Characteristics Research On Electromagnetic Scattering From Time-Varied Non-Gaussian Sea Surfaces And Breaking Waves

With the actual requirements of a myriad of applications such as moving targets detection,oil spill characterization,and remote sensing in marine environment,study of electromagnetic scattering and Doppler spectra from 2-D time-varying oceanic surfaces has attracted more and more attentions recently.On the one hand,the non-Gaussianity of oceanic waves has a significant effect on the water particle movements,so the study of non-Gaussianity is essential.On the other hand,it is difficult to calculate the scattering field from electrically large target,so the fast algorithm for electrically large ocean is a hot topic.Meanwhile some studies have shown that breaking waves have influence on scattering waves,and breaking waves and sea spikes appear to go hand in hand.This dissertation systematically presents the research of the electromagnetic scattering from the time-varying oceanic surface and breaking wave.The effects of nonlinearity and horizontal skewness on the normalized radar cross section and the Doppler spectrum are analysed.A new electromagnetic algorithm is put forward for the steady nonlinear oceanic surface.The scattering characteristics of breaking waves are also studied.The main academic contributions of the current work are as follows:1.Based on the choppy wave model and the Creamer(2)model,two types of nonlinear oceanic surfaces are simulated.The method of second-order small slope approximation is adopted to compute their NRCS and Doppler spectrum respectively.The simulation results are compared and analyzed.The research results indicate that the nonlinearity of oceanic wave has a very low impact on the NRCS.However,the nonlinearity affects Doppler spectrum remarkably,and the Doppler width of the choppy wave model for large incident angles is larger than the counterpart of the Creamer(2)model.2.The crests of the waves are tilted towards the wind direction.Although some researchers have discussed the effect of horizontal skewness on the backscattering coefficient,to our knowledge,the effect of horizontal skewness on the Doppler spectra has not been discussed so far.In order to simulate skewed surfaces,the first order modified Lagrange model is adopted.The effect of horizontal skewness on electromagnetic scattering is studied by combination of the first order modified Lagrange model with the small slope approximation method.The analysis suggests that the effect of horizontal skewness on NRCS varies for different wavebands.Although the effect on the NRCS of L-band is negligible,the horizontal skewness makes the Doppler asymmetric along the wind direction.A reasonable explanation is given from the water particle movement angle.3.The phase of the scattering field is absent,so the classic two-scale model cannot be used to analyze composite scattering by objects above a rough surface or Doppler spectrum.This dissertation presents a field-based semi-deterministic model based on the combination of the two-scale theory and the small slope approximation.The analytic scattering amplitude of small-scale capillary waves based on the local configuration angles is conducted.Different from the conventional two-scale model,the model is a field-based rather than RCS-based model,which considerably widens its potential applications.In order to take into account the influence of nonlinear hydrodynamics on echo characteristics,the choppy wave model is adopted to simulate nonlinear surfaces.Several examples of bistatic NRCS and Doppler spectra are given with comparison and analysis,which validate the rationality of the model in non-mirror direction.4.Breaking waves and sea spikes appear to go hand in hand.One of the present research rubs is building the 3D breaking wave models.The 3D breaking waves in this dissertation are formed by superimposing a small-scale rough surface upon 3D smooth breaking waves.The 3D smooth breaking wave profiles are formed by azimuthally aligning the ID LONGTANK series.Although the method is lack of hydrodynamics theoretical foundation,it is more visually real and suitable for pre-research.The iterative physical optics(IPO)is combined with the forward-backward methodology to calculate the scattering field.The OpenGL technology is adopted to solve the shadowing in the integral of IPO.The backscattering and bistatic scattering are studied.The results show that breaking waves have a direct effect on the high HH/VV.