Global Positioning System Reference
In-Depth Information
It is representative the planimetric positioning achieved with the VRS generated by the
wider (“blue”) network, which has a percentage of about 20-30% of the data that lies outside
the maximum axis value (30 cms). In this case, the percentage of measurement sessions with
fixed ambiguities is 100% (“red” network) and collapse to 40% (“green” network) and only
to 10% (“blue” network).
9. Conclusions
In this chapter, the accuracy of geodetic and GIS receivers in small-, medium- and large-
sized networks of GNSS reference stations were analysed, comparing the results obtained
with different network products. The accuracies achieved with a 95% of reliability, referring
to a well-known rover position and using 24 hours of measurement, were considered.
Geodetic receivers can benefit from the VRS corrections transmitted by networks with inter-
station distances up to 100 kms, allowing it to achieve planimetric accuracies from 2 to 8 cms
and from 5 to 12 cms in elevation. A similar behaviour can be found when MAC corrections
are used. This network product, in fact, provides comparable results for the small- and
medium-sized networks (about 5 cm in planimetry and 10 cm in elevation).
If large networks are considered, the NRTK positioning is often inefficient and unreliable.
Due to their lower accuracy to model biases of large areas, FKP corrections are not suitable
for positioning even in medium-sized networks.
The performance of GIS receivers in real-time is poorly influenced by the size of the
network. Planimetric error achieves accuracies from 65 to 85 cms in the three considered
networks, and elevation error is always about 1 m. This improvement is noticeable when it
is compared to the stand-alone position, with planimetric accuracies of 1.7 m and 4.5 m in
altitude. Even with the EGNOS corrections it is possible to reach the same altitude accuracy
(1 m at 95%) and a planimetric accuracy of about 75 cms. Using the network differential
corrections, a planimetric accuracy of 50 cm can be achieved by averaging few minutes of
real-time positions.
Regarding the post-processing positioning, no substantial differences were noted in the
accuracy considering static session of 5 and 10 minutes long for geodetic receivers, and of 10
and 20 minutes long for GIS receivers.
For geodetic instruments, it is found that the positioning using a VRS RINEX file allows an
improvement only when small-sized networks are involved. For wider networks, the best
accuracies are always obtained using the RINEX file from the nearest reference station,
although the number of ambiguity fixes may drop up to about 30% of the epochs.
Considering GIS receivers, the best performance is obtained when the nearest station data
are used in a small-sized network (inter-station distances of about 50 kms), with a
planimetric error of 2 cms and an elevation error of 3 cms. A VRS RINEX file generated by a
large network does not improve the position accuracy with respect to the results obtained
from the nearest station, while some advantages can be found when a medium-sized
network is involved. The planimetric accuracy, in fact, goes from 10 cms, when data from
the nearest station are used, to about 4 cms considering virtual data generated by a GNSS
network. A similar behaviour can be also found when elevation accuracy is considered
(from 15 cms to 8 cms).
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