Global Positioning System Reference
In-Depth Information
Fig. 7. CDF of planimetric (left) and elevation (right) errors of the three geodetic receivers
For this reason, it is acceptable to consider the behaviour of an “average” receiver, focusing
on the variation in the positioning quality with respect to the size of the network.
The following sections show the results of these tests with the different NRTK correction
used.
7.1.1 VRS positioning
The VRS is without doubt the most used differential correction in real-time positioning, as
well as the easiest to manage for each receiver. As seen above, in fact, this type of
differential correction is based on generating, starting from the data of network CORSs, a
virtual reference station close to the measurement site.
It is therefore expected a deteriorating positioning quality with the increasing of the inter-
station distances. If this variable is increasing, in fact, there are numerous inaccuracies that
can be made by the interpolation step during the generation of the VRS correction.
The Fig. 8 shows the average behaviour of a geodetic receiver in the case of a VRS correction
broadcasted by networks of different sizes.
The CDF analysis brings out an effective increase of the errors (both planimetric and
altimetric) when the size of the GNSS network grows up. The planimetric error, for example,
changes from values below 5 cms (95% of reliability) considering the “red” network up to 10
and 15 cm with the “green” and the “blue” one, respectively. A similar behaviour can be
observed for the elevation error, with values from 6 cm (“red” network) to 10 cm (“green”
network) and to about 25 cm (“blue” network).
Even with regard to the cumulative moving average, VRS positioning with a “red” network
achieves a centimetre accuracy after few minutes, with a trend that remains constant over
the time.
It is also interesting to analyse the trend of the cumulative moving average when “green”
and “blue” networks are used: in both cases, there was a significant improvement of the
position quality when the measurement period increases. This trend allows to reach a
centimetre accuracy after a few hours-length measurement. This behaviour is evident for
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