,-'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