# Mass Processing of Sentinel-1 Images for
## 4 Results and Discussion
### Ship Detection and Correlation Results  
Table 2 presents the overall detection and correlation results of the study. The total number of detections in the 11,573 (excl. RFI) images was 667,746. SUMO automatically assigned the lowest reliability level to 59,420 of those. These detections are assumed to be false alarms (in most cases they are azimuth ambiguities) and are excluded from the fusion process. The number of reliable detections was thus 608,326. Most of these (599,278) were detected in dual-pol VV + VH or HH + HV products.  
Figure 6: Daily number of Sentinel-1A acquisitions used in this study and area covered. The gaps indicate periods of satellite unavailability. Average image area: 250 \(\times\) 170 km\({}^{2}\).  
Figure 7: JEODPP cluster CPU workload (472 cores): (**a**) for the SUMO workflow applied to 11,647 S1 products (19 TB); and (**b**) for the Matlab script-based meta-information extraction. The blue line denotes the load of the user-triggered processes and the orange line represents the system waiting time. Horizontal axis is time in HH:MM and vertical axis is per cent.  
Of all the targets detected in these products, 98% were detected in the cross-pol channel (HV or VH), while 54% were detected in the co-pol channel (HH or VV). These results show that the contribution of the co-pol channel in this study was low, i.e., most targets detected in the co-pol were also detected in the cross-pol. This was expected given the high detection threshold used in the co-pol channels, a measure that was needed to reduce the number of false alarms, as mentioned in Section 3.1.  
A total of 818,493 interpolated reported positions were counted within the image footprints at the image acquisition times. The fusion stage returned 366,081 correlations between detections and interpolated reported positions. Therefore, 242,245 ship detections and 452,412 interpolated positions remain uncorrelated. In percentage terms, 40% of all the ship detections (with higher than lowest reliability level) remain uncorrelated. Many of these uncorrelated detections are expected to be real ships that either do not transmit AIS or are located in areas of low AIS coverage (in this case their messages will not be received even if transmitted). Many others are detections that are flagged as recurrent; since they are either ambiguities or fixed structures that often do not transmit AIS, it is natural that they are not correlated. They are further analysed in the next section. Some of the uncorrelated detections are likely to be reporting ships in areas of good AIS coverage for which the fusion process fails to associate the ship detection with the interpolated position. This is expected due to the inherently uncertain nature of the fusion, which relies on a spatio-temporal interpolation, and especially affects ships that report with a low frequency. This situation also results in an uncorrelated reported position. Finally, some of the uncorrelated detections are expected to be false alarms (e.g., sea clutter), although this number is probably low due to the high detection thresholds used in this study.  
In terms of interpolated reported positions, 55% of them are not correlated to any ship detection. Most of these are likely to be ships docked at ports (i.e., inside the buffered land mask), which are areas where SUMO makes no attempt at detection, or small reporting boats that are beyond SUMO's fully-automatic detection capabilities (with the high detection thresholds employed in this study) or even beyond Sentinel-1's imaging capabilities.  
The quoted correlated/uncorrelated fractions are averages over the entire AoI. However, the AIS coverage is not uniform across the AoI. In the regions of good AIS coverage, which are the ones closer to the European coast, the correlated fraction will be higher, and vice versa.