## 4 Results
### Results with Dual-Polarimetric Data
#### 4.2.1 Enhancement Performance with X-Band Dual-Pol Data  
Figure 6 shows the results obtained with the dual-pol image of PAZ acquired in the HH and VV channels. As expected, due to the steep incidence angle of the acquisition, the sea response in both co-polar channels is very high, especially in the VV channel, which reduces the contrast between ships and their surrounding background. The rank-1 processing greatly enhances the contrast, as shown in Figure 6c, thereby improving the performance of a ship detector. From this result, we deduce that even though the quad-polarimetric space is not available, the optimization process still clearly cancels the signal from sea areas. Figure 6d shows an azimuth (i.e., vertical) transect of the ship, highlighted by the red rectangle in Figure 6a. As seen in the plots, the VV channel exhibits less contrast (around 15 dB) between the peak (ship pixels) and the lower intensity values (sea area). As expected, HH channel provides a higher signal-to-clutter ratio (around 30 dB) due to a stronger response from the ship (the peak is at 20 dB, compared to 10 dB in VV polarization), while the sea clutter values remain similar (around \(-\)10 dB). The optimum channel yields the best results, preserving the peak at a level comparable to HH polarization (around 20 dB) while greatly reducing sea clutter levels to nearly \(-\)30 dB. The second combination of polarizations is {HH, HV}, which corresponds to the two conventional detection channels. Figure 7 shows the optimization results. As in the previous image, HH intensity, which is shown in Figure 7a, exhibits less contrast, i.e., a low signal-to-clutter ratio between ships and surrounding sea areas. The cross-polar HV intensity, depicted in Figure 7b, shows a reduced signal level coming from the sea, thereby improving image contrast. However, note that just by visual inspection, the optimum channel in Figure 7c exhibits again the highest contrast. It is worth noting that, visually, the contrast in the optimum image is similar than to the one obtained in the previous combination. Figure 7d shows an azimuth transect through the ship highlighted by the red rectangle in Figure 7a. As observed, the peak of each plot, corresponding to the ship pixels, is similar across all three channels, with HH intensity being slightly higher at around 11 dB. However, the sea response is significantly lower in the cross-polar channel HV (around \(-\)20 dB) compared to HH (\(-\)5 dB) and is even further reduced in the optimum channel (around \(-\)30 dB). The last combination of polarimetric channels corresponds to VH and VV. Optimization results are shown in Figure 8. As can be seen in Figure 8a, VV intensity presents a very strong response from the sea, which importantly reduces image contrast. This is the reason why the VV channel is not normally used in vessel detection, as previously stated. The VH channel, which is equivalent to HV in monostatic SAR systems due to reciprocity, exhibits more contrast as shown in Figure 8b. However, the optimized image presents again the highest contrast, with a significantly reduced signal level coming from the sea areas, as shown in Figure 8c. Figure 6: Optimization results of the dual-polarimetric HH/VV image acquired by PAZ over the coast of Rio Grande (Brazil). Figures (**a–c**) represent the image intensity in logarithmic scale ranging from \(-35\) to \(5\) dB. Figure (**d**) represents a transect through the ship highlighted with the red rectangle in (**a**). Numerically, the improvement is quantified for the three combinations of channels by estimating the signal-to-clutter ratio between a ship and the surrounding sea area. Specifically, we use the same ships as those in the transects shown in Figures 6d, 7d, and 8d, highlighted with red rectangles in Figures 6a, 7a, and 8a. The signal-to-clutter ratios are summarized in Table 4. The optimum channels provide consistently the highest signal-to-clutter ratio, regardless of the combination of polarimetric channels.