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The imaging of a ground surface by means of a SAR system generates a two-dimensional map of the 
illuminated area. Here a distinction has to be made between the two dimensions, i.e. on one hand the azimuth 
direction, which is the direction of flight, and on the other hand the range direction, which runs perpendicular 
to the direction of flight or parallel to the sight of the antenna (Figure 1). 
The azimuth resolution depends on the length of the antenna and is therefore limited when using a radar with a 
real aperture. The SAR technique [Oliver et al, 1998] is used to synthesise a very long antenna by combining 
signals (echoes) received by the radar as it moves along a track. The synthetic aperture can be constructed by 
moving a small antenna to different positions through the whole length of the synthetic aperture. At each 
position a pulse is transmitted. Then the return echoes pass through the receiver and are recorded in an ‘echo 
store’. The range resolution is obtained by a short radar pulse or equivalent techniques. 
  
  
   
Figure 1. Imaging technique Figure 2. SAR interferometry 
Some important imaging properties of SAR appear due to the side looking geometry, namely layover and 
shadow effects. We can describe them as follows: 
Layover: The angle of inclination of the slope facing the sensor becomes greater than the angle 
of incidence. Information starts to overlap, which no longer permits a clear 
allocation of the signals (see area AB in Figure 3). 
Shadow: Radar shadow arises if the angle of inclination of the slope facing away from the 
sensor becomes greater than the angle of incidence. Areas occluded by the relief are 
not visible (see area CD in Figure 4). 
    
AB C A eco 
Figure 3. Layover Figure 4. Shadow 
A SAR image is generated by processing the recorded raw data [Curlander et al, 1991]. In the processing stage 
the raw data are range and azimuth compressed forming a two dimensional image in slant range (radar) 
geometry. 
SAR images in slant range need to be projected into a certain cartographic system. For this procedure the DEM 
information is mandatory. We take advantage of the coherence capability of the radar for doing an 
interferometric processing by using a second antenna. As in stereometry the height can be evaluated if the same 
area is imaged from two different directions. On the other hand interferometry [Gens et al, 1996] considers the 
phase of the signal and not the amplitude like stereometry. By measuring the phase, one gets a very accurate 
surface model that is independent of the area contrast. By measuring the phase difference of the two SAR 
images (V, - V, ) obtained from S1 and S2 (Figure 2), one can determine the range difference A , — (r; T») 
between the two antennas very accurately as follows: 
r 
À 
A z-—e(V -W, 1 
in (F, >) (1) 
The advantage of measuring the phase is that one can obtain A, within tenth of mm accuracy and also 
independent of the image contrast, i.e. the surface model of ice or water surfaces is also obtainable. 
  
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International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B1. Amsterdam 2000. 
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