Optimization of the Degree of Polarization for Enhanced Ship Detection Using Polarimetric RADARSAT-2

Abstract

The scattered wave is represented in terms of two independent and rotation invariant parameters: the degree of polarization (DoP) and the total scattered intensity $R_{0} $ . The scattered wave polarization signature is introduced as a convenient graphical representation of the variations of the two scattered wave observable parameters as a function of the transmitting antenna polarization. It is shown that the signature of the DoP and the total scattered intensity $(R_{0} ) $ provide information on ocean and ship scattering that is complementary to that provided by the conventional received wave intensity polarization signature. While $R_{0} $ polarization signature of the ocean's Bragg scattering is sensitive to wind speed and synthetic aperture radar illumination angle, the DoP polarization signature appears to depend mainly on ocean surface roughness. The large dynamic range and low pedestal height of ship DoP polarization signature, in comparison with that of the ocean, favors the use of DoP for ship detection in comparison with $R_{0} $ . Optimization of the scattered wave parameters appears to be a convenient tool for efficient ship detection. The scattered wave optimization technique introduced by R. Touzi et al. in the nineties, is reconsidered and applied for enhanced ship detection. It is shown that the excursion of DoP, $\Delta p$ , and the minimum DoP, $p_{\min} $ , permits a significant improvement in ship-sea contrast in comparison with conventional (i.e., scalar) single channel polarizations (HH, VV, and HV). The additional information provided by the maximum DoP, $p_{\max} $ solves for ship ambiguities with land targets. Quantification of the ship-ocean contrast is reconsidered in the context of nonstationary ship signal. The ship peak signal quantification appears to be the most suitable method for accurate measurement of ship-ocean contrast in the presence of a nonstationary ship signal. The local $p_{\min}$ performs better than the peak of $\Delta p$ and single polarizations (HH, VV, and HV) . The added value of polarimetric RS2 information for ship detection is demonstrated using wide swath (50 km) polarimetric RADARSAT-2 data collected at 29° and 40° incidence angle over vessels (validated with Automatic Identification System data) in the Strait of Georgia, near Vancouver, Canada.