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International cooperation and technology transfer
Mussio, Luigi

The DGPS corrections for the test have been obtained in
three different ways:
a) by the OMNISTAR system, in real-time mode;
b) using as base receiver the GPS permanent station of
Perugia University (mean distance from rover about 100
km), with a post-processing solution;
c) assuming as base point another GPS station closer to
the rover (in Ancona: distance from rover 0 to 25 km), in
First of all, a phase double differences kinematic solution
has been computed to get a reference result. On that
purpose, the well-known Geotracer OTF software by
TerraSat has been used. The mean accuracy of this
reference solution can be estimated as a few centimetres
over the entire path.
Thereafter, three different DGPS solutions have been
calculated in the three ways referred above. A compa
rison between the phase solution and the DGPS results
has been finally carried out, analysing the differences
The differences between the three different DGPS
techniques used are rather small, confirming the
substantial equivalence of the OMNISTAR "virtual base
station" to a real base station located at some tenths of
kilometres from the rover.
In figure 8 are shown the RMS of the rover co-ordinates in
the OMNISTAR solution, in the path Ancona-Jesi. Below
that, figure 9 shows a plot of the differences between the
OMNISTAR DGPS solution and the OTF one. Assuming
the OTF solution to be "true", one can notice the DGPS
path is rather "noisy". For most epochs, however, the
coincidence level between DGPS and OTF is quite good,
with differences less than 2 meters. Still, there are many
error peaks, some of which give more than 10 meter
difference in position.
The two major gaps in the plots correspond to road
tunnels situated along the path. A relatively "quiet" period
is that between the epochs 476300 and 476700,
corresponding to an almost horizontal road without
obstructions. The first part of the path interested an urban
zone of Ancona town. The last part, after the second
tunnel, is a hill road, with frequent curves and trees on the
road sides.
From the above results, the necessity of an appropriate
validation or filtering procedure for such DGPS kinematic
outputs is more than evident.
The b) and c) solutions have also been obtained using a
phase-smoothed pseudorange procedure. This way the
results have been much less noisy than in DGPS mode,
with a coincidence with the OTF solution at about 10 cm
general considerations can be formulated:
• Good results (accuracy about 1 meter or slightly less)
can generally be achieved through the OMNISTAR
DGPS technique under good field conditions (visibility
of 6-7 GPS satellites; continuous reception of RTCM
• Field operations are very simple, but a screening and a
validation of the DGPS output is advisable, particularly
for kinematic applications;
• In static mode, a relatively easy validation procedure
can be based on averaging the results obtained during
the permanence on a given point; this way, the
position accuracy can be improved to some tenth of
• In kinematic mode, specially when frequent
obstructions are present along the path, appropriate
filtering and validation procedures have to be studied
and experimented; an application of the Kalman
filtering could be suggested.
Dominici D., M.L. Pecetti, F. Radicioni, A.Stoppini (1999),
Analysis and validation of different DGPS techniques.
Proc. of DGPS Trieste Meeting, Trieste, March 1999. In:
Reports on Geodesy, Warsaw University of Technology.
The researches reported on this paper have been partially
financed with MURST funds pertinent to COFIN97 project.
4. Final remarks
On the basis of the experiences carried out until now at
Perugia University on the OMNISTAR DGPS, some