# Large-Scale Automatic Vessel Monitoring Based on Dual-Polarization Sentinel-1 and AIS Data
## 4 Discussion  
A first analysis of the experimental results of this study has been introduced in the previous section. The SAR detection rates have been evaluated by assuming the AIS positions as ground truth. Even if, as mentioned in Section 2.3, the AIS datasets can present gaps and therefore do not cover the entire maritime traffic, a sufficient number of AIS positions is available to enable the characterization of the SAR detection probability of the AIS-SAR matched vessels. In order to complement this analysis and under the assumption that there are uncooperative vessels in our areas of interest, in the following, we investigate the SAR detections with no corresponding AIS positions, i.e., SAR over-detections, through a comparison of the coherence- and intensity- SAR-based detection results. Moreover, the SAR vessel signatures with no corresponding AIS are compared to those of the targets with an AIS match in order to be able not to consider them as false alarms. Table 2 gives the number of the targets resulting  
Figure 7: Mexico test case: AIS positions (green squares) and SAR \(C_{VV-VH}^{un-norm}\)-based detections (orange circles). The positions correspond to the cumulation of SAR detections for all the Sentinel-1 images of this dataset and their associated interpolated AIS positions. from the AIS-SAR matching procedure that are assigned to three different classes: AIS-SAR matches, AIS with no SAR detections and SAR with no AIS corresponding positions. We notice that for the English Channel case study, about 40% of the total number of the \(C_{VV-VH}^{un-norm}\)-based detections do not have corresponding AIS positions, while for the Mexico case study the percentage of unmatched SAR detections is approximately 20%. The Mexico test case is located in open sea areas where the vessel density is low and composed of large size targets that can be usually monitored reliably by AIS systems. However, the limitations of AIS systems such as low coverage of open sea areas by the terrestrial stations or message collision/saturation for SAT-AIS along with the presence of dark vessels without active AIS transponders, can explain the non-matched SAR targets for both the Mexico and English Channel cases. Figure 8 illustrates the SAR ship signatures of six targets corresponding to \(C_{VV-VH}^{un-norm}\), \(I_{VV}\) and \(I_{VH}\) images. Two targets are matched with AIS positions, the first one is a large sized cargo while the second one is a small sized sailing vessel. For all SAR images, we notice that the cargo is associated with a SAR signature comprised of several pixels with high backscattering values. In contrast, the sailing vessel is characterized by a SAR signature composed of a reduced number of pixels with lower backscattering values. The signatures corresponding to \(C_{VV-VH}^{un-norm}\) present a larger amount of pixels due to the fact that an averaging sliding window is employed to estimate the dual-polarization coherence. The vessel signatures are characterized by high values of \(C_{VV-VH}^{un-norm}\), demonstrating that metallic targets are coherent between the two polarization channels, while the values of \(C_{VV-VH}^{un-norm}\) for sea clutter, considered as a symmetric environment, tend to be low. This result is validated for both large- and small-sized SAR vessel signatures that present AIS matches. The other four SAR targets represented in Figure 8 (T3, T4, T5 and T6) do not have an AIS matching position. However, we can notice that their associated SAR signatures present important similarities with those of the targets matched with AIS positions. For instance, the signature of T4 is similar to the ones of a large size vessel. Moreover, T4 has been identified by all SAR-based detection methodologies.