exercise extreme caution in applying GPS height determinations to projects that are based on
conventional orthometric elevations.
The heights obtained from GPS are in a different height system than those historically obtained
with geodetic leveling. GPS data can be readily processed to obtain the ellipsoidal height. This is the
height above or below a simple ellipsoid model of the earth. Geodetic leveling gives rise to a
orthometric height, often known as the height above the MSL. These heights are found on topographic
maps, stamped on markers, or stored in innumerable digital and paper data sets. To transform between
these height systems requires the geoid height. These height systems are related by the following
equation:
h
=
H+N
Where--
h
=
ellipsoidal height
H
=
orthometric height
N
=
geoid height
5-27. Differential Error Sources. Error sources encountered in using differential GPS positioning
techniques are the same as for absolute positioning. In addition to these error sources, the user must
ensure that the receiver maintains satellite lock on at least three satellites for 2D positioning and four
satellites for 3D positioning. When loss of lock occurs, a cycle slip (discontinuity of an integer number
of cycles in the measured carrier beat phase as recorded by the receiver) may occur. In GPS absolute
surveying, if satellite lock is not maintained, positional results will not be formulated. In GPS static
surveying, if satellite lock is not maintained, positional results may be degraded resulting in incorrect
formulations. In GPS static surveying, if the observation period is long enough, postprocessing software
may be able to average out loss of lock and cycle slips over the duration of the observation period and
formulate positional results that are adequate. If this is not the case, reoccupation of the stations may be
required. In all differential-surveying techniques, if loss of lock does occur on some of the satellites,
data processing can continue easily if a minimum of four satellites are tracked. Generally, the more
satellites tracked by the receiver, the more insensitive the receiver is to loss of lock. In general, cycle
slips can be repaired.
5-28. Differential Accuracies. There are two levels of accuracy obtainable from GPS using
differential techniques. The first level is based on pseudorange formulations, while the other is based on
carrier beat phase formulations.
a. Pseudorange formulations can be developed from either the C/A-code or the more precise P-
code. Pseudorange accuracies are generally accepted to be 1 percent of the period between successive
code epochs. Use of the P-code, where successive epochs are 0.1 millisecond apart, produces results
that are about 1 percent of 0.1 millisecond (about 1 nanosecond). Multiplying this value by the speed of
light gives a theoretical-resultant range measurement of around 30 centimeters. If using pseudorange
formulation with the C/A-code, results can be ten times less precise (a range-measurement precision of
around 3 meters). Point-positioning accuracy for a
EN0593
5-14
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