —
it will. There will simply be more "wandering" of the data shown on the
display.
GPS alone is plenty accurate for route navigation, but the U.S. Federal
Aviation Administration has special aircraft navigation needs that go
beyond 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 vertically
and horizontally, but it consistently delivers accuracies within 1-2 meters
horizontal and 2-3 meters vertical, according to the FAA. It does this by
broadcasting correction signals on GPS frequencies. Your unit automati-
cally receives both GPS and WAAS signals.