Global Positioning System Reference
In-Depth Information
With regard to the performance of the cumulative moving average, it is possible to see that a
positioning error always lower than 5 cm can be achieved only by averaging several hours
of data. The analysis of the average length of the lines shows the small number of epochs
with a fix ambiguity values (almost always less than 50% of the total measured times). This
is not due to NRTK corrections transmission problems, but to the use of FKP corrections by
the receiver (the flat model does not fit well with the rover measurement errors).
7.2 GIS receivers
The tests carried out on the three GIS receivers shown in Table 3 were designed to study
their accuracy within GNSS networks with different inter-station distances. The corrections
from a VRS (used by all receivers in this class) and from the nearest reference station (NRT)
were tested. Given the receiver category and the metric accuracies expectations, the EGNOS
1
corrections were also used, in order to assess whether could be, for GIS receivers, the
benefits of a network of GNSS reference stations compared with the area corrections
broadcasted by a geostationary satellites constellation.
In the following, the results obtained using VRS corrections are discussed. After that, the
comparison between these results and those obtained using corrections from the NRT
station and from the EGNOS satellites are presented.
7.2.1 VRS positioning
First, planimetric and elevation accuracies achievable with a GIS receiver into networks with
different inter-station distances are analysed. As said above, 24 hours of measurements (to
be independent of satellites geometry) and only positions with a HDOP index lower than or
equal to 4 (to exclude outliers) were considered.
The Fig. 11, in the next page, shows the results obtained considering an “average” receiver.
From the pictures analysis, it may notice that the positioning accuracy changes when the
inter-station distance increases. However, it is possible to see that, unlike the geodetic
receivers, in this case there is not a significant positioning deterioration with the increasing
network size (from the “red” network to the “blue” one). The planimetric error at the 95% of
reliability, for example, goes from 80 cms (“red” network) to 60 cms (“green” network) and
to about 1 m (“blue” network). The improvement obtained by considering a medium-sized
(“green”) network is not surprising, but it must be analysed considering the quality of GIS
receivers. This behaviour shows a substantial stability of the positioning accuracy, which
remains always around metric values. This trend is more evident when the elevation error is
analysed. Cumulative moving average lines achieve a sub-decimetric accuracy only after
about 5 hours, showing no particular differences between the three different networks.
7.2.2 NRT and EGNOS positioning
The analysis carried out considering the positioning quality with VRS corrections were
compared with these obtained using the corrections from both the nearest reference station
and the European geostationary satellites constellation EGNOS. In order to highlight the
benefits of differential corrections, the stand-alone positioning results are also reported in
the figures.
1
http://www.esa.int/esaNA/egnos.html
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