Lowrance electronic GlobalMap Baja 840C User Manual

Page 13

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5

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 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 ci-

vilian users jumped from 100 meters (330 feet) under SA to the present

level of 10 to 20 meters (about 30 to 60 feet).

A minimum of three satellites are required to determine a 2D fix.

Twenty-four satellites orbit 10,900 nautical miles above the Earth,

passing overhead twice daily. A series of ground stations (with pre-

cisely surveyed locations) controls the satellites and monitors their ex-

act 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.
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, this 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 for the

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