International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004 
  
4.1 Related work 
Approaches for monitoring traffic have been proposed for both 
spaceborne systems, e.g. RADARSAT-2 [Livingstone et al., 
2002], Shuttle Radar Topography Mission SRTM [Breit et al., 
2003] and airborne systems, e.g. [Ender, 1999], [Eckart et al., 
2000]. The moving target indication (MTI) requires vehicle 
detection in the SAR data and parameter estimation. Target 
detection and estimation can be performed either incoherently 
with a single SAR sensor, e.g. [Kirscht, 1998], [Livingstone et 
al, 2002], or coherently (along-track Interferometry), with 
much higher fidelity, with two [Gierull & Sikaneta, 2003] or 
more apertures [Ender, 1998]. 
For processing a SAR image from a single aperture all scene 
objects will be assumed stationary. The relative motion between 
sensor and scene causes a Doppler frequency shift, which is 
exploited to achieve a high azimuth resolution and to determine 
the correct azimuthal position of objects. The movement of 
objects causes artefacts. The radial velocity component of the 
movement leads to an azimuthal displacement (because of the 
additional Doppler frequency shift), the parallel component 
leads to blurring. This first effect can be observed in Fig. 5 
comparing a SAR image and an aerial image acquired 
simultaneously. A cargo ship entering a lock (motion from left 
to right = in range direction) is mapped on the ground in the 
SAR image. 
  
Figure 5. Azimuth displacement of a cargo ship entering a lock. 
a) SAR image, b) aerial image 
4.2 Detection of single vehicles 
Some image based approaches exploit this azimuth 
displacement of detected vehicles from the expected position to 
estimate the radial velocity component relative to the carrier. 
Using the information about geometry of the route they try to 
derive the velocity of a vehicle. One-channel implementations 
may register only fast transverse or parallel movements. 
Other approaches exploit SAR data from two or more channels 
resulting from several antennas or subapertures oriented in the 
direction of flight which observe the same scene at different 
times. A coherent processing of the data allows calculation of 
along-track interferograms showing moving objects by phase 
differences. Introducing knowledge, (e.g. typical size or shape) 
moving objects can be detected in the interferogram by model- 
based image analysis [Schulz et al., 2003]. 
SAR-MTI (moving target indication) processing has shown 
promising results in rural and suburban area. Figure 6 shows an 
example of a SAR-MTI result which depicts an Autobahn exit 
and automatically determined positions of vehicles after 
complete MTI processing including clutter suppression, moving 
target detection, high-precision azimuth position estimation and 
target tracking [Ender, 1999] 
  
Figure 6. SAR-MTI result after target tracking (Autobahn exit 
Ingolstadt/Nord) [Ender, 1999] 
4.3 Exploiting context 
However, in urban areas SAR specific illumination phenomena 
like foreshortening, layover, shadow, and  multipath- 
propagation burden the interpretation. The so-called layover 
phenomenon occurs at locations with a steep elevation gradient 
facing towards the sensor, like vertical building walls. Because 
object areas located at different positions have the same 
distance to the sensor, like roofs, walls and the ground in front 
of buildings, the backscatter is integrated into the same range 
cell. In general, the signal mixture cannot be resolved from a 
single SAR acquisition. The signal contributions of different 
objects inside a resolution cell can be separated using e.g. full 
polarimetric or interferometric SAR data or even a combination 
of both [Guillaso et al. 2003]. Layover areas appear bright in 
the SAR image (see Fig. 8b). Perpendicular alignment of 
buildings to the sensor leads to strong signal responses by 
double-bounce at the dihedral corner reflector between the 
ground and the building wall. All these double-bounce signals 
have the same time-of-flight. This results in a line of bright 
scattering in the azimuthal direction at the building footprint. 
At the opposite building side, the ground is partly occluded 
from the building. This region appears dark in the SAR image, 
because no signal returns into the related range bins. 
Layover and radar shadow (e.g. caused by tall buildings) may 
hinder the visibility of neighboured objects of interest, like 
roads. The sizes of the layover areas /, and shadow areas sg On 
the ground in range direction (see Fig. 7) depend on the viewing 
angle 0 and the building height A. 
    
  
   
    
  
  
  
  
  
    
  
    
   
    
     
    
    
   
    
     
    
    
    
  
    
    
  
   
   
   
   
    
    
   
   
     
   
   
   
   
   
   
  
    
  
  
  
    
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