Based on the scattering mechanisms of multipath, a bistatic synthetic aperture radar (Bis-SAR) image intensity model for the composite ship-ocean (or other metallic targets with ocean) scene is developed in this paper through reasonably dealing with the path lengths of transmitter and receiver to the ship target. To get a Bis-SAR image intensity model for the composite ship-ocean scene, the Bis-SAR image intensity distribution for the ocean surface is analyzed, and the facet scattering model is used to give the individual returns from separate facets. In addition, the influence of velocity bunching (VB) modulation of long ocean waves on the distribution of radar cross section is also discussed. Finally, Bis-SAR imagery simulations of ocean wave and the composite ship-ocean scene are illustrated, and the results show that the proposed Bis-SAR image intensity model can be reasonable and the interaction effects between the ship and sea surface can make significant contributions to the Bis-SAR intensity.

Scale model measurement provides an alternative approach for predicting the RCS of electrically large targets. The basic principles of scale measurements are that the scale conditions must be satisfied between the two electromagnetic systems: the scale model and the full size prototype. This paper makes an attempt to change the salinity and temperature of model and deduces scale conditions for permittivity of sea water based on the single Debye model. The scale conditions for geometry of a rough sea surface and the simulation of electrically large real sea surface using the scaled sea surface generated in a water basin are also presented. To verify the validity of the scale conditions, some comparisons for the permittivity and geometry of rough sea surface are implemented between model and prototype. The great agreements on the normalized radar cross-section calculations, clutter time series, and Doppler spectrum also confirm the presented scale conditions.

A full-polarized facet based scattering model (FPFSM) for investigating the electromagnetic (EM) scattering by two-dimensional electrically large sea surfaces with high efficiency at high microwave bands is proposed. For this method, the scattering field over a large sea facet in a diffuse scattering region is numerically deduced according to the Bragg scattering mechanism. In regard to near specular directions, a novel approach is proposed to calculate the scattered field from a sea surface based on the second order small slope approximation (SSA-II), which saves computer memory considerably and is able to analyze the EM scattering by electrically large sea surfaces. The feasibility of this method in evaluating the radar returns from the sea surface is proved by comparing the normalized radar cross sections (NRCS) and the Doppler spectrum with the SSA-II. Then NRCS results in monostatic and bistatic configurations under different polarization states, scattering angles and wind speeds are analyzed as well as the Doppler spectrum at Ka-band. Numerical results show that the FPFSM is a reliable and efficient method to analyze the full-polarized scattering characteristics from electrically large sea surface within a wide frequency range.

A hybrid model based on the geometrical optics and physical optics (GO-PO) method and facet-based asymptotical model (FBAM) for the composite EM scattering from sea surface with ship has been previously developed. But, the GO-PO method faces a difficult problem that the lower efficiency in testing the visibility of patches to the incident and reflected waves. Hence, to improve the computation efficiency of the GO-PO method, a kd-tree accelerated hybrid model is presented in this paper. The kd-tree method firstly carries out the intersection test between the ray and the bounding box where the patch is located before the operation between the ray and the patch. If the distance parameters of the bounding box do not meet the requirement, the ray is not necessary to enter the bounding box, which can reduce the needless intersection tests. Numerical results show that the proposed kd-tree accelerated hybrid model could improve the efficiency about three times and could be applied to the study of composite EM scattering characteristics of electrically large sea surface with a ship target.

This paper deals with the effects of horizontal skewness on Doppler spectra. A.K. Fung et al. pointed out that the crests of the sea waves are tilted towards the wind direction, which is referred to as horizontal skewness. Although the nonlinear sea surface is more realistic than the linear one, it cannot embody the statistical front-back asymmetry of real sea waves due to the influence of the wind. Thus, the impact of horizontal skewness must be considered. The first-order modified Lagrange model for asymmetric ocean waves is combined with Monte Carlo method to produce the two-dimensional (2-D) time-varying skewed sea surfaces which are nonlinear and skewed simultaneously. Moreover, the second-order small-slope approximation method is used to calculate the scattering field and study the Doppler spectral characteristics from skewed sea surfaces. Horizontal skewness makes Doppler spectra broader in upwind case, but narrower in downwind case. In addition, the impact of horizontal skewness on Doppler spectra is dependent on wind direction and incident elevation angle.

In general, it is more practical to detect ship wake under the background of a complicated sea state than the ship directly. Thus, in this paper, the Doppler spectra of time-varying Kelvin wake on a time-varying sea surface are numerically investigated by considering the change of ship wake with time in ocean environments. For this purpose, the linear superposition model of a time-varying sea surface and a time-varying Kelvin wake is established. Combined with the facet scattering field model of sea surface and Kirchhoff approximation (KA), the Doppler of the radar scattering echo signal of the time-varying wake on the sea surface is simulated and analyzed under different polarizations, incident angles, ship speeds, and wind speeds, as well as wind directions. It can be observed that the Doppler spectrum changes as the conditions change. This study provides a reference for extracting motion features of ship wakes in a sea clutter environment.