| The radar characteristics of target include the properties of the target scattering and the composite scattering properties of target with environment.The factors affecting the electromagnetic scattering characteristics of the target mainly contain geometric shapes,materials,and surface morphology of the object.In this thesis the electromagnetic scattering properties of coated targets with roughness at the terahertz band are studied with high frequency approximation algorithm.The near-field scattering characteristics of the targets illuminated by the actual horn antenna are discussed.The electromagnetic scattering of the rough targets at the terahertz band is studied considering the far field patterns of the transmitting antenna.The scattering of the precisely modeled domain decomposition model of target and rough surface is calculated by the Method of Moment(Mo M)and the Physical Optics(PO)hybrid algorithm.At the same time,the hybrid method of Semi-Deterministic Facet Scattering Model(SDFSM)combined with Shooting and Bouncing Ray(SBR)and Physical Theory of Diffraction(PTD)considering impedance boundary conditions is applied to determine the scattering fields of the target associated with the large scale rough sea surface.The main research works are described as following:1.Modeling of the axial symmetry rough surface targets and the calculations of terahertz scattering are described in this thesis.The surface of a general object shows slight fluctuations in the terahertz band and cannot be treated as a smooth surface.Different from the classical empirical formulas correction scheme,the basic idea of random rough surface modeling was used to establish a rotationally symmetric rough surface target.The scattering characteristics of axial symmetric coating target with a fractal rough surface are calculated with the Physical Optics method.Firstly,The rough target with a rotationally symmetric structure such as surface rough circular cones and conical columns with roughness are established by using the fractal rough surface theory;Secondly,the rough surface targets are meshed to meet the physical optics criteria;Thirdly,the Fresnel's law and boundary conditions are used to obtain the surface electromagnetic currents,and then the radar cross section of the coated rough target is calculated.The scattering results of rough and smooth surface targets are also compared.The terahertz scattering characteristics of the circular cones and conical columns are obtained in different coated materials,different coated thickness,different incident angles,different incident frequencies,and different unevenness.The terahertz scattering characteristics of conical columns with different fractal and Gaussian rough surfaces are further analyzed and compared.2.Modeling and calculation of terahertz scattering of three-dimensional random rough surface targets are described in this thesis.Three-dimensional random rough surface theory was brought into the modeling of rough surface targets.First of all,according to the shape of the target,a coordinate transformation is performed on the geometric data of horizontal rough surface to establish a simple target model with a Gaussian rough surface.For the complex shape target,smooth geometrical profile is created and meshed using Computer Aided Design(CAD)software.Then,each meshed facet is projected into the coordinate plane,and the projection facets are roughened respectively.On this basis,the rough surface target is obtained with the superposition method.Finally,the physical optics method is used to calculate the scattering characteristics with different incident angles,unevenness and frequencies.3.The calculation of the near-field scattering of the target considering the radiation characteristics of the antenna.Firstly,the division criterion of the far and near fields is introduced.Secondly,the distributions of the aperture fields of a conical horn antenna are obtained by using FEKO software.The fields at any point in all regions outside of the conical horn antenna are calculated by radiation theory of surface antenna.The scattering fields of the target with different distances from aperture to target center and the different angles from the maximum radiation direction to the target center are calculated and compared with far field scattering.Then,the near field scattering characteristics of rough coated target in terahertz band considering the effect of antenna pattern are calculated by using the physical optics method with modified distance term.4.The calculation of composite scattering of complex floating target and rough surface.Firstly,a random rough surface is created based on the surface-point cloud data,and a Boolean operation is performed on the target and rough surface with a CAD software to obtain a composite model of the floating target and the rough surface;Secondly,the target and the rough surface which is adjacent to the target are grouped as region?,and the rest facets are arranged as region?.As the electromagnetic interaction between the target and the rough surface in region I is strong,so the Mo M is used in this region;the PO method is used to perform the scattering calculation in the region II for weak coupling between the target and rough surface in this region.This hybird method is called Mo M-PO,which is used to compute the composite scattering of complex floating target and rough surface.The difference of the composite scattering results of target and the rough surface is compared and analyzed in different rough surface parameters and different regional division schemes.5.The composite scattering of target with large scale rough surface is evaluated.The number of the facets increases with the increasing coverage of the calculation area.As a result,the number of unknowns in electromagnetic calculation sharply increases,which leads to difficulty in solving equation.In this thesis,a high-frequency hybrid method is proposed.To calculate the scattering of the large scale rough surface,the Semi-Deterministic Facet Scattering Model(SDFSM)based on the two-scale method was used.The hybrid method of Shooting and Bouncing Ray(SBR)and Physical Theory of Diffraction(PTD)considering impedance boundary conditions was performed to calculate the scattering of the target.Based on the works we have done before,a composite model of a large-scale sea surface with an airplane above is established.The SBR method with different coupling paths is used to calculate the composite scattering of target with large scale rough surface in different sea surface conditions,different heights of target and different incident angles. |
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