Clarke, Tim 
  
Figure 1. DSP electronics and cased systems 
Some characteristics of the system are: 
* Location precision: 1/70 of a pixel for 1000 measurement of stationary retro-reflective target 
* Local precision - straight line fit to retro-reflective target moved over a straight line = 1/38 of a pixel 
* Global precision, e.g. rms image residual from a bundle adjustment - between 1/30 and 1/50 of a pixel 
depending on configuration 
Resistance to image saturation - a few hours without failure 
Number of image processed without error - approx. 0.5 Million 
Warm up effects - approx. 1/16 of a pixel maximum variation in the x direction, stability after 15 minutes 
Number of targets processed in real-time - 170. 
3. TARGET IDENTIFICATION 
The unique identification of targets in each image is a necessary prerequisite for calculation of the six degrees of freedom of 
objects. To achieve this reliably two approaches are used. The first is the use of coded targets. A number of these targets are 
placed on the object to be measured such that any other targets on the object can be computed using their projection onto the 
image planes of the cameras and matching with the image locations of the target images. Subsequent to this initial start up 
procedure, correspondence in maintained by tracking. 
The coded targets have been rigorously tested and have been shown to be highly resistant to variations in scale, orientation, 
position, illumination, and occlusion. In addition the targets can also be picked out of noisy images. 
4. PHYSICAL DESIGN 
An example task chosen for investigation is the positioning of a wing leading edge rib with respect to a spar. A three or four 
camera system would be preferred from the point of view of redundancy, however, the physical constraints of the 
application are such that this arrangement would be too bulky. A stereo configuration was chosen as a good compromise for 
this application. The physical arrangement of the system takes into account the trade-off between accuracy in the X and Y 
direction and the Z axis. The arrangement prefers to compromise the Z accuracy in favour of the beneficial effect of a 
compact system that will be able to view targets at a wider range of angles of orientation. 
  
138 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000. 
  
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