# Large-Scale Automatic Vessel Monitoring Based on Dual-Polarization Sentinel-1 and AIS Data
## 3 Results
### English Channel Test Case
Since the vessel's speed cannot be extracted from SAR images at a large scale and the AIS vessel speed information is not always available in our dataset, for this study the correction of the SAR shift is not carried out. Figure 3: Illustration of the AIS-SAR cross-comparison for a Sentinel-1 IW image acquired over the English Channel area (24/10/2017): (**a**) Vessels detected by the \(C_{VV-VH}^{un-norm}\)-based detector and the CFAR applied to \(I_{VH}\), (**b**) signatures of vessels detected by only constant false alarm rate (CFAR) \(I_{VH}\) (left column) and by both \(C_{VV-VH}^{un-norm}\)-based detector and CFAR \(I_{VH}\) (right column), (**c**) vessels detected by the \(C_{VV-VH}^{un-norm}\)-based detector and AIS interpolated positions, (**d**) vessels detected by CFAR applied to \(I_{VH}\) and AIS interpolated positions. The number of AIS-SAR targets matched with a distance close to 2 km is very low, i.e., less than 5% of the total number of targets. These targets were associated with vessels for which the AIS messages cannot be accurately interpolated at the SAR acquisition time, e.g., a sparse number of AIS messages are available only before or after the SAR acquisition time. Figures 3c,d present the qualitative cross-comparison results of the \(C_{VV-VH}^{un-norm}\)-based detections and AIS positions and of the CFAR \(I_{VH}\) detected vessels and AIS positions, respectively. We can notice that overall there is a high agreement between the AIS and SAR positions for both SAR detection methodologies. For the selected SAR frame, at total number of 140 AIS interpolated positions are available. After applying the AIS-SAR matching procedure, it comes out that 112 AIS positions have been matched with the \(C_{VV-VH}^{un-norm}\)-based detections while for the CFAR \(I_{VH}\) 123 AIS positions were matched. The latter correspond to small size vessels for which the \(C_{VV-VH}^{un-norm}\) has low values, presumably due to the fact that the VV vessel signature was mixed with the sea clutter. These first results indicate that the detection performances were similar for the two SAR methods except for small size vessel, for which the CFAR \(I_{VH}\) performed better. However, if we compare the number of SAR over-detections for both methodologies we observe that the number of detected targets for \(C_{VV-VH}^{un-norm}\) (16) was lower than for the CFAR \(I_{VH}\) (37). A visual analysis of these targets shows that the \(C_{VV-VH}^{un-norm}\) over-detections are also detected by CFAR \(I_{VH}\) and the majority of their signatures are similar to vessels of different sizes that were validated by the AIS. The over-detections corresponding only to CFAR \(I_{VH}\) were small size objects of 1-2 pixels, with very low backscattering values, similar to those of the sea clutter. We argue that these correspond to false alarms rather than to dark vessels. The visual analysis confirms that at least half of the additional CFAR \(I_{VH}\) detections corresponded to false alarms due to the clutter noise and land ambiguities generated by land structures located near the coastline. A similar analysis is carried out for two more images of the English Channel dataset and two additional images from the Mexico dataset. The results presented similarities with the ones obtained from the analysis of the image presented in Figure 3, thereby allowing to conclude with some certainty that the \(C_{VV-VH}^{un-norm}\) helps in reducing the number of SAR over-detections. Thus, even if, on the one hand, the coherence seemed to have some difficulty in detecting small vessels, with a size close to the spatial resolution of the SAR sensors, on the other hand, it seems to be more robust with respect to false alarms. From this analysis, we could argue that for images with a higher spatial resolution, the coherence-based detectors could perform the best. Moreover, although the CFAR \(I_{VH}\) detected small vessels better than \(C_{VV-VH}^{un-norm}\), its reliability was low given the comparatively high amount of false alarms.