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.
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, this 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 for the unit 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 plenty accurate for route navigation, but the U.S. Federal Aviation Administration has special aircraft navigation needs that go
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