Eagle Electronics II - Manual (Page 6)

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GPS - HOW IT WORKS
The Global Positioning System (GPS) is perhaps the
best approach to navigation that has ever been devised.
Conceived by the Department of Defense (DOD) and
the United States military, the GPS system is an answer
to their needs of 24 hour global positioning, 365 days a
year.
Basically, the system works by using a constellation of
satellites orbiting Earth 11,000 miles in space. There will
be 24 satellites in orbit when the system is fully opera-
tional. When all satellites are in place, at least four of
them will be in view nearly anywhere on Earth twenty-
four hours a day. The GPS receiver requires at least
three satellites to give a “2D” fix. (A 2D fix is your position
in latitude/longitude. A 3D fix is your latitude/longitude
plus altitude.) When it locks on to at least four satellites,
it displays a 3D fix.
It takes three satellites to determine position.