Geoscience Reference
In-Depth Information
tAble 9.5
Achievable dGpS Accuracy Compared to Static and Rtk
GpS Accuracy (http://www.smi.com/downloads/
catalogs/2005catalog.pdf; modified here)
Correction type
horizontal Accuracy
vertical Accuracy
Single-frequency WAAS
3-7 m
a
3-7 m
a
Dual-frequency WAAS
<50 cm
<70 cm
StarFire( (dual frequency)
<10 cm
<15 cm
IGDG (dual frequency)
<50 cm
<70 cm
Static (using NGS CORS)
5 mm
b
5 mm
b
Static (using base and rover)
5 mm
b
5 mm
b
RTK
1 cm
b
2 cm
b
Network-based RTK
5 mm
b
10 mm
b
a
According to the WAAS specifications; however, much better accuracies
(<2 m, even up to 30-70 cm) were reported, as explained in Section 9.7.2.1.
b
Increases with the baseline length.
•
Horizontal service availability:
•
Requirement—95 percent threshold of 13 m, 99 percent of the time or better
•
Vertical service availability:
Actual—4.49 m
•
•
Requirement—95 percent threshold of 22 m, 99 percent of the time or better
•
User range error:
Actual—6.43 m
•
•
Requirement—6 m or less, constellation average
•
Actual—1.47 m
Other factors affecting the GPS positioning accuracy depend on (1) whether the user is station-
ary or moving (static versus kinematic mode), (2) whether the positioning is performed in real time
or in postprocessing, (3) the data reduction algorithm, (4) the degree of redundancy in the solution,
and (5) the measurement noise level. The currently achievable GPS accuracies, provided as two-
sigma, corresponding to 95 percent confidence level, are summarized in Table 9.6. The lower bound
of the relative positioning accuracy listed in Table 9.6 cannot be stated with precision, as it depends
on several hardware and environmental factors, as well as the survey geometry, among others (the
symbol → indicates the increase of the values listed). Thus, the accuracy levels listed in Table 9.6
should be understood as the best achievable accuracy.
9.10 GpS InStRUMentAtIon
During the past two decades, the civilian as well as military GPS instrumentation evolved through
several stages of design and implementation, focusing primarily on achieving an enhanced reli-
ability of positioning and timing, modularization, and miniaturization. In addition, one of the most
important aspects, especially for the civilian market, has been the decreased cost of the receiv-
ers, as the explosion of GPS applications calls for a variety of low-cost, application-oriented, and
reliable equipment. By far, the majority of the receivers manufactured today are of the C/A-code
single-frequency type. However, for the high-precision geodetic applications, the dual-frequency
solution is standard. Even though the civilian and military receivers, as well as application-ori-
ented instruments, have evolved in different directions, one might pose the following question:
Are
