Study On The Target Echo Algorithm In Large-scale Complex Environment Based On Cylindrical Coordinate Parabolic Equation

The prediction of target echo signal in large-scale complex environment has important application value in some fields,such as the target recognition,SAR imaging,long-range radar surveillance,missile fuze.In terms to the development of radar system and the development of target detection,recognition and imaging algorithm,how to obtain radar target echo data with ergodic and representative multidimensional radar characteristics of targets is of great significance.With the development of electronic information science and technology,radar types and application scenarios continue to expand,and the radar target echo gets increasingly complex.Especially when the target is in a complex environment,the diffraction and reflection effects caused by irregular terrain,such as mountains and hills,attenuation effects caused by lossy media such as forest vegetation,scattering effects caused by rough sea surface,refraction effect caused by atmospheric duct and so on,can not be ignored.In order to meet the requirements of the target echo prediction in large-scale complex environment,the dissertation studies the electromagnetic wave propagation model based on cylindrical coordinate parabolic equation(CPE)method and its target echo algorithm in large-scale complex environment.The main contents of the dissertation are summarized as follows:Firstly,the method of CPE is investigated.The forward parabolic equation(PE)in cylindrical coordinate system is derived from Helmholtz equation.On this basis,the exponential term of the PE is decomposed into two terms that change independently along the height and azimuth angle by Tappert approximation of the pseudo-differential operator.The exponential term is discretized based on the finite difference method,and thus the radio wave propagation model based on ADI solution method of the cylindrical coordinate PE is proposed.Besides,according to the general solution formula of electric fields in cylindrical coordinates and the orthogonality of the trigonometric function,the electromagnetic wave propagation model based on SSFT solution of PE in a cylindrical coordinate is derived.Numerical experiments are carried out to verify the “wide angle” in azimuth,the effectiveness of the simulation of inhomogeneous atmospheric refraction effects,and the correctness of the simulation of complex terrain environment.Finally,aiming at the problem of radio wave propagation in real complex geographical environment,the CPE digital map modeling technology is studied.Combined with digital map,the electromagnetic reconstruction method of three dimensional(3D)real environment and the visualization method of electromagnetic situation in 3D real scenes are studied.The reconstruction and visualization of 3D undulating terrain based on digital map are realized.Combined with CPE-SSFT model,the radio wave propagation prediction and 3D visualization display of electromagnetic situation in 3D real scenes are realized.Secondly,the efficient grid technology and its application of the CPE are studied.Based on the SSFT solution of PE in cylindrical coordinates,the non-uniform mesh model and sub grid model are proposed.Aiming at the problem of radio wave propagation in complex multi-scale environment,a CPE non-uniform grid model is proposed.According to the complexity of the environment and the intensity of its change with space,the environment is divided into different computing regions with different scales,and the corresponding grid is used for each region.The field values are refined by the “band limited interpolation” method to realize the matching between different grid systems.The model can reduce the computer hardware consumption and computing time,and thus improves the computational efficiency.In terms to the problem of radio wave propagation in the local multi-scale environment with key areas or key targets such as ships and aircraft,a CPE sub-grid model is proposed based on the non-uniform grid.According to the complexity and fineness of the environment,the calculation area is divided into integral and multiple local regions,and the grids of different scales are used for the calculation in each region.Therefore,it is able to realize the simulation of radio wave propagation in complex multi-scale environment efficiently.Its computational performance depends on the ratio of the integral fine area to the total area in the environment.The smaller the ratio is,the more obvious the efficiency of the non-uniform mesh model shows.In this dissertation,the calculation time is reduced about 15 times,which validates the efficiency of the proposed method.Besides,combined with the radar equation and CPE-SSFT model,an improved radar equation model based on CPE is proposed,which can be used to predict the 3D radar coverage in complex environment.In the end,the radar coverage simulation and 3D visualization display in real scene are realized by combining with a digital map.Thirdly,the scattering-field calculation method of target in a large-scale environment based on cylindrical coordinate parabolic equation is researched.The PE and its hybrid algorithm are studied for the calculation and visualization of the distribution of the target echo signal energy in complex environment and the integrated modeling of target and the complex environment.In order to solve the problem of electromagnetic scattering from a simple electrically large target,an electromagnetic scattering model based on CPE is proposed.The correctness and efficiency of the model are verified by comparing it with the method of moments(Mo M)and the 3D PE model.On this basis,a two-way cylindrical CPE model is proposed and applied to the calculation of the field energy distribution scattered by a simple electrically large target in a large-scale and complex environment.In order to solve the problem of energy distribution of echo signal scattered by a fine target in complex environment,a hybrid algorithm of 3DPE-Mo M based on the 3D vector PE and Mo M is proposed.The Mo M is used to process the scattering characteristics of the target,and the 3D PE method is adopted to calculate the propagation characteristics of the scattering signal.The model can calculate the scattering field distribution of the target in the paraxial rectangular region.In order to further calculate the energy distribution of the target scattering signal in a wide angle region,a hybrid CPE-Mo M algorithm based on the vector CPE and Mo M is proposed.Compared to the 3DPE-Mo M hybrid algorithm,the CPE-Mo M hybrid algorithm has "wide-angle" characteristics in azimuth.The calculation angle can reach 360 degrees,which can more intuitively show the scattering characteristics of targets and the distribution characteristics of scattering signals in complex environment.Finally,the target time domain echo algorithm based on the PE/FDTD hybrid method is studied.The complex environment and the target are regarded as a linear time-invariant system,and the target echo signal can be regarded as the output response of the transmitted signal after it passes through the system.Based on the PE and FDTD,the calculation method of transfer function for the integrated system of environment and object is investigated.On this basis,combined with the theory of the signal and system transmission,a PE/FDTD hybrid model is proposed to predict the echo signal of single station radar in large-scale complex environment.The correctness of the model in complex environment,such as half space,fog,rain and atmospheric duct,is verified by numerical experiments.Besides,combined with the "stop-to-go" method,a moving target echo model,which is suitable for the echo signal prediction of moving targets,is proposed.The model is used to predict and analyze the echo signal of a moving metal ball target in a numerical example.The velocity and position information obtained by the parameter estimation of the echo signal are basically consistent with the theoretical value,which verifies the effectiveness of the model.