Sea Surface Scattering Characteristics And Ship Detection By Space-born Full-polarization SAR

During the past few years in the world, several satellites have been launched andinto operation, onboard which there are full-polarization SARs (Synthetic ApertureRadar). However, the utilization of multi-polarization SAR for oceanic applicationhas not been cared enough. One of the reasons relates to the polarization agility of thesea surface scattering mechanisms. In this study, the sea surface scatteringcharacteristics and different scattering mechanisms, including the relation withpolarization, has been analyzed and described over all. Based on these analyses adecomposition of different microwave scattering mechanisms have been performedfor sea surface, to extract scattering information including short gravity wave, windsea, swell as well as ship targets, etc.A full-polarization conceptual analytical model has been proposed for scatteringmatrix by fully polarized SAR, based on the decomposition of radar backscatteringcoefficient into polarization related Bragg and non-polarization related scalar, whichwas proposed by Quilfen et al.[52] in1999. Scattering coefficients of fourpolarization channels have been considered as the composition of polarized Braggcontribution and non-polarized scalar contribution in this model. The tilt Bragg modelhas been utilized for Bragg contribution, while the non-polarized contributionconcerning to the Rayleigh scattering from bubbles and foams introduced by seasurface wave breaking and the specular reflection by the longer waves, as well as seasurface ship target, etc.The interaction between i.i.d. scatterers has been considered within each pixelarea using the random walk model [107], with the assumption that the Braggcontribution is the vector sum of the random walk by all the Bragg scatterings, as wellas another assumption that modeling the non-polarized contribution as orientedcylinder [3]. A summation expression, as well as the expression of complexbackscattering coefficients for four polarization channels, has been derived by thefull-polarization SAR scattering matrix of each pixel, to establish the composition inthe full-polarization conceptual analytical model. A Bragg related polarization ratio, β,is determined by an iteration algorithm, hence the separation of Bragg contributionfrom the non-polarized contribution is established.Based on complex backscattering coefficients of full polarization SAR, and to beanalogous to the complex covariance matrix in the Wishart distribution [77], the eigenvalue λ is introduced into the generalized K distribution [84], replacing thebackscattering coefficient σ, to generate the pol-K distribution. Pol-K distribution has been proved to have a better approximation to the real statistical sea surfacemicrowave scattering.According to the separation of the scattering mechanism as well as the pol-Kdistribution, one eigenvalue λbrafor Bragg contribution and another λscafornon-polarized scalar contribution have been considered. The latter is used in the pol-Kdistribution for scalar, the PN(λsca), together with which the Bayesian detector candetect ship targets from the non-polarization contribution. The principle lies in thedifferent between PN(λsca)s for pixels with and without ship target, where a muchlonger tail appears on the former.