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Rail SAR data was analyzed to determine the statistical correlation of background clutter between different UHF frequencies and between passes taken on different days. The purpose of this study was to determine the potential gain from change detection algorithms and multifrequency clutter cancellation algorithms using a high-quality near-ideal SAR data set. The polarimetric radar data used for this study spans a frequency range from 300 to 1200 MHz and had been processed into separate channels with 400 MHz bandwidth. The data is very low noise because of time integration to reduce radio frequency interference (RFI). In this paper, the rail SAR correlation coefficients calculated on data taken on two separate days at the same center frequencies and bandwidths. Secondly, the data was used to explore frequency-to-frequency correlations with correlations calculated for all possible frequency combinations of data taken simultaneously. Both overlapping and nonoverlapping frequency bands were processed. Finally, subtraction studies were performed whereby data from one pass is subtracted from a second pass at the same frequency and data between two frequencies were subtracted using weighted differencing processing. Generally, pass-to-pass correlations on successive days are highest at low frequencies. Correlations approaching .96 between two passes for the lowest frequency band were achieved. These high correlations mean that change detection algorithms can be used and that differencing will result in processing gain. Registration problems will exist with airborne acquired data, that are simplified with rail SAR data. The individual HH and VV polarizations are more highly correlated than the cross polarizations, probably because of higher signal levels relative to noise. Significant clutter reduction (over 10 db) and whitening were demonstrated on data taken in two passes two days apart. The frequency-to-frequency correlations were found to be lower than the pass-to-pass correlations with the level of correlation decreasing with increasing frequecny separation. Some minor whitening was demonstrated for differencing of registered SAR images in nonoverlapping frequencies but no overall clutter reduction was seen.
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