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In this test the Litton 90-100 was tested on a rotating 
platform which gives independent attitude with an accuracy 
of 3 arcsec. Figure (7) shows the difference between the INS 
attitude parameters and those produced by the rotating 
platform. The figure confirms that the short term INS attitude 
errors are of the order of 10-20 arc seconds for roll and pitch, 
and 30-50 arcsec for azimuth. This results in errors of 1 cm 
or less for points 50 m away. Another test is planned to 
confirm such accuracies under survey conditions. 
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Angular Accuracy (arcsec) 
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Angular Velocity (deg/sec) 
Figure 7: INS Attitude Accuracy as a Function 
of Angular Velocity 
The updated INS positions will be mainly used for 
interpolating camera coordinates and detecting GPS cycle 
slips. Inertial systems combine very high short-term 
accuracy and high data rate, and, thus, are ideally suited for 
these tasks. The accuracy of using INS positions in 
interpolating the camera coordinates depends on the 
position drift rate between updates. Figure (8) shows a 
typical drift behavior between update intervals of 20 sec in 
static mode. The figure confirms a drift of 3 cm/10 sec in 
latitude. This means that for a GPS updating interval of 1 
second, the INS position accuracy is at the level of few 
millimeters, better than the GPS accuracy for this time 
interval. 
  
0 20 40 60 80 100 
Time (sec) 
Figure 8 : INS position drift in static mode 
The idea of detecting cycle slips is based on comparing the 
measured integer cycle number with the number predicted 
from the INS derived position. If a cycle slip occurred, the 
respective carrier phase can be corrected by resetting the 
integer value to the integer value closest to the predicted 
value. Obviously, the method will work best for short time 
intervals. In environments were cycle slips are frequent, as 
for instance inner cities, the success of cycle slip detection 
depends mainly on the INS position accuracy. In order to 
correct cycle slips at the one cycle level, INS positions must 
be accurate to 0.5 cycle, or better, for the outage interval. 
The accuracy of the positions derived from INS are a function 
of the update rate and the vehicle dynamics. Detailed tests are 
currently under way to determine the time interval in which a 
stand-alone INS will give 10 cm accuracy. 
Camera errors contribute to the error budget through 
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pointing errors in r . The accuracy of r depends on the 
distance between the target and the cameras, the size of the 
target, and the accuracy of pin-pointing the target. To check 
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the accuracy of r we use the orientation parameters obtained 
from the bundle adjustment to determine the object 
coordinates from the image coordinates of stereo pairs. This 
is equivalent to positioning points with respect to the 
stereo-pairs, independent of GPS/INS data. The test was done 
by establishing a test field of ground control points (GCP) 
and taking some images for this test field in kinematic 
mode. Figure (9) shows the difference between the distances 
computed from the computed local coordinate and from the 
GCP coordinates. The figure shows that these errors are 
distance dependent, as expected, and reach a magnitude of 
10-15 cm for objects 50 m away from the van. 
  
  
  
  
  
  
  
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Distance from camera (m) 
Figure 9 : The accuracy of measuring distances 
with CCD cameras 
The georeferencing equation (5) contains four unknowns (3 
coordinates and one scale factor) in three equations. Having 
a second camera of the same scene will add three extra 
equations and one unknown for the same point (1). A least 
squares solution of the space intersection between the two 
rays from the two images is computed. With a one second 
image data rate and a speed of 60 km/h, the van moves 16 m 
between exposures, and it is expected that the same object 
will , therefore, appear in four consecutive images. This adds 
extra redundancy and geometry to the spatial intersection 
problem. Figure (10) illustrates the error reduction resulting 
from adding redundancy and geometrical constraints to the 
minimum configuration. Figure (10a) shows the geometry of 
the situation with the target point visible in four pairs of 
images. If the four individual pairs are used for positioning, 
the upper curve in Figure (10b) is obtained. It shows errors 
which grow rapidly with distance from the target and are 
considerably larger than expected. This is almost certainly 
due to stability problems of the camera mount in this 
specific run. When combining all images to determine the 
target point, the positioning error is reduced to 0.25 m 
which is well within the required accuracy. It should be noted 
that the combination is not just a weighted mean of the four 
individual determinations but indirectly introduces the 
coordinate differences between exposure stations, measured 
by GPS/INS, as a constraint into the equation. Instead of 
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