Geoscience Reference
In-Depth Information
GPS Positioning Strategies
In Geodesy/Surveying
In Navigation
Absolute
Differential
Differential
Absolute
Post-Processed
Real Time
Static
Stop and Go
Carrier Phase, Single
Baseline (RTK)
Pseudorange
(DGPS)
Pseudokinematic
Rapid Static
Network-Based
RTK
Kinematic
fIGURe 9.17
Global Positioning System (GPS) processing strategies. (Modified from Langley, R. B.,
GPS
World
, September, pp. 70-74, 1998. With permission.)
tAble 9.4
Comparison of Real-time and postprocessing Scenarios
positioning Mode/
Attribute
Remote
locations
Accuracy
time
navigation
Cost
portability
Postprocessing
Advantage
Advantage
Advantage
Real time
Advantage
Advantage
Advantage
9.9 hoW ACCURAte IS GpS?
The positioning accuracy of GPS depends on several factors, such as the number and the geometry
of the observations collected, the mode of observation (point versus relative positioning), the type
of observation used (pseudorange or carrier phase), the measurement model used, and the level of
biases and errors affecting the observables. Depending on the design of the GPS receiver and the
factors listed above, the positioning accuracy varies from 10 m with SA turned off (about 100 m
with SA turned on) for pseudorange point positioning, to better than 1 cm when carrier phases are
used in relative positioning mode. In order to obtain better than 10 m accuracy with pseudoranges,
differential positioning, DGPS services, or PPP techniques must be employed.
In summary, the current GPS constellation performance significantly exceeds the system design
specifications (in point positioning) and can be summarized as follows (SPS, 2001):
•
PDOP availability:
•
Requirement—PDOP of 6 or less, 98 percent of the time or better
•
Actual—99.99
