Finite-element Method For Composite Electromagnetic Scattering Of Random Rough Surface And Target

The sea surface is a typical random rough surface,and the detection of low-altitude targets above the sea surface and semi-embedded sea surface targets is a difficult point in the field of radar reconnaissance.Because the real rough surface has the characteristics of ultra-large scale,scholars at home and abroad use the electromagnetic field highfrequency approximation method to conduct research,and the full-wave simulation method is rarely used because of its huge memory consumption.Large ships at sea are electrically large-scale scatterers,and their radar echoes will not be obviously covered by sea clutter.Therefore,the calculation results using the approximation method in the main scattering direction have relatively small errors and can be accepted in engineering.However,the geometric dimensions of typical targets such as low-altitude cruise missiles,amphibious combat vehicles,and semi-submerged submarines are comparable to the wavelength of ocean waves,so their radar echo signals are easily covered by sea clutter.There is a strong electromagnetic coupling relationship between this type of target and the sea surface,and the accuracy of the calculation of the strong coupling area between the target body and the sea water directly affects the imaging accuracy of the corresponding radar imaging unit.The full-wave numerical method has the incomparable high-precision characteristics of the high-frequency approximation method,and highresolution SAR imaging simulation,large incident angle clutter simulation and other fields are very sensitive to calculation accuracy,so the full-wave numerical method still has application value.In this thesis,the finite element method is used as the core to study the bistatic radar scattering of offshore targets and rough surfaces at different roughness levels and different incident angles.Firstly,the solution process of finite element method to two-dimensional and threedimensional scattering problems is studied.In order to solve the problem of low efficiency of grid edge numbering in the process of matrix preprocessing,a fastnumbering method that can be widely used is developed,which theoretically reduces the complexity of the numbering process and improves the speed of preprocessing.In order to reduce memory consumption,the generation of system matrix can be accelerated by combining sparse matrix storage method with fast numbering method.Second,considering the limited truncation accuracy and high memory consumption of absorbing boundary conditions,this thesis studies the method of mixing finite element method and integral equation boundary.In order to fully reflect the advantages of the finite element method in dealing with complex media,seawater is regarded as a lossy medium in the simulation.The hybrid method is used to solve two types of targets at low altitude and semi-embedded sea surface.Numerical results show that the method is accurate when calculating large-angle incidence.The simulation results of backscattered waves under different sea conditions are consistent with the actual situation.The method cannot accurately simulate the deficiency of large-angle illumination.Finally,this thesis deeply researches the fast finite element method from two aspects.On the one hand: Considering that high-resolution SAR imaging simulation needs to do scanning calculation in a certain frequency range.In the traditional broadband simulation,multiple discrete frequency points are used to solve independently,which is inefficient.In order to overcome this low-efficiency problem,this thesis adopts the Reduced-basis Method(RBM),which uses limited basis vectors to generate a low-dimensional solution space,and only needs to solve at fewer frequency points to quickly obtain a wide frequency band The simulation results.Numerical results show that the method is accurate and has a good speedup ratio.On the other hand,considering the scatterer of metal-dielectric hybrid structure,in order to investigate the influence of the electrical parameters of the dielectric on the radar scattering cross-sectional area,the electrical parameters of the dielectric body will be set to change within a certain range in the simulation.In order to improve the simulation ability of radar scattering cross-section of variable electrical parameter targets,an improved Embedded Local Macromodel Method(ELMM)is proposed,which compresses the unknown amount of the medium region and reduces the system matrix dimension,thereby reducing the total calculation time of the radar scattering cross-section within the range of dielectric parameter changes.