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International cooperation and technology transfer
Fras, Mojca Kosmatin

D.I.T.S - Facolta’ di Ingegneria - Università’, di Roma "La Sapienza
* D.I.C. Facolta’ di Ingegneria -Università’ di Parma
** Trimble Italia
Commission VI Working Group 3 - Commission IV Working Group 3
KEYWORDS: highway surveying system, kinematic GPS, odometer, differential RTCM GPS, quality assessment.
A number of tests involving GPS receivers in various kinematics settings were conducted to assess the performance of GPS used in
support of highway surveys. The main objective was to experiment the recently developed GPS Pro XR/XRS Trimble receiver for
determining trajectories and reference point positions of a land vehicle dedicated to road maintenance. The performance of this single
frequency receiver, equipped with internal antenna for receiving satellite transmitted RTCM correction, was evaluated by using, as
reference, precisely determined positions. Different surveys were performed both in continuos kinematic and in Stop and Go mode. In
order to define an accuracy range for RTCM corrected positions, the tests were conducted mounting the Pro XR/XRS system on a vehicle
of the Societa’ Autostrade equipped with an odometer and a geodetic dual frequency GPS receiver. Chainage values, conventionally
adopted for localising objects during road surveys, are extracted from the GPS co-ordinates and compared with those derived by the
odometer. Preliminary results from the tests are presented and discussed.
The present work examines the use of GPS for the kinematic
positioning of a land vehicle dedicated to road survey; in
particular, focus was placed on the necessity of the highways
survey, where different vehicles are used to measure parameters
about road surface characteristics.
On the basis of these parameters, the related maintenance
activity can be planned; therefore it is necessary that the
employed vehicles are able to assign a spatial position to every
data samples, which should correspond to a unique point on the
road. The spatial position is usually assigned by means of a
one-dimensional cumulative co-ordinate, know as mile
chainage. The origin of the co-ordinate is set on conventional
points, which can be different for each path of highway.
In most cases the vehicle dedicated to road survey are able to
measure the mile chainage through an odometer. Where
possible, a number of reference points are established in
correspondence of a stable and well visible object (as road signs
or junctions).
In a typical highway survey the vehicle proceeds at a constant
speed (about 70 km/h) on the right lane of the road, as close as
possible to the border line; when passing by the reference
points the operator bits an impulse which is recorded by the on
board computer. Additional points in support of a better
localisation of the measured technical parameters can be
determined on the way; the position of these points is computed
relatively to the main reference points.
The most advanced systems include also a video camera on
board of the vehicle, which acquires images to help in the
positioning processing.
The described measurement system has the following
1. the mile chainage is a relative measure without geographic
meaning; therefore the collected data cannot be either
overlapped to data from other source or referred to a
cartographic reference system;
2. the accuracy of the vehicle’s positions is in the range of 50
to 100 meter every kilometre (the lower errors can be
obtained when the route is quite regular);
3. the durability of the reference point is not guarantied.
With the aim to define an alternative procedure for positioning
data collected on the road, a number of field experiments were
carried out in this work. Initially, the performance a recently
developed GPS receivers (Trimble Pro XR/XRS) was
investigated in different environments. Secondarily different
tests were executed on highways where a vehicle equipped with
an odometer was adopted. Finally, a hardware solution is
proposed to be adopted for operational survey.
Recently developed models of GPS receivers (single frequency
only code) results particularly suitable for accurate and reliable
kinematic survey. In these new receivers the signal acquisition
system is very robust against the noise that is very frequent in a
typical dynamic environment. Moreover the achievable
accuracy (about 1 -2 meter), is quite satisfactory for most of the
applications in the road sector.
Among these new types of receivers, the Pro XR/XRS Trimble
system was selected to carry out the experiment described in the
following. This receiver uses twelve channel for tracking
Ll/CA code with carrier phase filtered observations. It is an
integrated GPS/Beacon/Satellite Differential receiver with
Everest™ multipath rejection technology. The given accuracy
(RMS) for differential correction is of 50 cm plus 1 ppm for
horizontal components and submeter plus 2 ppm for the vertical