| Sea clutter analysis under complex sea conditions is of great significance in microwave remote sensing field such as radar early warning,targets detection and ocean parameter inversion.In this paper,we mainly focused on the scattering characteristics of complex sea surfaces with internal waves,breaking waves and foam layers.The composite scattering between the sea surface and the target above it is also investigated.A composite scattering model of complex nonlinear sea surface is established,and the time-varying Doppler characteristics and SAR imaging are simulated.It provides a theoretical basis for sea surface detection,target recognition and radar performance improvement in complex sea conditions.The main contents and innovations are as follows:1.The Kd V model is used to simulate the geometric profiles and propagation characteristics of one-soliton oceanic internal waves.The Variable-Coefficient Gardner model is used to analyze the geometric profiles and propagation characteristics of one-soliton and two-soliton internal waves with the consideration of high-order nonlinear terms.The modulation effect of internal waves on the sea spectrum is obtained via the wave number balance equation,and the scattering characteristics of deterministic sea surface with internal waves are calculated by the semi-deterministic facet two scale method.The effects of different internal wave parameters(amplitude,pycnocline depth and density difference)on the geometric characteristics and scattering characteristics of internal wave are analyzed.Furthermore,both overtaking-type and head-on-type internal waves are presented and the effect of the perturbed term on internal wave profiles and the scattering characteristics of both one-and two-soliton cases are discussed.2.“Sea spike” phenomenon at low grazing angle has drawn much attention in recent years.The local scattering characteristic of a single breaking wave is analyzed,and the influence of multipath effect and Brewster angle effect for VV polarization are discussed.Combined with the crest and static whitecap coverage,a geometric model of nonlinear sea surface with breaking waves and foam layers is established by a wedge-like model covered with random distributed bubble particles.Based on the ray theory,the multipath scattering of breaking waves and its surrounding sea surface are analyzed.The wedge diffraction is also considered by the impedance wedge equivalent edge currents method.The influence of the foam layer is calculated by the vector radiative transfer theory.The backscattering coefficient and its spatial distribution characteristics of the sea surface with breaking waves and foam are obtained by the semi deterministic facet two scale method.The scattering coefficient of sea surface with and without breaking waves and foam layers are compared,and the variations of the scattering coefficient with the wind speed are presented.The inflence of breaking waves and wake bubbles on surface-ship wake scatterin at low grazing angles are also investigated.3.Time-varying sea surface scattering field model is established by capillary wave modification facet scattering model.The Doppler spectrum corresponding to the complex sequence is obtained by spectral estimation method,and its statistical characteristics are obtained by averaging multi-sample.The Doppler spectrum of linear sea surface and nonlinear sea surface are compared.On the other hand,the Doppler spectra of sea surface echoes are determined directly based on the NRCS and its variance.This Doppler spectra model has a high computational efficiency compared with the former method.4.The sea surface electromagnetic scattering model and the velocity bunching effect are combined to simulate SAR image of the time-varying nonlinear sea surface.In addition,the SAR imaging of the sea surface disturbed by the internal wave and the SAR imaging of the sea surface with the breaking waves and foam layers are simulated.In addition,the interferometric SAR images are obtained and its application in the detection of sea surface flow field is verified.5.The scattering characteristics of electrically large conducting objects are analyzed and the scaling theory is verified.A numerical matching method is introduced to calculate the diffraction coefficient of the impedance wedge,and the impedance wedg equivalent edge current is derived.The diffraction field of the coated object with a wedge structure is investigated.The total scattering field of the electrically large coated dielectric object is obtained by adding the physical optics field.Using the modified four-path method and the equivalent edge electromagnetic current method,the composite scattering problems of two-dimensional sea surface and simple target,sea-sweeping missile and ship are discussed respectively. |
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