# A Systematic Review of Ship Wake Detection Methods in Satellite Imagery
## 3 Results
### Overview of Ship Wake Characteristics
While it is widely recognized that Kelvin waves appear in remote sensing images with meter-level resolution, valuable information contained within these images is sometimes overlooked. For instance, the speed of a ship generating a transverse wave matches the propagation velocity of the wave's center. Regarding the optical sensing of ship wakes, there is a fervent interest in the use of infrared bands. This is because there is a difference, both in surface roughness and temperature, between the wake structures generated by the passage of a ship and the background. In this context, researchers speak of a \"thermal\" wake, formed by the cooling water and exhaust heat discharged by a ship, framed by a cold wake formed by the cool water in the lower part of sea that rises to the surface due to the ship propeller and the rolling vortexes generated by the hull [20]. [PERSON] et al. [21] characterized the infrared response of a ship wake through simulations using a ray tracing method with different detecting conditions; [PERSON] et al. [20] developed and tested different simulating models of infrared imaging of ship wakes based on different sources of radiance and their transmission through the atmosphere. One general output from all of these studies is that there is a strong dependence of the wake radiance on the observing geometry (i.e., the observer zenith angle). [PERSON] and [PERSON] [22] modeled the infrared and radar signatures of wakes, in particular the characteristic V-shaped Kelvin wakes, comparing them in different wind and vessel speed conditions. They concluded that the Kelvin wake is more visible at low wind speeds with a flat sea surface, and that higher vessel speeds are needed the higher the sea state becomes. They also noted that, in general, radar observations allow for a better signal than infrared for higher sea states. Another characteristic of wakes that can be exploited for detection comes from the application of the Fourier transform. When analyzing the frequency spectrum, Kelvin wake and turbulent wake have stable X-shaped and linear distribution patterns, respectively [19; 23]. An example of this pattern is shown in Figure 2. While this pattern is predictable given the geometrical nature of the structures under discussion, analysis in the frequency domain can be useful to isolate the aforementioned components of wakes from sea clutter. In particular, the shapes of these patterns do not significantly change due to varying sea states or illumination conditions, contrary to sea clutter, which exhibits a more distributed pattern in the frequency domain [10]. As is true for all spaceborne remote sensing, and more specifically for object detection, spatial resolution is a critical parameter. In this sense, ship wake detection has a clear advantage when compared to simple hull detection: the wake of moving vessels is, in general, a larger target. In fact, as previously discussed, some of the structures of a wake can extend for kilometers behind the generating ship. This characteristic is especially interesting when dealing with the detection of small ships. In optical imagery, spatial resolution has been shown to be significantly more important than spectral resolution [11], with wake detection methods performing better in high-resolution panchromatic imagery than multi-spectral imagery, with the previously discussed exception of thermal infrared bands. This has been shown to be true for SAR imagery as well [24]. When referring to maritime surveillance, there is also something to be said for revisit time requirements. [PERSON] et al. [25] analyzed the re-observation capabilities of complex satellite systems involving different radar constellations. Their study found that re-observations occur rapidly, at fractions of the orbital period, but there is an average \"blind\" interval of about \(12\) h due to the inherent limitations of the commonly selected sun-synchronous orbits.