Radar imaging of Kelvin arms of ship wakes

The wave pattern generated by a moving ship is formed by two dominant featu res: the turbulent wake and a 'V'-shaped pattern trailing the ship, consist ing of the two Kelvin arms. In this paper we investigate the radar imaging mechanism of Kelvin arms, which are formed by the cusp waves. A composite s urface model for the radar backscattering at the ocean surface is used. The radar signatures of Kelvin arms can be attributed to tilt and hydrodynamic modulation of Bragg waves by the cusp waves. The proposed model allows the computation of the normalized radar backscattering cross-section (NRCS) as a function of radar frequency, polarization, incidence angle, wind speed a nd direction, and wavelength, direction, and slope of the cusp waves. By using this imaging model, radar signatures of cusp waves are calculated for several spaceborne Synthetic Aperture Radars (SARs): (1) the SEASAT L-b and HH-polarized SAR, (2) the ERS-1/-2 VV-polarized SAR, (3) the RADARSAT C -band HH-polarized SAR, and (4) the X-, C- and L-band multipolarization SAR s of the Space Radar Laboratory flown on the space shuttle during the SIR-C /X-SAR mission in 1994. The results of the simulations are compared with SE ASAT and SIR-C/X-SAR imagery of ship wake patterns. It is shown that the de pendence of the observed radar signatures of Kelvin arms on radar look dire ction is consistent with the proposed imaging theory and that the measured relative mean NRCS values induced by Kelvin arms can be fairly well reprodu ced by the proposed model. The simulations indicate that ship wake signatur es should be more clearly visible on SEASAT L-band SAR than on ERS-1/-2 or RADARSAT C-band SAR images. The radar signatures of Kelvin arms are stronge st at low wind speeds and are not very sensitive to wind direction.