Full text: XVIIIth Congress (Part B4)

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Figure 3: Detail from an ET3 image (nadir channel) 
showing the summit of the Kronplatz in the Swiss Alps. 
The blocky appearance (blow-up in the upper left 
corner) is due to the effects of DCT data compression. 
Note that although the diameters of the cables 
(assiciated with cable cars going to the Kronplatz) are 
below the nominal resolution of the camera, they are 
clearly visible in the image. The distance from the 
camera location to the summit was about 40km 
    
  
    
  
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frequency [%] 
t2 
0 50 100 150 200 250 
DN 
Figure 4: Histograms of panchromatic channels nadir, 
stereo 1, and photometry 1 and 2, respectively. 
3.3. Performance of the Flat-Field Correction 
During the analysis of data from HRSC Extended Test 3 all 
images were corrected for the different responsitivity of 
sensor elements (Fig. 5). These "flat-field errors" are 
visible in the uncorrected images as characteristic vertical 
stripes, which, after the flat-field correction, are effectively 
  
  
  
  
     
  
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0.8 LL 1 i | L | 1 I Li Hd ] LL il | 11-1 ] 1 
0 1000 2000 3000 4000 5000 
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Figure 5: Flat-field correction factors for sensor 1N 
(nadir) as determined during the radiometric calibration. 
351 
Figure 6: Detail of an ET3 imaging sequence (nadir 
channel) before (left) and after (right) flat field 
correction. Note that the contrast in the images is 
enhanced. The difference between adjacent pixels is 
typically less than 2-3 DN values, i.e. only about 1% of 
the total DN range. The horizontal line through the 
image shows the location of the profile plotted in Fig. 
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removed. The performance of this correction has been 
verified plotting the DN values along a profile in the 
direction of the CCD sensor line before and after the flat- 
field correction. To minimize the effects of texture, a profile 
was selected which contains a relatively homogeneous sky 
region (Figs. 6 and 7). The profiles show that much of the 
high-frequency noise in the image is removed. 
  
    
  
  
  
| ET3 Imaging 
195 f Sequence 149 — 
nadir channel 
S 190 
> 
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L1 p d ad L3 LL eel 
0 50 100 150 20 
pixel (sample) number 
  
Figure 7: Brightness profile through the sky (see Fig. 6) 
in the nadir image: The pixel (sample) number is plotted 
against the respective DN values before and after the flat- 
field correction. It is evident that the high-frequent noise in 
the uncorrected image (dotted line) is effectively removed in 
the corrected image (solid line). 
3.4. Simulation of Imaging Conditions in Mars 
Orbit 
During selected imaging sequences, the rotational speed of 
the turntable was gradually increased in order to simulate the 
changing spacecraft speed as expected in the highly 
elliptical Mars orbit. The scan rate of the camera was 
changed accordingly in several steps, resulting in 
increasingly smaller exposure times for the scan lines and a 
decrease in image brightness. 
To verify this type of camera operation, profiles of 
greyvalues (DN values) through homogeneous sky regions 
were sampled in flight direction (Fig. 8). The profiles show 
indeed a correspondence between exposure times and 
observed brightness. Additionally, this test could be used to 
verify the correct operation of the software which computes 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996 
 
	        
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