# A Systematic Review of Ship Wake Detection Methods in Satellite Imagery
## 3 Results
### Overview of Ship Wake Characteristics
Although less common than turbulent wake, Kelvin wakes are imaged in higher-resolution SAR imagery. It is often the case that only one Kelvin arm is visible, or even only its cusp waves. This is often due to the relative geometry between the SAR and the wake, as previously stated. A narrow-V can sometimes be seen in SAR imagery, usually at low wind speeds (less than 3 m/s). Finally, internal wave wakes are only observed in regions of shallow water stratification, under moderate wind conditions (3 to 10 m/s). The internal waves generated by a moving ship mostly occur in coastal waters where water stratification is strong because of the mixing of freshwater and seawater [16]. They are more apparent in L-band imagery than they are in X-band or C-band imagery, and they are significantly more distinct if the SAR sensor is perpendicular to the ship track. Another characteristic of wakes in SAR images is the azimuth offset of the wake and the hull [3,5]. This effect is proportional to the Doppler shift effect of the back-scatteredsignal in SAR, and, thus, can be re-conducted to the slant range component of the speed of the ship. Some studies have exploited this effect to obtain an approximate evaluation of ship velocity, proving an interesting capability. [PERSON] et al. studied the detectability of wake components using different SAR sensors through quantitative and qualitative analyses [17], as well as the non-linear dependencies of oceanographic characteristics and the said detectability [18]. They found that near-hull turbulence and turbulent wakes are not significantly influenced by C-Band or X-Band frequencies or by slant range variations. However, Kelvin wakes are more detectable at shorter slant ranges, due to the scattering effects of tilt modulation, hydrodynamic modulation, and velocity bunching. This effect diminishes with increasing slant ranges, making Kelvin wakes less detectable. Narrow-V wakes, primarily caused by Bragg scattering, are better detected by X-Band sensors than C-Band sensors, as X-Band's shorter Bragg wavelengths align more effectively with the wakes. The study also confirmed that ship wakes are generally more detectable in TerraSAR-X imagery compared to RADARSAT-2 and Sentinel-1, likely due to the prevalence of Bragg-based wake components. In optical imagery, wake appears very differently. Because of an easier interpretability, there are less studies on the ways ship wakes appear in optical images. In 2018, [PERSON] and [PERSON] [16] provided basic information for ship detection and identification by observing and summarizing the features of different types of ships and their wakes in optical images. Their focus was mainly on turbulent wakes and Kelvin wakes in Gaofen GF-1 satellite images. The turbulent region appears as a bright trail in optical images, since bubbles significantly increase the reflectivity, almost equally in each band. This is especially true for the smooth regions of turbulent wakes, and for parts of Kelvin wakes. [PERSON] et al. [19], while proposing a hull detection method with cascaded wake detection, conducted a statistical analysis of a large amount of ships with varying Froude numbers. They found that fishing vessels with short hulls often only generate turbulent wakes visible in satellite imagery, while motorboats (with very short hulls) also produce two visible bright narrow V-shape Kelvin arms. Bigger vessels, like cargo ships and warships, can produce visible Kelvin wakes, and, depending on their speed, they may also generate striped wakes and internal waves. Kelvin waves are of particular interest, as they contain a plethora of valuable information on ships. For example, the propagation of a transverse wave, if one is generated, can be related to the speed of the ship that generated it (the propagation velocity of the wave's center is the same as the speed of the ship). Furthermore, the reflectance of Kelvin waves is deeply influenced by the draft of the ship, as well as its speed, width, and length [13].