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Reinhard Schuster 
3.4 Measurement of the Geometric Registration 
The purpose of the registration calibration is to assign to each sensor 
pixel the angular position of its seen ground pixel 
element. 
OTF Measurement 
2 LIN, 
ÿ 
50 100 150 200 250 300 350 400 450 500 
position (pixel) 
  
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1 T T T 
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o] met re m eS d MTF>0.6: v<22.8 Ip/mm 
MTF>0.3: v< 60.9 Ip/mm 
2 50 100 150 200 250 300 MTF>0.1:  v« 150.7 Ip/mm 
frequency (Ip/mm) 
Figure 10. (a) code pattern before (red) and after (blue) imaging through a test lens 
(b) transfer functions MTF (blue) and PTF (green) deduced from (a) 
Unlike film cameras, where film shrinking requires the need of artificial fiducial marks, the situation is much easier for 
the digital camera. Each CCD-pixels is a fiducial per se. This allows to reduce the calibration task to assign to each 
pixel the value of its 'down to ground looking' direction vector a, mostly expressed by two polar angles, relative to a 
lens based co-ordinate system. 
Owing to the stringent accuracy requirements of 1 arcsec for both angles, special electro-optical means are foreseen in 
the CVG to control mechanical motions of swivel and x-scanner in real time. Powerful algorithms, specially adapted to 
the stochastic properties of the code structure, allow determination of the actual position of the code at the CCD-lines 
with sub-jum resolution. 
3.5 Radiometric Calibration 
The radiometric calibration is done with a large Ulbricht sphere, located directly beside the CVG. The test bench is still 
under construction, but operates very similarly to the DLR equipment. At the time of writing the first test runs with the 
engineering model of the ADS40 are complete. 
4 COMPARISON OF THE GEOMETRIC CALIBRATION MEASUREMENTS 
Photogrammetric cameras must be geometrically calibrated with an accuracy of parts of one pixel. Therefore very 
precise angular measurements are necessary to meet the stringent requirements. Using the first working ADS40 model, 
the engineering model, geometrical calibration was performed at both calibration facilities of DLR and LH Systems for 
the purpose of comparison. The outputs of the geometrical calibration process are two polar angular co-ordinates, 
assigned to each measured pixel. The first results indicate that it is sufficient to measure pixels every 2-5 degrees within 
the field of view, depending on the length of the CCD-line and on the focal length of the camera. The angular co- 
ordinates for pixels in between are interpolated numerically. The geometrical calibration results can be presented in 
different forms and units, e.g. in multiples of a pixel. The conversion to conventional units e.g. SI units, is then a simple 
multiplication with the pixel size. 
In the preceding chapters we outlined the different measurement modes at both locations. At DLR in Berlin the ADS40 
is mounted horizontally on a rotation-tilt stage and the direction of the pixel through the optics to the fixed collimator 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part Bl. Amsterdam 2000. 293