Reinhard Schuster 
  
  
  
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1 
Figure 5. Result of geometric calibration in degree (1) before and 
in units of pixel-size (2) after transformation 
The second requirement is met by measurements of the Point Spread Function (PSF), from which the Modulation 
Transfer Function (MTF) can be calculated by a Fast Fourier Transform (FFT). It is useful to register at the same pixel 
the PSF and geometry. Because the PSF is a two-dimensional function, we measure in the two main directions 
corresponding to the flight direction and perpendicular in the line direction. From these measurements the MTF can be 
calculated at an arbitrary number of points along the line and used as input for the image post processing. Examples for 
PSF/MTF measurements with the engineering model of the ADS40 are shown in figure 6. The MTF curves characterise 
the optical performance of the digital camera system, which is mainly determined by the pixel size and the optics. The 
resolution always seems to be better in the flight direction than in the line-direction. 
flight-direction swath direction MTF 
  
  
  
  
  
  
  
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0° 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
+10° 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Figure 6. Examples of PSF/MTF measurements (nadir line of engineering model of the ADS40) 
The use of modern digital sensors together with a flexible electronic read-out channel also leads to new possibilities for 
the radiometry. If the total field of view (FoV) of a line camera is illuminated homogeneously we get a line-signal 
  
290 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B1. Amsterdam 2000. 
  
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