Global Positioning System Reference
In-Depth Information
In PPP-RTK, the function of the network is to provide the user with satellite clocks and
interpolated ionospheric delays. When these precise estimates are passed on to the user, the
above given definition of these clocks ensures that the ambiguities of the user are also
integer and ambiguity resolution is available at the user side. Satellite clocks for each epoch
are added as pseudo-observations, with appropriate variance matrix. The precise IGS orbits
are used. For the network processing in Teunissen et al., 2010, a Kalman filter is used,
assuming the ambiguities are time-invariant, while for the user, an epoch-by-epoch least-
squares processing is used, thus providing truly instantaneous single-epoch solutions. The
integer ambiguity resolution of both network and user is based on the LAMBDA method
(Teunissen, 1995), with the Fixed Failure Ratio Test (Teunissen and Verhagen, 2009).
7. Network RTK applications
In this section important applications that can benefit from the few cm-level positioning
precision and accuracy achievable by using a single GNSS receiver with NRTK are
presented.
7.1 NRTK in engineering surveying
Surveying works in construction sites are usually dependent on determination of accurate
coordinates and heights. The 3D positioning versatility and accuracy achievable from NRTK
encourages the use of this technique for construction surveying works, particularly for large
sites when a rapid survey is needed. The method helps in reducing field expenses and time
due to reduction of the size of surveying crew, elimination of the need for frequent setups of
the surveying instruments, and the reduction of the need for accurate local traverses or
multiple control stations within the site. Studies showed that RTK GPS and the traditional
techniques employing total stations gave statistically compatible results (El-Mowafy, 2000).
With a typical accuracy of 1-5 cm, the NRTK GPS technique can be utilised for medium
accuracy construction survey works such as:
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grading,
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staking out of marks with medium accuracy, such as roads, footings, pipelines, utilities,
landscaping, fences etc.,
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cadastral surveys,
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mapping,
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checking of the as-built structures,
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site exploration for new projects.
A NRTK GNSS system can be integrated with the total station for instantaneous
determination of the total station location by mounting the GNSS antenna directly on top of
the total station alidade in open sites. Thus, the need for establishing permanent horizontal
control stations onsite can be minimised. For orientation determination, the total station can
be sighted at a back station, where its coordinates can be instantaneously determined using
the NRTK-GNSS technique. This process improves the economics of surveying work, and
reduces the overall surveying time, including the time required for the initialisation of the
total station at each setup. However, one should note that performance of surveying with
RTK GNSS in construction sites are affected by satellite availability, multipath errors
resulting from working near buildings, and latency of the reference data. The influence of
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