a value of 0.5 for _ will produce the best results without dramatically increasing the runtime of the algorithm. When
Swas set at 1.0 for Figure 3(b), Figure 3(d), and Figure 3(f), the images appear to have lost some data points
from the images that used a loop gain of 0.5. This result should be expected, as the loop gain serves to normalize
the dirty beam pattern that will be removed from the dirty map at each iteration of the algorithm.
CONCLUSIONS
As shown, SAR data collected at a reduced bandwidth using proper spectral estimation methods may yet be used
to form a coherent image. FFT methods for the more bandlimited case produced an image that was plagued
with sidelobes. By windowing the data, the sidelobes were reduced, but the resolution was degraded as well.
When the bandwidth was increased, the FFT algorithm produced images with narrower spectral peaks and thus
more resolved features. The application of CLEAN proved beneficial for the bandlimited case, as sidelobes
were removed and resolution was improved. In addition, lower values of the loop gain _ were shown to
better preserve the continuity of line features in the image.
Other more sophisticated methods should also be applied to the limited bandwidth scenario. APES (Amplitude
and Phase EStimation) [7] and CAPON [8] are adaptive filtering techniques that have been proven to
successfully eliminate sidelobes and to produce high resolution images. These methods would achieve
better amplitude estimates than the CLEAN and FFT algorithms used in this experiment. Furthermore,
extended adaptive filtering (EAF) techniques have recently been developed for APES and CAPON spectral
estimators for the case that the data sets are non-rectangular [9]. Although other methods exist for the missing
data case, EXAPES and EXCAPON avoid discarding further data (to achieve rectangular data sets) while
preserving good second order statistics. CLEAN is able to improve the quality of SAR images efficiently and
without computational complexity, but EXAPES and EXCAPON are the techniques best suited for this
limited bandwidth case.
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