Ships often become visible on SAR images because the large backscattered radar power from the metal structure of the ship gives rise to a bright spot in the radar image. Travelling ships also become visible on SAR images by their wake. Often the wake of a ship can be delineated on a SAR image, but not the ship itself.
The wake of a ship consists of the turbulent or vortex wake and the Kelvin wake. The turbulent wake trails the ship in the direction of the ship's heading while the Kelvin wake, which consists of two arms (Kelvin arms), trails the ship in the form of a V-shaped pattern. On ERS SAR images, most often the turbulent wake is visible but not the Kelvin wake. At distances larger than a few shiplengths, the turbulent wake is usually imaged as a black line. This is because the turbulence (generated by the propeller of the ship) damps the surface waves and thus causes a reduction of the backscattered radar power. However, sometimes the turbulent wake is imaged as a bright line. This can occur when the sea surface is covered by (natural) surface slicks. In this case the two counter rotating vortices within the turbulent wake (see Fig. 1) push aside the slick material. In the slick-free area and sufficiently far from the ship where the turbulence has weakened, the wind can generate short surface waves.
The Kelvin wake is formed by cusp waves which typically have wavelengths between 10 and 40 m and amplitudes between 0.2 and 1.0 m (see Fig. 2). Thus they cannot be resolved by the ERS SAR. The visibility of the Kelvin arms on SAR images depends strongly on the radar look direction (azimuth angle) relative to the ship's heading as discussed in detail in the paper by Hennings et al. (1999). The radar signatures of Kelvin arms are strongest when the look direction of the radar is normal to the direction of the crests of the cusp waves, and weakest when it is parallel to this direction. This is the reason why on ERS SAR images often only one Kelvin arm is visible while the other is only faintly or not at all visible. Furthermore, the radar signature (i.e., the change of the NRCS relative to the background) of Kelvin arms depends strongly on wind speed: The larger the wind speed, the smaller is the radar signature. But there are also other factors contributing to the magnitude of the radar signature of Kelvin arms: the size, form and speed of the ship, the radar frequency and polarization, and the incidence angle.
Fig. 1: Schematic drawing of two counter-rotating vortices generated by the propeller of a ship. They persist in the turbulent wake and give rise to convergent flow regimes at both sides of the wake. This is the reason why the radar signature of a ship's turbulent often comprises of two lines at the rim.
Fig. 2: (a) Aerial photograph showing the components of a ship wake pattern: a, bow wave; b, stern wave; c, transverse wave; d, turbulent wake, and e, turbulence region adjacent to the ship's hull. This photograph was taken by a camera with a fisheye lens aboard a low-flying aircraft (courtesy of R. Doerffer, GKSS). (b) Sketch of the different components of the ship wake pattern shown in (a).
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