Geoscience Reference
In-Depth Information
Reference
Station
Reference
Station
Raw Data
GPS
Network
CMR+/RTCM
LAN
Router
NMEA Position
Virtual Ref.
Station
Reference
Station
Reference
Station
fIGURe 9.15 Basic infrastructure and data flow in the virtual reference station (VRS) network. (Courtesy
Trimble Navigation Ltd.)
The primary benefits of the network-based RTK, from the user perspective, are as follows:
No need to establish base or reference station
Reduction of time and chance for errors inherent in setting up a base
Savings in equipment and time cost
Extension of the operating range with improved initialization and accuracy
The primary applications are monitoring and modeling of the movement of man-made and
natural structures using GPS and integrated sensors, and monitoring the integrity of high-order
geodetic networks. The market is still expanding as the number of VRS installations is growing
nationwide; the evolving applications include oilfield subsidence, dam deformation monitoring,
landslide monitoring, volcano monitoring, construction site surveying (Figure 9.16), and other engi-
neering applications (www.trimble.com/vrs.shtml).
9.7.4 P R e c i s e P o i n t P o is i t i o n i n g
As explained in Sections 9.7.1 through 9.7.3, relative or differential GPS techniques are used to elim-
inate the atmospheric, receiver-specific, and satellite-specific error sources. A promising alternative
to relative positioning and DGPS is precise point positioning (PPP)—an absolute GPS positioning
technique with all major error sources carefully modeled (i.e., removed from the direct pseudorange
or carrier phase observables). For example, the errors in GPS broadcast ephemeris and clock correc-
tions can be successfully reduced using the precise IGS (http://igscb.jpl.nasa.gov/) orbits and clocks
(accuracy of the final IGS orbits and clocks is better than 5 cm and 0.1 ns, respectively). Ionospheric
corrections can be provided by IGS or by the local or regional CORS networks, as explained in Sec-
tion 9.7.3. The influence of the troposphere may be reduced in several ways. The simplest solution
is to use the standard atmospheric models; however, the resulting accuracy may not be sufficient.
Better results are obtained if meteorological data (temperature, pressure, and relative humidity) are
used in the atmospheric models instead of the standard atmosphere. The best results can be achieved
if the tropospheric zenith delay corrections are derived from the local or regional CORS networks.
The receiver clock correction remains an unknown in the PPP mathematic model, similar to the
standard point positioning technique.
Search WWH ::




Custom Search