Improvement And Application Of Small Slope Approximation Method In Rough Land/Sea Surface Scattering

Research on electromagnetic scattering or three-dimensional sea surface is of great significance in both civilian and military applications.Under the action of the wind,a large number of foam layers will be generated above the sea surface,which will interfere with the detection of targets at sea.The traditional Small Slope Approximation method(SSA)is difficult to directly solve the foam layer-sea surface composite model and needs to be corrected accordingly.The desert terrain is a special rough surface with sand dunes above it.Therefore,it does not satisfy the applicable conditions of the traditional SSA,and the SSA needs to be corrected accordingly.This thesis focuses on the electromagnetic scattering of the foam layer-sea surface composite model and the barchan dune model in desert areas.The research has important application values in remote sensing,early warning of complex environments,and detection and identification of foam layers above the sea surface.The main works of the thesis are as follows:1.Based on the theoretical value and experimental value are not consistent of the electromagnetic scattering of the foam-containing sea surface,the traditional first-order SSA method was improved.The equivalent dielectric constant and vector radiative transfer theory are applied to the study of electromagnetic scattering from foam-containing sea surface.In the case of small incident angle(less than 40°),the scattering coefficient of the sea surface will be influenced by the mirror effect of the foam layer.The theory model of the equivalent dielectric can be used to modify the calculation results of the simple sea surface.When the incident angle is large(large than 60°),the scattering coefficient of the sea surface will be influenced by the non-Bragg scattering effect of foam layer.Therefore,the theory model of the vector radiation transmission is used to further correct the equivalent constant model.The results show that the corrected theoretical values are in good agreement with the relevant experimental values,and the error is within the allowable range,which indicates the practicability and effectiveness of the two modified models.2.The effect of the foam layer on the Doppler shift and spread characteristics of the sea echo is studied.Based on the two modified models,the difference between the Doppler spectrum of the foam-containing sea surface and the Doppler spectrum of the pure sea surface under the conditions of different incident wave frequency,incident angle and wind speed are discussed respectively.The results show that the Doppler spectrum of foam-containing sea surface will produce a new peak on the basis of the Doppler spectrum of the pure sea surface.With the frequency of incident waves increases or the wind speed increases,the peak will increase continuously and may eventually replace the original,which means the frequency shift of Doppler spectrum has occurred.3.Based on the wind-blown theory,a barchan dune model is established for desert terrain and the dune model is divided into 5 regions.Using transformation matrix to improve the traditional first-order SSA,and a Multi-zone Small Slope Approximation is proposed.The accuracy of the method is verified by comparing the results with the traditional SSA.This method is used to analyzed the scattering characteristics of barchan dune and calculate the backscattering and bistatic RCS of barchan dune with different wind speeds,wind directions,electromagnetic incident angles and frequencies.The calculation results well compensate for the limitations of the traditional SSA which is not suitable for solving the electromagnetic scattering of the dune model.