## 4 Results
### Results with Dual-Polarimetric Data
#### 4.2.2 Enhancement Performance with C-Band Dual-Pol Data  
The quad-pol image gathered by Radarsat-2 over Barcelona (Spain) has been also exploited to evaluate the performance of all combinations of two polarimetric channels \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Processed Polarimetric** & \multicolumn{3}{c}{**[HH, VV]**} & \multicolumn{3}{c}{**[HH, HV]**} & \multicolumn{3}{c}{**[VV, VH]**} \\
**Channels** & HH & VV & OPT & HH & HV & OPT & VV & VH & OPT \\ Ratio [dB] & 31.5 & 21.2 & 42.1 & 19.7 & 24.9 & 35.4 & 15.8 & 25.4 & 34.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Signal-to-clutter ratio between a ship and its surrounding sea area after applying the optimization method with different combinations of polarizations using X-band PAZ data. Figure 8: Optimization results of the dual-polarimetric VH/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**). corresponding to dual-pol data. Unlike the previous case with PAZ data, the three combinations {HH, VV}, {HH, HV}, and {VV, VH} are available at a single date (see Table 1). To ensure a fair comparison between the different cases, we manually select the same reference sea pixel, which is also the same one we used for the quad-pol optimization of this image analysed in Section 2.1. Additionally, we select the same ship to plot the transects and calculate signal-to-clutter ratios for the three dual-pol cases and the quad-pol case. Figures 9, 10, and 11 illustrate the results for the polarimetric combinations {HH, VV}, {HH, HV}, and {VV, VH}, respectively. As shown in Figures 9c, 10c, and 11c, the optimized images exhibit enhanced contrast, with a notable reduction in sea clutter. This suppression is particularly pronounced when the cross-polar channel (VH or HV) is employed, as seen in Figures 10c and 11c. In addition, contrast enhancement is evident in the transects across the same ship, shown in Figures 9d, 10d, and 11d. In all channels (original and optimized), the peak intensity of the transects, corresponding to the ship pixels, remains around 5 dB. However, the sea level is at \(-10\) dB in the co-polar channels (HH and VV) and drops to \(-25\) dB in the cross-polar ones (HV and VH). Comparing the red plots in Figures 9d, 10d, and 11d clearly reveals that sea clutter reduction is more effective when the cross-polar channel is considered. As observed, it decreases to approximately \(-25\) dB for the HH, VV combination and further to around \(-35\) dB for the HH, HV and VV, VH combinations. In summary, the quantitative improvement with the estimation of the signal-to-clutter ratio between the ship and its surroundings for the three combinations of channels is detailed in Table 5. The optimum channel always provides the highest signal-to-clutter ratio compared to the original channels. As an additional example in the C-band, we apply the contrast optimization technique to a freely available Sentinel-1 dual-polarimetric image acquired over the Strait of Gibraltar in 2018. Polarimetric channels are VV and VH, which correspond to conventional polarizations acquired over land and coastal areas. Optimization results are shown in Figure 12.  
As can be seen by comparing Figure 12a,b, VV polarization exhibits, again and as expected, a stronger response of the sea surface compared to VH, leading to reduced contrast between vessels and their surrounding areas. It is clearly observed that contrast is improved in the intensity of the optimized channel, as shown in Figure 12c given the significant reduction in sea clutter level and preservation of signals coming from vessels. An improvement for the application of automatic detection techniques is further supported by analyzing the transect through the ship shown in Figure 12d.