Lowrance electronic LCX-17M - Manual (Page 17)

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The Global Positioning System (GPS) was launched July 17, 1995 by
the United States Department of Defense. It was designed as a 24-
hour-a-day, 365-days-a-year, all weather global navigation system for
the armed forces of the U.S. and its allies. Civilian use was also
available, 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 that 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 civilian users jumped from 100 meters (330 feet) under SA
to the present level of 10 to 20 meters (about 30 to 60 feet.)
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
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
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.