Michael Breuer 
  
measurement means are available there are always noise effects which have to be modeled for error estimation and 
quality management. Often combined GPS/INS is not available because these systems are very expensive if they reach 
high precision. Therefore low-cost navigation systems or simple GPS receivers providing a positioning accuracy of 30 
meters or more are widely used. Such data serve only for approximation. Sometimes they turn out to be useless to 
achieve the desired correction accuracy. 
2 THE PROBLEM 
No remote sensing acquisition system can render 
a true geometric reproduction of the earth’s 
surface without corrections (Binnenkade, 1993). 
This is why methods for geometric processing of 
the data are needed. The discussed problem in 
this paper deals with the elimination of all dis- 
turbing effects from the image data. These effects 
are superimposed (see Fig. 1) and some of them 
are correlated (Rose, 1984). The main effects are 
briefly described in the following paragraphs. 
    
  
flight track 
A B C 
2.1 Panoramic Distortion 
Figure 1. Schematic diagram of distortions 
The cause of panoramic distortion is the 
cylindrical projection (see 1) across the flight track. For that reason the scale as well as the spatial resolution varies 
within one scanline. From the nadir (the place of maximal resolution) it decreases towards the sides. Furthermore the 
changes are different in x and y directions. A pixel is quadratic at the nadir and becomes a trapezoid at the sides. The 
panoramic distortion is sometimes also referred as tangential scale distortion (Buiten, 1993, Richards, 1986). 
2.2 Underscan 
The aim of hyperspectral scanning is to capture the terrain surface without any gap. The rotation velocity of the mirror 
(res. the scan rate) must be consistent with the velocity of the airplane along the flight track to meet this demand. It 
seems imaginable to pitch the scan rate constantly with the along track velocity during a flight mission. However the 
scan rate is constant in most practical applications. Therefore the pilot has to take care of keeping the along track 
velocity constant so that no gap could occur in the image data. If the gaps could not be avoided the resulting effect is 
referred to as an underscan (sometimes also referred as undersampling). In this case the information about the terrain is 
lost (see Fig. 1, section C). 
2.3 Overscan 
An overscan (res. oversampling) is the opposite of an underscan. It occurs if the along track velocity of the airplane is 
smaller than the allowed threshold. An overscan results in an increase of the along track scale of the image data. This is 
less dangerous than an underscan because no information is lost (see Fig. 1, section A). In the ideal case all nadir pixels 
are in touch (see Fig. 1, section B). But even then the margin pixels overlap especially if the airplane moves straight 
ahead and if the field of view is large. 
2.4 *S-shape" distortion 
This distortion results from the fact that the airplane moves ahead during the time one line is captured. This results in an 
S-shaped curvature (see Fig. 1). This effect is highly correlated with the yaw angle and the crab distortion. Each scan is 
mostly approximated to be a straight line because the rotation velocity of the mirror should be high compared with the 
along track velocity of the airplane. The "S-shape" distortion is sometimes also referred as sensor scan nonlinearities 
(Richards, 1986). 
2.5 Crab distortion 
The crab distortion results in a skewed image and occurs when extreme crosswind is encountered during data 
acquisition. In this case the axis of the airplane must be oriented slightly away from the flight axis to counteract the 
wind (Lillesand and Kiefer, 1994). 
  
94 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000.