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GPS
Theory
DRMS method. Typically, the accuracy of most good GPS receivers
under normal conditions will be within 15 to 30 meters.
Position Fix
A 2D position fix is possible with only three satellites visible. Four
satellites are required for a 3D fix and at least five satellites (with good
geometry) are required to provide RAIM. Pressure altitude data can
sometimes be substituted for the fifth required satellite in order to
provide RAIM. Your Apollo NMC allows use of an altitude input to
replace one of the required satellites for a position solution. This will
yield a 2D, or horizontal, position with only three satellites, but is not
as accurate as when four satellites are used. A three-dimensional fix
(3D) includes altitude information. GPS altitude is calculated
according to an ideal model (WGS-84 spheroid). The GPS altitude is
determined by the distance calculated to be the height above an ideal
ellipsoid model of the earth designed to closely approximate the
surface of the earth. The translation applied to GPS altitude to convert
to MSL altitude changes from area to area due to the uneven shape of
the earth. The designed accuracy of GPS for civilian-use can have a
typical accuracy of 500 feet depending on the cumulative error
sources. So, don't depend on your GPS altitude for navigating.
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