Investigation On Polarimetric Electromagnetic Scattering And Doppler Spectra From Time-Varying Sea Surfaces Driven By Wind

This dissertation presents a systematic investigation on the polarimetirc electromagnetic scattering and Doppler spectra characteristics of time-varying sea surfaces driven by wind?The emphasis is put on the analysis of the influence of ocean nonlinear hydrodynamic interactions,ship-induced Kelvin wakes as well as sea slicks on the polarimetirc electromagnetic scattering and Doppler spectrum characteristics of sea surfaces.The investigation in this dissertation is of great value in many fields such as sea surface field observation,ship surveillance and indentification,sea slicks surveillance,etc.The main works are listed in the following:1.A tapered plane wave is introduced in the second-order small-slope approximation(SSA-?)model,and its validity is verified by comparing with method of moments(MoM).Based on the SSA-II model with the tapered plane wave incidence combined with nonlinear sea surface model,the influence of the nonlinear hydrodynamic interactions on polarimetric scattering Doppler spectra signatures is investigated in detail.Simulation results indicate that the Doppler shift and spectral bandwidth of CWM nonlinear sea surfaces are significantly larger than those of linear sea surfaces at moderate to large incidence angles,especially for low grazing angles.Moreover,the Doppler signatures show distinct polarization dependence,and most importantly significant differences of Doppler signatures between co-and cross-polarization can be observed in our simulations.In addition,the simulation results indicate that the co-polarized Doppler shift increases obviously with wind speed increasing,whereas the cross-polarized Doppler shift looks less sensitive to wind speed variations.2.Based on the polarimetric scattering model of SSA-II with tapered wave incidence,monostatic and bistatic scattering from two-dimensional dielectric rough sea surface is investigated with emphasis on analyzing the polarization signature of the scattered wave under circularly polarized wave incidence such as GPS signal.Numerical simulations show that for backscattering case,the left-hand circularly polarized scattered wave is predominant under right-hand circularly polarized wave incidence.For bistatic configuration under circularly polarized wave incidence,the polarization state of scattering wave strongly depends on incident angle,scattering angle,as well as their relation to the Brewster angle,which is associated with medium permittivity.The simulation results of the scattering wave polarization state under circularly polarized wave incidence are potentially valuable for choosing the polarization channel of GPS receiver.3.A polarimetric electromagnetic(EM)model of ship wake embedded in rough sea surfaces is established.On this basis,the influence of ship-induced Kelvin wake on monostatic and bistatic polarimetric scattering signatures of sea surface is investigated in detail.For both monostatic and bistatic configuration,simulation results indicate that electromagnetic scattering signatures in the presence of a ship-induced Kelvin wake dramatically differ from those without ship wake.In addition,it is also indicated that under low wind conditions a larger ship speed gives rise to a stronger enhanced backscattering and enhanced depolarized scattering,which is a result of multiple scattering arising from a larger sea surface roughness due to the presence of ship-induced Kelvin wake.4.Based on Marangoni damping theory,the influence of natural sea slicks on sea surface roughness spectrum and sea surface geometrical structure such as RMS height and RMS slope is examined.On this basis,the influence of natural sea slicks on the Doppler spectrum signatures of backscattered radar sea-echo is systematicly investigated in detail based on the SSA-? model with tapered wave incidence.A narrowing of the Doppler spectra in the presence of oil slicks is observed in numerical simulations.Moreover,the numerical results indicate that the Doppler shift in slicks can decrease or increase,depending on incidence angles,oil characteristics and polarization state.5.Based on Green's theorem and Helmholtz equation,the coupling integral equations are derived for calculating EM scattering from layered rough surfaces with an arbitrary number of rough interfaces.The influence of thick oil films on EM scattering from rough sea surfaces are investigated by utilizing MoM with emphasis on analyzing the influence of oil thickness and oil parameters on bisatic scattering characteristics of sea surfaces.Simulation results show that EM scattering characteristics are significantly affected by oil characteristic parameters.However,EM scattering characteristics of sea surfaces are weakly influenced by oil thickness,which is due to the fact that oil film is rather transparent at microwave frequencies.