,-'c V 
2.3. Land-based tests 
The integrated GPS/INS/CCD system was also tested in 
terrestrial applications. The sensor assembly was 
mounted on the top of the van for acquisition of the data 
needed for a topographic survey of the roadways. The 
van-based system, currently under development, is 
designed for precision monitoring of the highway edge- 
and the centerlines. The system’s current hardware 
configuration, similar to the one described in section 2.1 
is presented in Figure 10. The single, side-looking 
camera, tilted downwards by 5°, was mounted rigidly 
on top of the INS, offset from the GPS antenna by 
~1 m. The imagery was collected along the surveyed 
road, and the subsequent stereo-pairs (formed by the 
time-offset succeeding images) were formed with the 
directly acquired orientation parameters. The ground 
pixel size was 2 mm at the target area. The quality of 
the direct orientation parameters, represented by the 
standard deviations, is presented in Figures 11 and 12. 
The spikes that can be observed in the position standard 
deviations correspond to partial or total losses of GPS 
lock when the vehicle was passing under the foliage or 
close to the buildings. The otherwise low level of the 
standard deviations indicates the quality solution, but an 
independent check is always needed to assess the 
system’s absolute accuracy. This is usually 
accomplished by an independent photogrammetric 
solution or survey of the control points visible on the 
imagery. 
Figure 10. Hardware configuration for land-based 
applications. 
The overall accuracy indication can also be achieved by 
examining the repeatability of the solution obtained for 
the control points measured on different directly 
oriented stereo pairs. The statistics of such a 
comparison, based on 44 stereo pairs, is presented in 
Table 2. The results indicate that the direct orientation 
parameters were indeed estimated with high quality. 
Another repeatability test was performed by comparing 
the ground coordinates of 15 check points measured on 
the directly oriented stereo pairs from two different 
passes, as shown in Table 3. The GPS/INS/image data 
for those passes were collected with slightly different 
GPS constellation; the pass one observed six to seven 
satellites, whereas pass two was able to collect GPS 
data from only five and less satellites. This is reflected 
in the differences in the positions listed in Table 3. 
35 
30 
£ 25 
20 
-2 15 
.2 10 
pitch 
roll 
heading 
0 
4.165 
4.17 4.175 4.18 4.185 
GPS time of week [sec] 
Figure 11. Attitude standard deviations. 
GPS week time [sec] 
Figure 12. Position standard deviations. 
x 10 
Statistic 
Easting [m] 
Northing [ml 
Height [m] 
Mean 
0.015 
0.004 
0.008 
Median 
0.006 
0.003 
0.006 
Maximum 
0.050 
0.025 
0.035 
RMS 
0.019 
0.007 
0.010 
Table 2. Ground coordinate difference for the control 
points measured from different stereo pairs. 
2-4-6