Study On The Electromagnetic Scattering Characteristics Of Wakes Induced By Marine Targets And Its SAR Imaging

The research on the electromagnetic scattering characteristics of marine target wakes and their synthetic aperture radar(SAR)images are hot issues that have attracted much attention in recent years.For the work of marine target detection,in addition to direct target detection,the detection of the waves induced by the target can also determine the existence of marine targets.Especially for an underwater target or a target of non-conducting materials,it is difficult for a radar to receive their echo signal directly.The images of wakes contain the information of target's track,size,and motion parameters,which are of great significance in the marine environmental monitoring,the detection,tracking,and motion parameter estimation of the target.The study on the simulation of SAR imagery of various wake waves in a complex ocean is a subject with both great application prospects and challenges.Although the electromagnetic scattering characteristics and SAR imaging of marine target wakes are only relatively small engineering applications in the subject of microwave remote sensing,related theories involve almost all aspects of remote sensing and detection of marine target's characteristics.The establishment of the model should be combined with the theories and methods of multiple disciplines,including oceanamics,hydrodynamics,electromagnetic scattering and SAR imaging theories.The dissertation studies the electromagnetic scattering characteristics and SAR imaging of various marine target wakes in the microwave frequency band systematically.Focusing on the application of the electromagnetic scattering and SAR imaging of the multi-source waves,a modulation facet scattering model is established.The dissertation focuses on the modeling and electromagnetic calculation of the electromagnetic scattering characteristics of various nonlinear waves,including breaking waves,turbulence,and of underwater target wakes,in ship wakes.Combining the CFD technology and various electromagnetic algorithms,a procedure of coupling simulation for the electromagnetic scattering from multi-source wave is established.This procedure can be also used for scattering calculation and SAR imaging of other multi-source waves.The main works are as follows:1.A modulation facet scattering model is proposed in combination with hydrodynamic and electromagnetic theory.By solving the wave action spectrum equation,the influence of the velocity fields of the wake on the ambient wind waves is calculated.The nonlinear effects of wakes are fully considered,so that the visibility of the wake in ambient waves can be described accurately in the SAR imaging.Although there have been some classical electromagnetic scattering models which can estimate the scattering from sea surfaces accurately,when the models were applied to SAR imaging models,the nonlinear effects of wakes are often ignored,which will lead to an underestimation of the visibility of the target wakes.The modulation facet scattering model solves this problem,and then it can be used to simulate the SAR imaging of the wakes on the sea surface very well.2.A numerical wave model,including a turbulence model,is implemented to obtain the hydrodynamics of the wakes.The finite volume method(FVM)is used to discrete and solve the current field of the wakes,and the volume of fraction(VOF)method is used to capture the geometry and current field of the wake.A complete ship far-field wake including the turbulent wake is obtained.Comparing with the methods based on the thin ship and the potential flow theory,this method can obtain more comprehensive information about various target wakes more flexibly,and nonlinear components in wakes are considered.The accurate geometry and velocity field of the wake will play an essential role in the subsequent electromagnetic scattering and SAR images.3.In the combination of the advantages of the modulation spectrum facet scattering model and the numerical wave model,a simulation process for the electromagnetic scattering distribution and SAR imaging of wakes is proposed.By solving the equilibrium equation of the active spectrum,a SAR imaging model that fully considers the three modulation effects of tilt,hydrodynamics and velocity bunching is obtained.This model can fully consider the modulation effect of the flow field of wakes on the roughness of sea surface and the motion of waves in the wake during the SAR imaging process.Based on the SAR method,a comprehensive far-field wake SAR image containing turbulence on a dynamic sea surface is simulated.4.A new method for calculating the electromagnetic scattering of the near-field ship wake is proposed.The geometry of the near-field ship wake is obtained by the computational fluid(CFD)simulation.The forward-backwards iterative physical-optical(IPO)method is used to evaluate the multiple scattering effects between the ship waves and hull model.In order to improve the convergence of the algorithm,the forward and backward sub-relaxation iteration method is implemented,and the Z-Buffer technology to handle the shadow effect of the model.The influence of breaking ship waves on the electromagnetic scattering distribution of ship-sea composite scenes under different radar perspectives is investigated.The results show that the corner reflector formed by the steep breaking waves and hull may cause new strong scattering areas of the composite scenes.5.Aiming at the SAR imaging simulation of the wake induced by a submerged target,a method for modelling the submerged target wake in arbitrary density stratified condition is proposed.The scattering distribution and SAR images of internal wave wakes are studied.The results show that the pattern of the submerged target wake is affected by the target geometry,motion state,and ocean density stratification,etc.The patterns of the wakes induced by different targets or the same target with different speeds are different in electromagnetic characteristics and SAR images.The wakes are usually in the form of internal wave wakes when moving at low speed,and usually in the form of Kelvin wakes when moving at high speed.The dissertation improves and extends the simulation methods for the electromagnetic scattering from various target wakes.The hydrodynamic and electromagnetic characteristics of various typical wakes are studied systematically.The research focuses more on the simulations and analyzes of electromagnetic characteristics of the nonlinear ship waves,i.e.far-field turbulence,near-field breaking waves,and underwater target wakes,and the SAR images of the wakes induced by the ship targets and submerged targets are simulated and analyzed emphatically.