| In the environment of naval battlefield,interference information such as sea clutter,coupling between the target and sea surface and so on will seriously affect the accuracy of radar detection,and further reduce the ability of radar to detect and track targets.In order to adapt to the electromagnetic environment of complex battlefield,it is necessary to conduct indepth research on the electromagnetic scattering of targets and the sea environment,and establish a complex electromagnetic model of sea surface to provide a reliable theoretical basis for electromagnetic imaging,radar detection,target recognition,etc.In this dissertation,focusing on the problem of electromagnetic scattering of electrically large objects with sea surface containing broken waves,the improved algorithm of Shooting and Bouncing Ray(SBR)Method and its combination with Facet Scattering Method(FSM)are proposed.A composite electromagnetic scattering model of the target and sea surface with broken waves is established.On this basis of the previous work,ISAR imaging research based on the electromagnetic scattering echo data is carried out.The main research of this dissertation is as follows:1.An improved ray tracing method based on clue kd-tree and adjacent facet element relationship is proposed to improve the computational efficiency of SBR algorithm.The clue kd-tree preserves the link relationship both between child nodes and between child nodes and parent nodes.When traversing the nodes,it only needs to jump to neighboring nodes according to the clue instead to re-traverse the search from the root node every time.The cue-based kd-tree effectively reduces the redundant traversal query in ray tracing.In the leaf node,reverse ray tracing is performed according to the relationship between adjacent plane elements.On the one hand,the error caused by ray tube splitting is avoided,and on the other hand,the intersection test of unnecessary facet elements in the leaf node is reduced.Through these two improvements,the efficiency of ray tracing process is effectively improved.Finally,the physical diffraction theory is combined with the improved SBR algorithm,and the target edge diffraction is calculated by PTD method to improve the accuracy of the algorithm.2.Based on the two-scale model of the sea surface and its scattering mechanism,the computation method of electromagnetic scattering from sea surface with breaking waves is proposed by combining the improved shooting and bouncing ray method mentioned before and the facet scattering method.The electromagnetic scattering model of sea surface with breaking waves is established to improve the calculation accuracy of high sea conditions.The breaking waves are introduced to the facet model of the sea surface in the form of wedge structure.The number and location of breaking waves are determined according to the coverage of white crown and the tilting degree of sea surface,respectively.The scattering of the non-Bragg scattering,including the mirror reflection of the inclined facets describing the large-scale waves and the multipath scattering between the breaking waves and the surrounding undulating sea surface,are calculated by the improved SBR method.The shielding effect of sea surface itself is also considered in the ray tracing process.The perturbation method is used to calculate Bragg scattering caused by small fluctuation of sea surface.3.By combining the improved SBR algorithm with the FSM algorithm,the target scattering,the sea surface scattering and the coupled scattering of the target and the sea surface are calculated,respectively.Accordingly,the composite electromagnetic scattering model of the target and the sea surface with the breaking waves is established.The calculation method of coupling interaction of six different paths including target-breaking waves,target-sea surface-breaking waves and target-breaking waves-sea surface is proposed.Through the simulation of composite scattering between electrically large targets(missiles,ships)and the sea surface with breaking waves,the influence of the existence of breaking waves on composite scattering is analyzed.4.An ISAR image generation method based on electromagnetic scattering simulation echo data is proposed.The small bandwidth and small angle scattering echo data are calculated based on the improved SBR algorithm and FSM algorithm,and the ISAR image is then generated by the inverse two dimensional fast Fourier transform of the echo data.The proposed method is applied to ISAR imaging simulation of the composite model of the sea surface and the targets(such as missiles,ships).The influence of different illumination angles and sea surface wind speeds on the ISAR imaging is analyzed.The simulation results verify the effectiveness of the proposed imaging method based on electromagnetic scattering calculation.In order to improve the resolution of ISAR imaging and the definition of imaging contour,a multi-view ISAR imaging method based on sub-aperture synthesis is proposed.First,the echo data of each sub-aperture with large rotation angle and large bandwidth are collected in multiple incident directions,then multiple ISAR sub-images are obtained by two-dimensional inverse Fourier transform of the echo data of sub-apertures,finally,the subimages are fused to form the total ISAR image with equivalent large aperture range.Simulation examples of multi-view imaging for targets such as the missile,aircraft and ship are presented.The multi-view ISAR images of targets show high resolution and clear contour,which verifies the effectiveness of the algorithm. |
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