## 4 Results  
### Results with Quad-Polarimetric Data  
Figure 2 shows the results obtained with the quad-pol ALOS-1 dataset. Figure 2a,b represent the intensity of channels HH and HV, respectively, which are commonly employed as conventional detection channels. A lot of ships are clearly visible throughout the scene. As expected, the HV channel exhibits higher contrast between vessels and their surroundings compared to HH [25, 33, 34]. However, the response of some ships in HV is weaker than in HH, which might make their detection difficult. A drawback of the HH polarization is its higher signal level from the sea areas surrounding the vessels, which reduces the contrast (i.e., the signal-to-clutter ratio) between the vessels and the sea, thus limiting the application of automatic detection techniques to that channel. By manually selecting one the sea pixels as reference, we obtain the optimum image shown in Figure 2c. It is clearly seen that sea clutter level is drastically reduced and, simultaneously, signal level coming from the ships is increased. As an additional remark, note that the contrast between the urban areas and vegetated areas around the Bay of Tokyo is also improved. Figure 1: RGB false colour images formed with the backscattering coefficients of each available polarimetric channel. **(a,b)** ALOS-1 and Radarsat-2 quad-polarimetric images with R = HH, G = HV, B = V. (**c,d**) Sentinel-1 and PAZ dual-pol image with R = VV, G = VH, B = VV/VH. (**e**) PAZ dual-pol image with R = HH, G = HV, B = HH/HV. (**f**) PAZ dual-pol image with R = HH, G = VV, B = HH/VV. The improvement can also be visualized in Figure 3, which shows a range transect passing through the ship depicted in the magnified region at the top of Figure 2. On the one hand, by comparing the transects for the HH and optimum channels (blue and red plots), we observe that in the optimum channel, the ship intensity is approximately 3.5 dB higher, while (simultaneously) the sea clutter intensity is notably lower, dropping from around \(-8\) dB to \(-40\) dB. On the other hand, a similar effect is observed when comparing the transects corresponding to the optimum and HV channels (red and green plots, respectively). In this case, the ship intensity is about 12 dB higher, while the sea clutter intensity is observed to be 28 dB lower. The values of the signal-to-clutter ratio between the same ship and its surrounding background clutter are detailed in Table 2. The optimum channel exhibits the largest ratio, which indeed benefits the application of any vessel detection algorithm compared to the use of HH or HV. In the specific case of using a CFAR detector, the higher the signal-to-clutter ratio (equivalently, the higher the contrast) the more accurate detection of vessels with any value of the probability of a false alarm. The second example corresponds to the quad-polarimetric image of Radarsat-2 acquired over Barcelona. The results are shown in Figure 4. As in the previous example, the steep incidence angle of the acquisition causes a strong response of the sea surface, particularly evident in the HH polarization, as depicted in Figure 3(a). This results in a reduced signal-to-clutter ratio (contrast) between ships and their surrounding background, thereby limiting the effectiveness of detection techniques. HV provides again higher contrast than HH. However, upon closer inspection of the ships, their response appears to be scattered \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Polarization** & **HH** & **HV** & **Optimum** \\ \hline Signal-to-Clutter & & & \\ Ratio [dB] & 15.26 & 21.37 & 43.32 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Signal-to-clutter ratio between ship shown in upper part of Figure 2 and its surrounding sea clutter for different polarimetric channels. Figure 2: Results with L-band quad-pol image acquired over Bay of Tokyo (Japan) by ALOS-1. All figures represent image intensity in logarithmic scale ranging from \(-35\) to \(0\) dB.