# Towards Automatic Recognition of Wakes Generated by Dark Vessels in Sentinel-1 Images
## 3 Results  
### Dataset  
In this study, VV-polarized Interferometric Wide (IW) swath mode Sentinel-1 C-band images are obtained by Copernicus Open Access Hub. The images correspond to Level-1 Ground Range Detected (GRD) products [34]. All the images are acquired in ascending phase over the Gulf of Genoa, Italy, with a ground range and an azimuth spacing of 10 m. Table 1 lists the image IDs along with wind conditions, derived from [35; 36; 37] as in situ measurements at Genoa and at La Spezia stations, respectively. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Date** & **6 July 2020** \\ \hline Image ID & SIA\_IW\_GRDH\_1 SDV\_20200706T171436\_20200706T171501\_033337\_03 DCC6\_1122 \\ Image ID & SIA\_IW\_GRDH\_1 SDV\_20200706T171411\_20200706T171436\_033337\_03 DCC6\_3302 \\ Wind@Genoa & Speed: 2 m/s & Direction: 204\({}^{\circ}\)N \\ Wind@LaSpezia & Speed: 5 m/s & Direction: 134\({}^{\circ}\)N \\ \hline
**Date** & **18 July 2020** \\ \hline Image ID & SIA\_IW\_GRDH\_1 SDV\_20200718T171436\_20200718T171501\_033512\_03E220\_DEFC \\ Image ID & SIA\_IW\_GRDH\_1 SDV\_20200718T171411\_20200718T171436\_033512\_03E220\_1765 \\ Wind@Genoa & Speed: 2.2 m/s & Direction: 259\({}^{\circ}\)N \\ Wind@LaSpezia & Speed: 2.9 m/s & Direction: 149\({}^{\circ}\)N \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dataset details. The satellite images are completed by the maritime traffic data, provided by the Automatic Identification System (AIS) and used as ground truth. The data are acquired by the AIS ground station of La Spezia-Castellana (Station #890) at latitude and longitude of 44.07\({}^{\circ}\)N and 9.82\({}^{\circ}\)E, respectively, and from an altitude of 200 m. The AIS data were stored with a time step of 1 s between successive messages of the same ship, so they are particularly suited for SAR matching in a coastal area. The AIS data are exploited to select ships with characteristics of length-to-beam ratio (LBR) and velocity similar to GoFast class. In detail, hereinafter, all the ships with LBR larger than 3.75 [27] and velocity larger than 15 knots are classified as GoFast-like ships. Three different test scenarios are processed by the implementation in MATLAB\({}^{\circledR}\) environment of the proposed method. The scenarios can be described as follows:  
* Scenario 1: wakes generated by GoFast-like ships in a homogeneous wind speed area. Figure 2a is a tile of the image acquired on 6 July 2020. Four ships are included in the tile: ship 247056100 has a velocity of 22.6 knots and LBR of 7.5, so it is classified as GoFast-like ship, whereas the other three ships moves slower than 15 knots (ship 636014645 has a velocity of 13.1 knots and LBR of 3.5, ship 232003476 and ship 2473622320 have LBR of 3.5 and velocity of about 7 knots). Figure 2b shows the wind field, whose speed values over the tile range from 1.6 to 4.5 m/s, whereas Figure 2c portrays the location map of the scenario. * Scenario 2: wakes generated by GoFast-like ships in a sea area affected by natural surface film [38]. The tile (Figure 4a) of the image gathered on 18 July 2020 is characterized by a sea area with natural surface film and three distinguishable wakes. The leftmost ship (2470036100) has a velocity of 25 knots and LBR of 7.04, and the rightmost ship has a velocity of 18.4 knots and LBR of 5.6, so they can be both considered as GoFast-like ships. In addition, a non-collaborative ship is moving between the previous two ships. Figure 4b shows the estimation of wind field over the tile, whereas Figure 4c portrays the location map of the scenario. * Scenario 3: wakes generated by GoFast-like ships close to a low wind speed area. In Figure 3a, ship 255805983 has a velocity of 21 knots and LBR of 7.9. It moves along an almost azimuthal direction (162\({}^{\circ}\)N) close to an area with low wind speed. In details, the wind speed (Figure 3b) reduces from about 2 m/s around the ship until 0.3 m/s in the dark area of the tile.