# Mass Processing of Sentinel-1 Images for
## 2 Study Definition and Datasets
### Sentinel-1 Data  
During the campaign period, Sentinel-1 primarily used the IW acquisition mode in the AoI, in accordance with its observation and planning strategy [20]. The EW and SM modes were occasionally used in the first months after the start of the operational phase, but they are no longer used routinely in the AoI. Consequently, and to facilitate the interpretation of the results, only IW acquisitions have been used in this study. Furthermore, only images that contain at least 10% of their extent inside the AoI have been analysed. This is to prevent images that will only marginally contribute to the results of the study from putting an unnecessary burden on the analysis platform. On the other hand, images that stretch outside the Med (into the Atlantic Ocean beyond the Strait of Gibraltar, into the Black Sea beyond the Bosphorus and into the Red Sea beyond the Suez Canal) are kept in the analysis and contribute to the final statistics. A total of 97 Sentinel-1B images were acquired over the AoI between 26 September 2016 (start of Sentinel-1B operations) and 30 September 2016 (end of this study). Although the A and B satellites are identical and all the components of the processing chain have successfully been tested with Sentinel-1B products, for better guaranteed uniformity it has been decided to exclude those 97 images from the study since those images will not have a sizable impact on the results reported in this paper. Table 1 presents the main details of the Sentinel-1 images used in this study. The overall number of Sentinel-1 IW images used in the study is 11,647. All of them are GRDH products, with a resolution of around 20 m. As can be seen in the table, the vast majority (11,421) of them are dual-pol VV + VH. Figure 2 plots the number of observations for each geographic location in the area (the maximum number of observations of a single location is 225). The diamond patterns are due to the overlap of images acquired in ascending and descending orbits and in neighbouring orbit tracks. A number of these 11,647 images are affected by radiofrequency interference (RFI). In most cases this interference originates from ground sources, and more rarely from the Radarsat-2 SAR satellite, which operates in the same frequency band as Sentinel-1 [23,24]. Figure 3 shows typical examples of how this interference is manifested in Sentinel-1 images. The visible bright stripe artefacts can lead to a large number of false ship detections in very narrow sea areas, which can be misinterpreted as unusual shipping activity. To mitigate this problem, some of these images have been identified and  
Figure 1: Study area (Mediterranean Sea). labelled as RFI-affected. No automated identification algorithm has been implemented and, instead, the process relies on a manual intervention explained next. Due to the very large number of images in this study, the identification cannot be done by visual inspection of each image. Instead, once the ship detection analysis has been applied to all the images, the detections have been aggregated and displayed on a map. Visual inspection of this detection map has shown several conspicuous long and narrow areas with high density of detections. The images these detections originate from have then been visually examined and, if the presence of RFI is confirmed, the images are labelled as RFI-affected. The ship detection analyses of these images are not included in the results presented in this paper. It is clear that the images labelled as RFI-affected contain large RFI-free areas with genuine ships, and that these genuine detections will be missed in the results. However, in this paper it is preferable to miss those targets than to include large amounts of false alarms in spurious linear features that could lead to misinterpretation of the detection maps, with the additional constraint that human assistance in the detection process is to be kept at a minimum. A total of 74 images have been labelled as RFI-affected.