Wim van Rossum 
  
2 RESOLUTION ENHANCEMENT 
2.1 Azimuth resolution 
Since PHARUS is a phased array system, it is very well suited for spotlight mode operation. A Spotlight Beam Steering 
Control mode was implemented by the National Aerospace Laboratory (NLR. In this mode the beam is steered 
electronically to a point on the ground resulting in a larger synthetic aperture and consequently in a higher resolution as 
compared to the normal scan mode of PHARUS. 
Focussing of the image proved to be difficult. The phase consistency across the synthetic aperture was investigated. 
During the acquisition of the synthetic aperture 
several beam position changes occur. These 
changes, with a minimum step size of 0.5°, 
keep the beam pointed to the designated spot. 
Inspection of corner and transponder signals 
indicated that there were no significant phase 
discontinuities associated with beam switching 
[Nijenboer, 1999]. 
RE 
    
The same investigation also showed that 
sometimes motion compensation using only 
the on-board recorded motion data was not 
sufficient. Improvement of this compensation 
was achieved with the use of an autofocussing 
technique. The technique used is the Phase 
Gradient Autofocus algorithm {Wahl., 1994]. 
In this algorithm the phase error caused by an 
unknown movement of the platform is 
estimated using the data by means of an 
iteration loop. In Figure 3 images obtained 
using the PGA algorithm are compared with 
the images obtained using the range-Doppler 
algorithm only. The azimuth resolution could 
be improved significantly as compared to the 
normal SAR mode. The PGA algorithm then 
further improved the resolution, e.g. from 87 
cm to 37 cm in the lower two images of Figure 
3. 
An azimuth resolution better than 30 cm has 
been achieved. However, with a range 
resolution of 3 m, it was more sensible to 
perform azimuth multi-looking in order to 
make the azimuth and range resolution 
comparable than to show the full resolution in 
Figure 3. Extending the synthetic aperture. In the left column the ~~ azimuth. The multi-looking accounts for the 
motion compensation is performed using the motion data only, in the reduction in speckle in Figure 3. 
right column an extra phase estimation is performed using PGA. 
  
2.2 Range resolution 
From the previous sub-section it could be concluded that also an improvement in the range resolution is needed. The 
normal bandwidth of PHARUS is 45 MHz which allows for a range resolution of 3 m. The data acquisition chain has 
been designed for this bandwidth. However, the radar antenna hardware allows for the use of 100 MHz bandwidth. By 
transmitting two different chirps of 45 MHz, spanning a 90 MHz band, the effective bandwidth can be doubled without 
any changes to the radar or data-acquisition hardware. This doubling leads to an increase of the range resolution bya 
factor two, from 3 m in slant range to 1,5 m. 
This approach was tested using two transponders. In Figure 4 the response of one transponder in the range direction is 
shown for both the original 45 MHz chirp and the response for the total 90-MHz chirp. The increase in the range 
resolution is clearly visible. 
  
342 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B1. Amsterdam 2000. 
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