International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004 
  
scatterers are involved, the signal contributions of different 
objects can hardly be separated analysing a single SAR image 
alone. The InSAR DEM is unreliable in layover areas. Layover 
is a major hindrance for the analysis of SAR data in urban 
areas, e.g. for building reconstruction. 
Fig. 3: a) ground range magnitude image and building layer 
(red); b) DEM and vector map (roads: green). 
The choice of a large viewing angle minimizes layover effects. 
But, large viewing angles lead to extended occlusion areas, 
caused by the cast shadow from elevated objects on the ground 
behind. From shadow regions, no signal is returned to the 
sensor. Therefore, these areas in the SAR data contain no 
information but noise only. The choice of the viewing angle in a 
dense urban area is a trade-off between layover and shadow 
effects [Soergel et al, 2003a]. 
Since the viewing angle is known, the building height can be 
determined from the size of layover or shadow areas. However, 
these features are often disturbed by the signal of other objects. 
Parallel to the sensor track orientated building walls together 
with the ground in front of them build a so-called dihedral 
corner reflector: the signal is reflected back to the sensor by 
double-bounce scattering. The path length is identical for all 
double-bounce signals, causing bright lines in azimuth direction 
in the SAR image located at the building footprints (e.g. at 
buildings F and G). At building / a dihedral corner is build from 
the tall main building and the flat roof of the entrance hall. 
Another kind of dominant scattering frequently observed in 
urban areas is specular reflection, e.g. at the roof structures on 
buildings B and J. These single-bounce signals can be 
  
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discriminated from double-bounce scattering by their 
polarimetric properties [Guillaso et al., 2003]. 
In case of a small radar signal wavelength like X-band, there is 
a significant influence of trees on the visibility of buildings. For 
example, building K is occluded from a large tree, which also 
covers some small buildings of group / in front by layover. 
4. CHOICE OF OPTIMAL SAR ACQUISITION 
PARAMETERS 
It is essential to determine a-priori the optimal SAR acquisition 
parameters in order to minimize the influence of layover and 
shadow for a selected area of interest or an object class. For this 
purpose, GIS data are required. Here, we focus on best effort 
mapping of buildings and roads by SAR for the test area shown 
in Fig. 3b. 
Based on the DEM, layover and shadow are simulated [Meier et 
al., 1993]. From the sensor position, the DEM grid is sampled 
in range direction. Layover and shadow regions are detected 
analysing the distance and the viewing angle. By intersection of 
these results with the map data, the affected areas of buildings 
and roads are identified. 
In order to find the best SAR parameters the simulations were 
systematically repeated with varying aspect angles a and 
viewing angles 9 (Fig. 4). About 650 simulations have been 
carried out for the entire test site [Soergel et al, 2003b]. 
Different viewing angles 0 
o 
Different aspect angles a 
Flight & 
trajectory 
(0,9) * 
  
  
Fig. 4: Simulation of SAR phenomena layover and shadow 
With the given sensor parameters, only 43% of the building 
roof area and about 20% of the road area can be sensed without 
distortions by layover and shadow. These numbers increase to 
86% for roofs and 62% for roads in case of a combination of 
four chosen SAR acquisitions (Tab. 1). 
  
SAR measurements ] 2 3 4 
Undistorted roof area .|51.6 172.8 181.5.186.5 
Undistorted road area | 39 47 55.6 | 62 
  
  
  
  
  
  
  
  
  
Table !. Portion of roof and road areas visible without 
distortions for several SAR acquisitions in per cent. 
Two simulation results for the same area as shown in Fig. 3b 
are depicted in Fig. Sa,b. The first simulation was carried out 
with the parameters of the acquired InSAR data and the other 
one for an aspect angle from the right with same carrier altitude 
and viewing angle. The dependence of SAR data acquisition 
from the aspect angle can clearly be observed comparing e.g. 
the shadow and layover areas caused by the buildings D and E. 
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