Poli, Daniela 
  
track stereo viewing, provided by a high resolution nadir-looking lens and two off-nadir lenses, forward (+21.4 degrees, 
channel 6) and backward (-21.4 degrees, channel 7) looking. 
The two images used in this work were taken from 
channel 6 and channel 7, with a time delay of 40 
seconds and a ground resolution of 18 m. The nadir 
image could not be used because channel 5 on Priroda 
was defocussed. Each image has a dimension of 2976 
pixels across-track and 10472 pixels along-track and 
consists of a combination of two overlapping images in 
the flight direction. Clouds are generally visible, but 
they can not be always recognized in both images (Fig. 
2) or they have different lightness and contrast (Fig. 1), 
because of the different illumination-to-sensor angle. 
  
igh Moreover, some other smaller structures (e.g. buildings) 
wer also appear different in the two images. The time 
rom difference between the two acquisitions causes some 
and additional displacement and deformation of the clouds. 
and 
ring The radiometric quality of the images was improved by 
red reducing the noise and applying a Wallis filter (Wallis, 
d to 1976) for contrast enhancement and radiometric 
m), equalization. À further preprocessing was necessary to (a) (b) 
tric detect and reduce vertical stripes due to the saturation of 
red the even columns in cloud regions. A 3x1 template Figure 1. MOMS-2P stereopair: channel 6 (a) and channel 
tric window moves with step 2 in x-direction and step 1 in y- 7 (b). 
ing direction. If the grey mean value in the window is bigger 
ing than 230 and the correlation with a template containing 
es, typical stripes is bigger than 0.9, a filter is applied: the grey values are calculated again using a 5x1 mask. As a result, 
ces regions with vertical stripes became more homogeneous than before and some details appeared, as shown in Fig. 3. 
sors 
oud 
1ese (a) 
1sor 
uügh 
the 
  
  
  
  
  
  
  
  
  
  
  
s at 
ts is 
nel 
size 
on 
ight 
jade (b) 
Figure 3. Zoom in a saturated region before (a) and after (b) 
applying the filter. 
  
ion, 
ng- Figure 2. Example of cloud illumination 
differences, particularly with thin 
clouds, between channel 6 (a) and 7 (b). 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000. 1163