Eagle Electronics SeaCharter 502cDF - Manual (Page 16)

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so you can mount your unit and plug in the power. Or you might want
to see how our text formatting makes the manual tutorials easy to
skim. If that's the case, move on to How to Use This Manual on page
11. But, if you want to understand the current state of satellite naviga-
tion, look over this segment describing how GPS and its new companion
WAAS work together to get you where you're going.
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 avail-
able at first, 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 govern-
ment 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 satel-
lite and its position above the earth. Three of these satellites are spares,
unused until needed. The rest virtually guarantee that at least four satel-
lites 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.