Optimal Polarization of Passive Millimeter-Wave Imaging for Sea Scene Observation

Passive millimeter-wave (PMMW) imaging is one of the important approaches for remote sensing and target detection in the ocean background. In general, seawater always fluctuates under the influence of factors such as wind, earthquakes, tidal forces, and ship movements. The undulating waves produce many bright or dark spots of brightness temperature (TB). This complex background may increase the difficulty of object detection for observers and postprocessing algorithms. In this article, we comprehensively assess the polarization properties of PMMW imaging for sea scene observation, both theoretically and experimentally. According to the millimeter-wave radiation propagation model, the TB difference characteristics between ship and seawater under various conditions are calculated and analyzed. Based on the simulated and measured experimental data, the polarization TB and the multipolarization parameter images are qualitatively and quantitatively discussed using performance metrics such as statistical histograms, differential signal-to-noise ratio (DSNR), receiver operating characteristic (ROC) curves, and survey scoring. The analysis results show that vertical polarization is usually the optimal polarization observation mode under most conditions. For the long observation distance or very small angle of incidence, different polarization responses might be comparable. In addition, if the system can obtain multipolarization images at the same time, polarization fusion can bring more advantages for target enhancement, detection, and recognition.