Research On Electromagnetic Scattering Characteristics Of Nonlinear Sea Surface

The electromagnetic(EM)scattering calculation of nonlinear sea surface and the analysis of echo characteristics have always been difficult and hot issues in the field of sea surface EM scattering.Presently,numerical methods or approximate methods are often used in the EM scattering of sea surface calculation.Generally,Numerical methods have high precision,however,there is a general problem of low computational efficiency when the sea surface has electrically large size.Compared with numerical methods,approximate methods have higher computational efficiency,but the higher the incidence angle is,the worse the calculation accuracy is.On the other hand,the geometric modeling of the electrically large sea surface is the premise and the key point of the research on the EM scattering characteristics.While the linear sea surface is only rough simulation of a simple swell or wind wave.How to add the nonlinear effect of wind and waves,and the interaction between waves to realize the geometric modeling of sea surface is a critical foundation.As the wind speeds up,sea surface with foam or breaking waves is a typical nonlinear sea surface that can reflect the interaction between waves.Yet whitecaps are divided into the crest foam(Stage A)and the static foam(Stage B)two stages.And different foam stages have different foam thickness and coverage.How to adopt effective methods to determine the coverage ratio of stage A and stage B in the same sea state,and establish the EM scattering model in this case plays an important role in the comprehensive and correct analysis of the EM scattering of foam covered sea surface.As the ocean waves spread from the deep-water region to the shallow-water region,the interaction between the tidal current and the underwater bottom topography will lead to different changes in the roughness of the sea surface in the spatial position.How to establish the geometric model of shallow sea surface and the modulation mechanism of the interaction between tidal current and underwater bottom topography is the basis of analyzing the EM scattering characteristics of the sea surface for shallow water.In this dissertation,the EM scattering and echo characteristics of nonlinear sea surface are systematically studied in order to quickly and accurately calculate the EM scattering requirements of nonlinear sea surface.The application range of the approximate methods is analyzed,a fast and accurate EM scattering model for the electrically large sea surface is established.In addition,the EM scattering characteristics of foam covered sea surface under high sea state are also analyzed.The modulation mechanism of the sea surface for shallow water by the interaction of underwater bottom topography and tidal current is studied.The main research work of this dissertation is carried out from the following aspects:1.The numerical methods have high precision but low efficiency in the calculation of EM scattering for electrically large sea surface.The approximate method TSM(Two Scale Model)hybrid model which is commonly used combines KA(Kirchhoff Approximation)and SPM(Small Perturbation Method),while different cutoff wavenumber has a great influence on the results.Based on the premise of ensuring the calculation accuracy,the IEM(Integral Equation Method)is used to replace SPM of TSM.And the calculation method of adaptive cutoff wavenumber is proposed.Compared with the traditional TSM,the new model has higher calculation accuracy.2.Based on the idea of the facet-based EM scattering,a facet-based TSM model for the electrically large sea surface is established.The IEM is used to calculate the Bragg scattering for the elementary radar returns from each facet.KA is still kept in the model to compute specular scattering from each facet.As wind speeds up,the facetbased EM scattering model is adopted to calculate the small facet which has no foam covered of sea surface.The multi-layer medium EM scattering model is used to calculate the small facet which has foam-covered of sea surface.Finally,the scattering fields of all facets are superimposed to obtain the total scattering field of the sea surface.And the percentage proportion of the adaptive stage A and stage B is proposed.The EM scattering model of the sea surface covered with stage A and stage B under different sea states is eatablished.3.When the sea waves travel from the deep sea to the shallow sea,the geometric model of the shallow sea is different from that of the deep sea.The method of sea spectrum combined with regular waves is proposed to generate shallow sea surface affected by underwater bottom topography.It reflects not only the random characteristic of sea waves,but also reflects the refraction effect of the shallow sea.Based on the mechanism of Bragg scattering,the interaction between tidal current and underwater bottom topography in the shallow sea area is analyzed.The variation trends with spatial position for the modulation effect of the sea surface by underwater bottom topography under different parameters are realized.The modulation effect of underwater bottom topography on sea surface is predicted accurately.4.Based on the measured sea echo data,the Doppler spectrum and distribution characteristics of sea clutter in different sea states are analyzed.Meanwhile,based on the facet-based TSM model proposed in this dissertation,the imaging analysis of different scenes including sea surface with different wind fetch,submarine internal waves and so on are simulated.