Study And Optimization Of The Algorithms On The Composite Scattering From The Rough Sea Surface And The Object

The research on the composite scattering from the object located at the rough sea surface has found extensive applications in oceanic communication, radar surveillance and target detection, etc. This dissertation presents the systematical research on the evaluation of the composite scattering from the object and rough sea surface in microwave frequency band, and proposes an analysis and improvement on the solutions of the scattering from the complex and electrically-large object, the scattering from rough sea surface and the composite scattering from the object and rough sea surface repectively.The main contributions in this dissertation are outlined as follows:1) Based on the deeply researching of the basic theory of method of moment(Mo M)and multilevel fast multipole algorithm(MLFMA), the MLFMA analysis of the scattering from the electrically-large perfectly conducting object is given. In order to accelerate the solution procedure of scattering problems with MLFMA, a pre-corrected MLFMA(PCMLFMA)is proposed. Applying the Physical Optics method(PO), the approximated PO currents of the object could be obtained as the pre-corrected currents. With Galerkin’ method, a rather sparse impedance matrix is generated to evaluate the PO surface current. Based on the fact that the difference between unknowns and the PO currents is closer to the zero vectors than the unknown currents themselves, it will need less iteration steps by using the difference as the unknown vector to get the solution. Numerical results demonstrate that PCMLFMA could significantly decrease the number of iteration steps and greatly improve the computational efficiency.2) With the deeply investigation of the Volume Integral Equation(VIE) and the Volume-surface Integral Equation(VSIE), based on the theory of Mo M, the expressions of VIE-Mo M and VSIE-Mo M are derived in detail. Aimed at the electromagnetic scattering characteristics of the complex dielectric objects the hybrid perfectly conducting-dielectric objects, the codes of VIE-Mo M and VSIE-Mo M are written and verified by numerical results compared to commercial software finally.3) The traditional graphical electromagnetic computing method(GRECO) is improved based on the further investigation of its theory and accomplishment. The shadow-corrected GRECO(SCGRECO) is presented to solve the bistatic scattering from complex and(very) electrically-large perfectly conducting(PEC) objects. Combined with GRECO to extract the illuminated and shadow facets in accordance with the incident direction, the contributions of the currents in the illuminated region are calculated by GRECO. Meanwhile, current marching technique(CMT) is employed to calculate the contributions of the currents in the shadow region in order to improve the conventional GRECO. The accuracy of the SCGRECO is well verified by exact numerical methods.4) The modeling of the rough sea surfaces is performed based on the sea spectrum model and the linear summation theory in this dissertation. Based on the Kirchhoff approximation(KA), the large-scale sea surface can be break into myriads of plane facets, then the Kirchhoff integration is derived analytically on each individual discretized facet. The analytical expression obtained, so-called the “facet-based Kirchhoff approximation(FBKA)”, is suitable for a quick scattering calculation on the electrically very large sea surface, since it breaks through the limit of the intensively refined meshes as the usual Monte Carlo implementation does. Numerical results show that the great accuracy and high efficiency are both obtained in the process of evaluation of scattering from dielectric rough sea surfaces.5) Based on the most popular four-path model,a hybrid algorithm combining FBKA and MLFMA and another hybrid algorithm combining FBKA and SCGRECO are presented to accurately evaluate composite scattering from a three-dimensional(3D) perfect conducting(PEC) object on a dielectric 2D rough surface with electrically large dimensions in resonance zone and high frequency zone, respectively. In order to obtain a fast solution of the scattering from the electrically-large rough sea surface, The FBKA is employed in both hybrid algorithms. In the hybrid FBKA-MLFMA model, the MLFMA is introduced to reduce the computational complexity from O(N2) to O(Nlog(N)) in the accurate solution of scattering from the electrically large PEC object.In the hybrid FBKA-SCGRECO model, in order to deal with the bistatic scattering from the electrically(very) large object in high frequency zone, The SCGRECO is used tosignificantly cut down the error in the bistatic scattering obtained by GRECO with little computation cost. The electromagnetic interactions between the target and the rough sea surface are estimated by the four-path model, which avoid the complicated and time consuming iterative solution. Several numerical examples have been presented to demonstrate the efficiency and accuracy of the proposed hybrid methods.6) The difference scattering and the composite scattering from the object located at dielectric rough sea surface are evaluated by using a novel corrected half-space multilevel fast multiple algorithm(CHMLFMA) and a novel hybrid FBKA and corrected half-space multilevel fast multiple algorithm(CHMLFMA) algorithm respectively in this paper. Based on the scattering characteristic of the rough sea surface, the spectral reflection coefficients in the spatial half-space Green’s functions are proved as novel complex reflection coefficients,leading to the corrected half-space Green’s functions for the half-space problems involving rough sea surface; by using the proved half-space Green’s functions, the spatial expressions of the generalized corrected half-space Green’s functions are derived based on the mixed-potential electric field integral equation(MPIE). The generalized corrected half-space Green’s functions are suitable for the Mo M analysis of scattering from the objects in half-space. With the generalized corrected half-space Green’s functions, the MLFMA is extended to CHMLFMA to calculate the difference scattering from the object on the dielectric rough sea surface by using a high-order approximation. The proposed CHMLFMA executes nearly a full-wave simulation, meaning more accurate than the four-path model(FPM). The proposed algorithm avoids the complicated and time consuming outer iterative solution between the object and the rough sea surface of iterative hybrid analytical-numerical algorithms. Futhermore, the demonstrated algorithm has the advantage of only calculating the region of the object. By using the FBKA to fast calculate the scattering from the rough sea surface, the novel hybrid CHMLFMA-FBKA algorithm could be obtained. Numerical results reflect that CHMLFMA and the hybrid CHMLFMA-FBKA algorithm are more accurate than the most popular four-path model and more efficient than the half-space method of moment(HMo M).