Study On The Scattering Mechanism Of Ship Wakes In SAR Imaging

The study on the scattering mechanism of ship wakes in SAR imaging provides important theoretical basis to the ship detection and classification under marine environment.Considering that the ship motion always leads to offset and blurring effects in SAR images,the ship wakes could sometimes offer the information to ship detection.According to the specific features,there are mainly three types of ship wakes classified in SAR images,including Kelvin wake,turbulent wake,and internal wave wake.Among these wakes,the estimations on ship velocity and direction could be conducted from the wavelength of Kelvin waves as well as the cusp wave direction,while the turbulent wake features give information about ship heading and beam.In the open ocean,the characteristic parameters of target on or under the surface can be identified through the internal wave wake.To better understand the relationship between wake features in SAR image and target parameters,this dissertation performs a systematic research on the electromagnetic scattering and radar imaging from ship wakes.With the hydrodynamic modeling of typical ship wakes,the numerical models are presented for their electromagnetic scattering and SAR imaging characteristics.The main works are as follows:1.According to the facet description of sea surface,the semi-deterministic facet scattering model is developed from the first perturbation theory with Kirchhoff correction to the near-vertically incident conditions.The model could implement the fast prediction of electromagnetic scattering coefficients from electrical large sea surface as well as its spatial distribution.For the nonlinear characteristic of sea waves,the small slope approximation model is utilized to study the Doppler spectra of scattering echo from the dynamic sea surface,which is combined with the introduction of the tapered incident field.The influence of nonlinear effect,radar and marine environmental parameters such as incident angle,polarization,and wind direction et al on Doppler shift and spectral width is analyzed in detail.2.On the basis of ship wave theory and the thin ship theory,the numerical model is presented for the simulation of Kelvin wake under various sizes and moving conditions.Then the effects of Kelvin wake on the scattering coefficient of sea surface are considered with the help of small slope approximation method,which is with emphasis on the variation of the scattering characteristic with the moving parameters.However,the scattering coefficient meets the limitation on the description for detail information of surface waves.Based on the facet scattering model,the scattering distribution of surface waves with Kelvin wake is studied to illustrate the tilt effect of ship wake on sea waves.3.In view of the specific stratification of seawater,the discrete internal layer and the diffuse internal layer are proposed with the corresponding dispersion relations and wave crest patterns.And the field models for ship internal wave wake are built by the combination of velocity potential theory.According to its field characteristics,the scattering coefficients for arbitrary bistatic configurations are discussed by means of region segmentation under low sea states.Moreover,the spatial scattering properties are investigated for the surface waves with internal wave wake,followed by a detail explanation about the effect of simulation parameters on the wake patterns.4.The interactions between short and long waves are presented according to the hydrodynamics of sea waves.Combined with real aperture radar imaging theory,the velocity bunching imaging model is constructed for the SAR imaging simulation of ship wakes,which considers the main modulation mechanisms to radar returns,such as tilt modulation,hydrodynamic modulation,and velocity bunching velocity.The proposed could achieve the fast simulation of SAR images for Kelvin wake and internal wave wake.And the condition for the visibilities of ship wake in SAR images are summarized through the comparisons among numerical results under various parameters.5.As a specific two-phase flow problem,the one-and two-dimensional turbulent wakes are simulated with numerical methods in the compute fluid mechanics.And a preliminary imaging model for turbulent wake is provided based on the flow field.The electromagnetic scattering characteristics for one-dimensional case are evaluated by the method of moments,in which the super scattering characteristics appears for the scattering coefficients.,With the simulated velocity field,the SAR imaging simulation for two-dimensional case is studied to describe the SAR characteristic of turbulent wake with different scales.