Research On Time-domain And Frequency-domain Scattering Analysis Of Electrically Large Targets

With the rapid development of information technology, the conventionalmechanized warfare has been changed into modern electronic warfare. The moderndetecting equipment and weapons are characterized by intelligence, accuracy andremote indication. The conventional weapons are especially vulnerable in terms of theviability, penetration survival ability and defense ability. The application of stealthtechnique has greatly enhanced the survival probability and fighting performance ofweapons. Development of the stealth technique starts with a deep research in thescattering analysis of targets. However, the electrically large characteristic of the targetmakes it a great challenge to predict the radar cross section (RCS) efficiently. With thewidespread application of short-pulse communication and ultra-wideband radar systems,increased attention has been focused on time-domain electromagnetic scatteringanalysis. Urgent demands are proposed towards the fast and accurate wideband RCSevaluation of electrically large targets. Being associated with the research projects, thisdissertation is mainly concerned with time-domain and frequency-domain scatteringcalculation algorithms of electrically large targets. The author’s major contributions areoutlined as follows:1. The trimmed NURBS modeling technique is studied thoroughly. Based on ananalysis of the Initial Graphics Exchange Specification (IGES) standard file format, thegeometrical information of the trimmed NURBS surface is extracted, including thesurface degrees, the control points, the control point weights, the knot vectors and thetrim curves in the3-D domain and the parametric domain. A re-development procedureof the IGES file is realized, which makes the modeling interface accord with that of theelectromagnetic computation.2. Based on a thorough analysis of the mathematical property of the trimmedNURBS surface, a method is obtained to judge whether an individual point is located inthe effective region of the trimmed surface. The judging process is accomplished in the2-D parametric region rather than the3-D space, thus reducing the complexity to a largeextent. This is the pre-processing step of the subsequent electromagnetic computation.3. In order to avoid the use of numerical integrations, the SPM-Gordon method isproposed to evaluate physical optics (PO) integrals over trimmed NURBS surfaces. Theinvalidity of the stationary phase method (SPM) applied to trimmed NURBS surface is demonstrated and analyzed. Then the problem is successfully solved by a hybridalgorithm of SPM with Gordon formula. A simple and fast shadowing techniquesuitable for trimmed NURBS surfaces is also proposed. It is illustrated that the runningtime of the SPM-Gordon method is less than one tenth of that of the numericalintegrations.4. A dielectric time-domain physical-optics (TDPO) is proposed to extend theconventional TDPO to the time-domain scattering analysis of electrically largehomogeneous dielectric targets. The Fresnel reflection coefficients are introduced intothe frequency-domain PO approximation, and then the formula of transient scatteredfield is derived through the inverse Fourier transform. Numerical results demonstratethe validity of the dielectric TDPO in the wideband RCS prediction.5. In order to eliminate several disadvantages of the conventional TDPO resultedfrom numerical integrations, an improved TDPO is proposed to avoid numericalintegrations utilized in the conventional TDPO. The time-domain scattered field in theimproved TDPO is expressed as a convolution between the incident pulse and thetime-domain PO integral. An accurate closed-form expression of the PO integral over atriangular facet is obtained through the Radon transform. The accuracy of theexpression is not dependent on the size of the triangular facet. As a result, the target canbe subdivided into lesser facets as long as the modeling requirement is satisfied.6. In the edge subdivision of the conventional TD-EEC, the subdivided straightsegments should be small enough to ensure the integration precision of the contourintegral. An improved TD-EEC is proposed to calculate the time-domain scattered fieldand the wideband RCS of targets composed of plate structures. The time-domainscattered field is expressed as a convolution between the incident pulse and the contourintegral of diffraction coefficients. Then an accurate closed-form expression of theintegral is proposed pertaining to straight integration lines by the use of Radontransform. The accuracy of the expression is irrelevant to the length of the integrationline. Thus when any straight diffracting edge is considered, subdivision is not requiredin the improved TD-EEC, which releases the heavy burden on the CPU and the memorystorage.