e. Field Processing and Verification. It is strongly recommended that GPS data processing and
verification be performed in the field (when applicable) so that any problems may be identified and
corrected before returning from the field.
5-34. Absolute Positioning. The accuracy obtained by GPS point positioning is dependent on the
user's authorization. The SPS user can provide an accuracy of 80 to 100 meters. SPS data are most
often expressed in real time; however, the data can be postprocessed if the station occupation took place
over a period of time. Postprocessing produces a best-fit point position. Although this will provide a
better internal approximation, the effects of S/A (when activated) still degrade positional accuracy up to
80 to 100 meters. The PPS user requires a decryption device within the receiver to decode the effects of
S/A. The PPS provides an accuracy reading between 10 to 16 meters when a single-frequency receiver
is used for observation. Dual-frequency receivers using the precise ephemeris may produce an absolute
positional accuracy on the order of 1 meter or better. These positions are based on the absolute WGS-84
ellipsoid. The PPS that uses the precise ephemeris requires the data to be postprocessed. The military
uses a GPS-S receiver that is capable of meter-level GPS point positioning without postprocessing.
5-35. Differential Positioning. Differential GPS surveying is the determination of one location with
respect to another location. When using this technique with the C/A- or P-code, it is called differential
code phase positioning. Differential code phase positioning has limited application to detailed
engineering surveying and topographic site mapping applications. Exceptions include general
reconnaissance surveys and operational military or geodetic-survey support functions. Additional
applications for differential code phase positioning have been on the increase as positional accuracy has
increased. The code phase tracking differential system is a functional GPS-S for positioning
hydrographic-survey vessels and dredges. It also has application for topographic, small-scale mapping
surveys. The collected data is used as input for a geographic information system database. A real-time
dynamic DGPS positioning system includes a reference station, a communication link, and remote-user
equipment. If results are not required in real time, the communication link can be eliminated and the
positional information is postprocessed.
a. Accuracy of Differential Global Positioning System Surveys. Differential code phase surveys
can obtain accuracies of 0.5 to 0.05 meter. This type of survey can be used for small-scale mapping or
as input to a geographic information system (GIS) database.
b. Reference Station. A reference station is placed on a known survey monument. It is an area
with an unobstructed view of at least four satellites, 10, above the horizon. It consists of a GPS
receiver, a GPS antenna, a processor, and a communication link (if real-time results are desired). The
reference station measures the timing and ranging information broadcast by the satellites and computes
and formats range corrections for broadcast to the user equipment. Using differential pseudoranging, the
position of a survey vessel is found relative to the reference station. The pseudoranges are collected by
the GPS receiver and transferred to the processor where PRCs are computed and formatted for data
EN0593
5-26
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