Xda

'irtual Ref. Station

'irtual Ref. Station

NMEA Position

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 engineering applications (www.trimble.com/vrs.shtml).

9.7.4 Precise Point Positioning

As explained in Sections 9.7.1 through 9.7.3, relative or differential GPS techniques are used to eliminate 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 corrections 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 Section 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.

Mobile phone transmitter

Mobile phone transmitter

Surveyor

FIGURE 9.16 Virtual reference station (VRS) example setup for construction area. (Courtesy Trimble Navigation Ltd.)

Surveyor

FIGURE 9.16 Virtual reference station (VRS) example setup for construction area. (Courtesy Trimble Navigation Ltd.)

The availability of IGS and CORS networks and orbital, atmospheric, and timing products that are derived from the GPS observables collected by these networks made the PPP technique feasible. The obvious benefits of this positioning approach are as follows:

• Single receiver operation (low cost)

• Can be applied anywhere and anytime (remote areas, space applications, etc.) under different dynamics

• Not limited by a baseline length (no baseline processing)

• Independence on GPS reference stations

• Can be applied for static and kinematic platforms

• Simple processing algorithms

For more details on the PPP error modeling and currently achievable accuracy (subdecimeter to submeter level, depending on the type of observables used, mode of positioning—static versus kinematic—and quality of corrections applied), the reader is referred to the literature (Gao and Shen, 2002; Kouba and Heroux, 2001; Ovstedal, 2002; Wielgosz et al., 2005).

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