in range are directly reflected in resultant coordinate errors. Differential positioning is not so concerned
with the absolute position of the user but with the relative difference between two user positions, that are
simultaneously observing the same satellites. Since errors in the satellite position and atmospheric-delay
estimates are effectively the same at both receiving stations, the errors cancel each other to a large
extent.
5-21. Correcting Errors. A pseudorange correction (PRC) can be generated for each satellite being
observed. If a second receiver is observing at least four of the same satellites and is within a reasonable
distance, it can use these PRCs to obtain a relative position to the known control point since the errors
will be similar. Thus, the relative distance (coordinate difference) between the two stations is relatively
accurate regardless of poor absolute coordinates. For example, if the true pseudorange distance from a
known control point to a satellite is 100 meters, and the observed or measured pseudorange distance is
92 meters, then the pseudorange error or correction is 8 meters for that particular satellite. In effect, the
GPS-observed baseline vectors are no different from azimuth/distance observations. As with a total
station, any type of initial-coordinate reference can be input to start the survey.
local-project datum coordinates. Since differential-survey methods are only concerned with relative
coordinate differences, disparities with a global reference system used by the NAVSTAR GPS are not
significant for topographic purposes. Therefore, GPS coordinate differences can be applied to any type
of local-project reference datum (for example, NAD 27 or NAD 83).
5-23. Carrier Phase Tracking. Differential positioning using carrier phase tracking uses a formulation
of pseudoranges. The process becomes somewhat more complex when the carrier signals are tracked so
that range changes are measured by phase resolution. In carrier phase tracking, an ambiguity factor is
added, which must be resolved to obtain a derived range. Carrier phase tracking provides a more
accurate range resolution due to the short wavelength (about 19 centimeters for L1 and 24 centimeters
for L2) and the ability of a receiver to resolve the carrier phase down to about 2 millimeters. This
technique has primary application to engineering, topographic, and geodetic surveying and may be
employed with either static or kinematic surveys. There are several techniques which use the carrier
phase to determine a station's position. These surveying techniques include static, rapidstatic, kinematic,
stop-and-go kinematic, pseudokinematic, and on-the-fly (OTF) kinematic/RTK. Table 5-2 lists these
techniques and their required components, applications, and accuracies.
5-11
EN0593
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