Research On Creeping Wave Tracing And Modeling Algorithms Of Electromagnetic Diffraction On Dielectric-Coated Target Surfaces

In recent years, the scattering by the perfectly conducting (PEC) targets coating with thin dielectric is of significance and has a very good engineering application prospect, due to its wide applications in military field, such as target characteristics, radar detection, electromagnetic wave propagation, electronic countermeasures and antenna radiation. Especially with the rapid development of material science and stealth technology, the coating of absorbing material on the surface of a target can not only absorb the incident waves, but also redistribute the scattering energy by controlling the principal axes of anisotropic material to reduce radar cross section (RCS) of the target. Based on the impedance boundary conditions (IBC), as an approximate model, the impedance target provides a feasible way for the theoretical research of the electromagnetic scattering of the target coated with thin medium. Hence, in order to study the property of surface diffraction of the target coated with thin medium, the study of surface diffraction of the impedance target is necessary. And the surface diffraction of the impedance target is a very important and basic problem in the study of electromagnetic scattering and target characteristics.The main content of this paper includes the following parts:1) For the geometric modeling, NURBS and Bezier technologies are introduced, and several NURBS geometric models are given. Compared with conventional modeling of the facets, the advantage of NURBS modeling are high precision, less curve patches, no virtual edges and easily acquisition of geometric parameters on surfaces. However, consider the complexity of the representation of NURBS surfaces and lack of the simple and stable numerical algorithm in calculating the derivative of NURBS expression, the NURBS patches are subsequently subdivided into a combination of rational Bezier patches by Cox-De Boor algorithm. And then targets are described by Bernstein basis function, which is more stable and simple for the tracing computations.2) The creeping ray tracing algorithm on the surface of PEC targets and isotropic medium coating targets are studied. According to the study, the creeping ray tracing by conventional Euler method is unstable and low precision, we present a creeping-ray tracing algorithm based on a novel adaptive variable step Euler method for improving the accuracy of creeping-ray tracing on arbitrarily-shaped NURBS surface. Because the adaptive variable step Euler method is based on the conventional Euler method, the efficiency is ensured. And then for a high precision, a novel creeping ray tracing algorithm is developed based on the property of geodesic. Both of two algorithms can provide a good foundation for the modeling method of surface diffraction on PEC target and PEC target coated with isotropic medium.3) Based on the accurate creeping ray tracing, the modeling method of surface electromagnetic diffraction on PEC targets and isotropic medium coating targets is studied. Based on the impedance boundary conditions, the PEC targets coated with isotropic medium can be equivalent to isotropic impedance targets. And after understanding the uniform geometrical theory of diffraction and obtaining the creeping ray tracing results, in order to validate the correction of the electromagnetic modeling method, some electric fields from PEC target and isotropic impedance target by UTD and impedance UTD are given respectively and compared with the analytic results.4) The trajectory of creeping ray on PEC targets coated with thin anisotropic dielectric is studied. After understanding the propagation characteristic of electromagnetic wave in anisotropic dielectric, the propagation characteristic on the surface of anisotropic dielectric is developed. Generally the creeping wave on anisotropic dielectric surface can be divided into two type modes, ordinary wave (OW) and extraordinary wave (EW). And then according to study, the paths of OW satisfy the geodesic equations, while the paths of EW are still to be decided. Hence, in order to determine the trajectory of EW propagation, a system of differential equations of EW trajectory on anisotropic dielectric-coated surfaces is derived on the basis of Fermat's principle, Euler-Lagrange equations and differential geometry. And when smoothly convex surfaces are coated by isotropic dielectric, the differential equation set can be reduced to geodesic equations. Finally, to verify the correctness of the differential equation set,4th-order Runge-Kutta method is introduced to solve the differential equation set. Then the correctness and generality of the differential equation set can be verified from the results of numerical creeping-ray tracing.5) Based on the accurate tracing of OW and EW, the modeling method of surface electromagnetic diffraction on PEC targets coated with anisotropic dielectric is preliminarily studied. Based on the impedance boundary conditions, the PEC targets coated with anisotropic dielectric can be equivalent to anisotropic impedance targets. In order to simplify the calculation of electromagnetic wave and directly using the impedance UTD method, an equivalent impedance ZABequi is introduced. The ZABequi is an impedance parameter of the propagating direction of creeping wave, and we can directly substitute ZABequi into isotropic impedance UTD method for the calculation of surface diffraction on anisotropic impedance surface. Finally, based on the creeping ray tracing algorithm and impedance UTD algorithm, the surface diffracted fields of some anisotropic impedance targets are given and analysed.