Study On Modeling Methods Of EM Scattering From Sea Surface And Ship Target

The theoretical modeling for the composite scattering of complex sea background and target has always been one of the research emphasis and difficulties in the field of electromagnetic field and microwave technology.When the target and rough surface are within the scope of electromagnetic waves,it needs to consider not only the scattering respectively from the rough surface and target but also the coupling effects between them.However,the randomness of rough surface,the intricate structures on target and the electrically large size make the coupling scattering mechanism between the target and rough surface more complex.In addition,the composite scattering of rough surface and target more focuses on the electromagnetic information of target in order to effectively detect and recognize the target from rough surface.The random echo from rough surface now becomes noise that must be eliminated.In view of the significance and application prospects of the composite scattering,this dissertation systematically presents the research on the modeling methods for the composite scattering from the electrically large ship target on sea surface.The main research and academic contributions of the dissertation are as follows:The traditional scattering model of sea surface mainly takes the sea surface as the research object and only gives the radar scattering coefficient of total echo.These models are often unable to give the spatial information of facet scattering field,so they are difficult to be applied to the composite scattering modeling of target on sea surface.Aiming at the application requirement of the composite scattering from electrically large ship on sea surface,this dissertation estabishs a refined facet scattering field model(RFSFM)based on the facetized treatment of the scattering contribution and the simplified representation of the capillary wave.This model divides the scattering area into mirrored scattering area and diffuse scattering area according to the corresponding local incident angle of the facets.Then the capillary wave modified facet scattering model(CWMFSM)is used to calculate the scattering of facets in diffuse scattering area.And the KA is used to calculate the scattering of facets in mirrored scattering area.Based on the dielectric constant of sea water,the scale conditions for the dielectric constant of sea water is deduced.The results show that a set of temperature and salinity for the scale model can be obtained theoretically to make the dielectric constant of sea water could remain equal at frequencies of scale model and prototype.But such a set of temperature and salinity for the scale model is nearly inacceptable in the laboratory.Thus,this dissertation analyzes the feasibility and error of the scaling measurement in the laboratory using the freshwater instead of seawater.By comparing the scattering coefficient,spatial distribution of scattering field,clutter time series and Doppler spectrum between the scale model and prototype,the presented scale conditions for the geometry of a rough sea surface are verified and using the freshwater instead of seawater to do the scaling measurement in the laboratory is feasible.According to RFSFM,the probability density function(PDF)and space-time correlation characteristic of the sea clutter under different conditions are statistically analyzed.For the composite scattering of target on sea surface,the method for determining which patches are direct illuminated by incident rays and which patches are illuminated by the first reflected rays is firstly given.And then,the GO-PO method is estabished to calculate the scattering from electrically large ship and the coupling scattering between the ship and sea surface.When dealing with the coupling scattering between the ship and sea surface,the GO-PO method can consider the mutual shaded effect between the patches and more reasonable calculate the second scattering contribution of patches.Finally,based on RFSFM and GO-PO method,the composite bistatic radar scattering characteristic and Bis-SAR imaging for the scene of ship on a sea surface are anlyzed.For the real ship target on sea surface,the ship will moves with translation and rotation under the action of water,namely,the motion of ship in six degrees of freedom.It can make the stance of ship on sea surface change constantly,and further lead to the scattering characteristic of ship and the coupling scattering characteristic between the ship and sea surface are changing over time.Thus,this dissertation presents a model for the motion of ship in six degrees of freedom in two dimensional(2-D)sea surface based on PDSTRIP.Then,the time-varying characteristics for the motion of ship in six degrees of freedom and the composite scattering echo of moving ship on sea surface are simulated.