35 
Figure 6 A mark on the ground in correspondence of a road 
sign 
Figure 7 The two GPS antennas mounted on the roof of the 
vehicle 
The test was realised in two different surveys: 
Stop and Go, with the vehicle stopping for few epochs on 
every reference point: 
Kinematic, with the vehicle acquiring the reference points 
while moving. 
The position of the surveyed points was calculated in the UTM 
mapping projection relative to GPS reference ellipsoid 
Figure 8: Map of the reference points established on the 
highway. 
4.1. Quality control of DGPS positioning 
The co-ordinates of the 21 kilometre road signs were measured 
during different surveys: 
1. Stop&Go/dual frequency receiver (4700)/post-processed; 
2. Stop&Go/single frequency/post-processed; 
3. Stop&Go/single frequency/ satellite transmitted RTCM 
corrections; 
4. Continuous/single frequency/post-processed; 
5. Continuous/single frequency/satellite transmitted RTCM 
corrections. 
The results are compared in Table 2 where the mean values and 
standard deviations of the differences between the reference co 
ordinates from the first survey and the other four are pointed 
out. 
Table 2 Mean values and standard deviations of differences 
between the reference co-ordinates and Pro XR/XRS results. 
The level of accuracy of the two Stop&Go positioning is on the 
order of few decimetre, while the continuos kinematic surveys 
show large errors to sighting of the reference points. This is 
evident in the North co-ordinates corresponding to the direction 
of navigation, which shows a systematic error of about 8 
meters. 
A quality control of the GPS co-ordinates can be obtained by 
means of repeated measurements of the fixed baseline 
established between the two GPS antennas mounted on the 
roof. The discrepancies between an accurate estimate of the 
baseline length and the one estimated raw data collected during 
the experiments in correspondence of each reference points 
were computed; the mean value of the horizontal component 
which is of primary interest in highway survey of the baseline 
were: 
Aj = + 0.54 ± 0.42m for Stop&Go/RTCM survey 
Aj = + 0.38 ± 0.32m for Stop&Go/Post-processed survey 
4.2 Quality control of the odometer 
The performance of the odometer for computing the mile 
chaînage was verified by repeated measurements of the 
reference point positions both in a moving mode acquisition 
and by stopping in correspondence of the signalised marks on 
the ground. This test pointed out a linear trend in the difference 
of mile drainage values, which result more evident in the 
continuos mode: the kilometric errors is about 1 meter in the 
moving mode and 20 cm in the other (Figure 9). This confirms 
the presence of accumulated errors in odometer measured 
chaînages. 
The more accurate mile drainages obtained by aligning the 
vehicle to the marks on the ground are considered as reference 
values against which evaluating the one derived in a standard 
dynamic acquisition mode. The comparison of the 2 set of data 
provides differences scattered around the mean value within a 
range of 2.06 meters (Figure 10).