Passive Millimeter-Wave Polarization Imaging of Ship Kelvin Wake on the Sea Surface

The Kelvin wake, produced by the interaction between ships and the sea surface, manifests as alternating light and dark bands on the seawater, serving as distinctive features associated with ships and enabling the indirect detection of maritime targets. Owing to the geometric disparity between the Kelvin wake and the surrounding sea surface, there exist pronounced differences in their radiative properties. Millimeter-wave radiometers exploit these differences to detect the wake. As a valuable complement to optical, infrared, and radar imaging modalities, passive millimeter-wave (PMMW) polarization imaging of Kelvin wakes has garnered considerable interest. Nonetheless, research on millimeter-wave radiation modeling of Kelvin wakes remains relatively underdeveloped, and the intrinsic relationships among Kelvin wake characteristics, ship parameters, observation conditions, and other influencing factors have yet to be systematically elucidated. This gap constrains the broader application of this technology in detection and identification tasks. To address this, our study employs a ray-tracing framework integrated with a geometric model of the Kelvin wake and the Kirchhoff approximation (KA) method to compute the brightness temperature (TB) distribution of the Kelvin wake, thereby enabling PMMW polarization imaging. Building upon this model, the effects of various ship and observation parameters, sea surface and atmospheric conditions on the Kelvin wake TB are comprehensively analyzed, and the optimal observation parameters for PMMW imaging are identified. These findings provide a theoretical foundation for Kelvin wake detection and ship parameter inversion. Furthermore, field experiments were conducted using a shore-based PMMW imaging system operating in the W-band. The experimental results demonstrate the promising capability of PMMW polarization imaging for Kelvin wake detection, thereby validating the effectiveness of the proposed model and the associated analysis of radiative characteristics presented herein.