33
Owning to the receiver hardware development a very
performing differential kinematic method can be presently
adopted for road survey. This technique can be applied with the
availability of a single receivers, of course the ROVER. The
differential positioning is possible thanks to corrections send to
GPS ROVER receiver trough a satellite communications link.
The corrections computed using a network of reference stations
are transmitted in a standard format known as RTCM-104
(Radio Technical Commission for Maritime Services, 1990).
The receivers is equipped with to different antennas, one for
receiving GPS signal and one for the differential corrections.
The performance of this technique, still within the range of 1 -s-
2 meters, is not distance dependent since it do not require any
reference station. The only limitation is the visibility of the
communications satellite.
This flexible and low cost positioning method was carefully
examined in this work in consideration of its potential
suitability for road survey.
3. FIELD EXPERIMENTS
In order to check the effective performance of differential GPS
techniques for highway survey, different field experiments were
carried out. Different models of TRIMBLE receivers were used
with particular attention to the Pro XR/XRS receiver, which
support the use of satellite transmitted RTCM-104 correction.
The receivers and the software adopted during tests are listed in
table 1.
RECEIVER
Trimble Pro XR/XRS (single frequency)
Trimble 4700 (dual frequency)
Trimble GPS TS 4800 (dual frequency)
SOFTWARE
ASPEN (acquisition and navigation)
PATHFINDER OFFICE (differential correction)
GPSURVEY (phase processor)
The dual frequency receivers were used to compute very
accurate co-ordinates adopted in the validation of results from
Pro XR/XRS receivers and as MASTER station.
The software Aspen was used by the operator during the
acquisition on the road, allowing to monitor different
parameters, such as the satellite constellation and the status of
satellite correction, to insert quickmarks and features and to
display corrected co-ordinate and the trajectory.
The performance of three different survey modes was
investigated by means of the following preliminary field
experiments executed with non-dedicated vehicles:
Testl - Static GPS in Real Time with RTCM-104 correction
transmitted via OMNISTAR to Pro XRS receiver;
Test2 - Differential GPS in Post-Processing with a Rover Pro
XR receiver and a 4700 Trimble receiver as Base
Station;
Test3 - Differential GPS in Real Time with RTCM-104
correction transmitted via OMNISTAR to Pro XRS
receiver.
The Post Processing differential mode was supported by a
MASTER station located in a selected site by the Centro
Rilevamento Dati of the Company Autostrade. The reference
point was monumented on the roof of a building and precisely
measured by means of a static session using two dual frequency
receivers.
3.1 TEST 1: Accuracy assessment of Pro XRS -RTCM
corrected position (transmitted via satellite)
The aim of this test was to define the absolute accuracy of co
ordinates determined by the Pro XRS receiver operating with
RTCM-104 satellite transmitted correction (via OMNISTAR).
The receiver was placed at the Reference Point DITS, located
on the roof of the Department 37 of the University "La
Sapienza" of Roma, which position was already known with
millimetre accuracy. The period of acquisition was five hours
long with a sampling interval of 30 seconds. More than 600
points were successfully collected providing horizontal
accuracy of about 70 cm (Figure 2). The bias observed in the
NE direction can be adduced to signal disturbances effective on
the site.
Figure 2 TEST 1 Distribution of points measured in a static
survey using RTCM satellite transmitted correction.
3.2 TEST 2: Performance of Post-Processed corrected
DGPS
This test was carried out along 3 kilometres of the urban
highway Roma EST. The route was covered twice back and
forth, including a cloverleaf connection, in order to examine the
consistency and reliability of the computed trajectory.
