| Microwave scattering and emission from the randomly rough ocean surfaces are the theoretical basis and prerequisites of marine microwave remote sensing.The in-depth understanding and analysis of the physical mechanism and properties of sea surface microwave scattering and emission has curcial scientific and engineering significance for guiding the design of marine microwave remote sensing payloads,improving the retrieval accuracies of ocean dynamic parameters,tracking and recognition of ocean targets,etc..In this thesis,the mechanism of scattering and radiation from the rough ocean surfaces is studied using analytic approximation models and numerical methods.The properties of ocean emission and scattering are analyzed with the simulations at various frequencies,polarizations and observation geometries.Furthermore,the impacts of ocean dynamic parameters,e.g.,ocean wind field,sea surface temperature(SST),sea surface salinity(SSS)and atmospheric condition,etc.,on the ocean scattering and emission,as well as the capability and potential of retrieving them,are studied.At the meantime,the improved ocean spectrum and numerical simulation methods are developed.The major research contents of this thesis have been outlined as follows:1.In order to study the unique negative upwind-crosswind(NUC)asymmetry of L-band sea surface backscattering in a low wind speed range,a new interpretation is presented,and an improved ocean wave spectrum is also proposed based on this interpretation.Furthermore,this newly developed spectrum is combined with the advanced integral equation method(AIEM)and the numerical Maxwell model in 3D(NMM3D)to simulate the L-band backscattering from the anisotropic sea surfaces.The simulation results show that both AIEM and NMM3D can successfully simulate the NUC and positive upwind-crosswind(PUC)asymmetry of L-band ocean backscattering at low and high wind speeds,respectively.The good consistency of model simulations and Aquarius observations indicates that our spectrum provides a better description of wave directionality,especially over wavenumber ranges from short-gravity waves to capillary waves,which have major interactions with the incident microwaves.In addition,a practical spectral integral range is given for the calculation of the ocean mean square height in AIEM model.2.For the sake of investigating the potential of retrieving ocean dynamic parameters utilizing the ocean microwave bistatic scattering,the fully polarized microwave bistatic scattering from the anisotropic ocean surfaces is simulated with the improved ocean spectrum and IEM/AIEM model.The spatial properties bistatic scattering and its sensitivities to ocean dynamic physical parameters,e.g.,wind speed,wind direction,sea surface salinity(SSS)and sea surface temperature(SST)are investigated in multiple polarizations and multiple incidences,and their combinations.The simulation results show that the bistatic scattering provides more spatial and physical information for the retrieval of wind speed and wind direction than the conventional monostatic scattering.This study expands the understanding of L-band ocean surface scattering in fully bistatic configuration and proves the usefulness of bistatic radar signals for the retrievals of geophysical parameters.3.The effects of sea surface temperature(SST)on ocean radar backscatter are investigated under both neutral and non-neutral atmospheric conditions.By studying the physical mechanism of SST's effects on ocean radar backscattering,the influence factors are determined.A physical-based model(PBM)consisting of the SST-enhanced KHCC03 ocean wave spectrum and the 2nd order small slope approximation(SSA-II)electromagnetic scattering model is developed and validated to evaluate the SST effects on ocean backscattering at L-,C-and Ku-band.The simulations show that effects of SST on ocean backscatter vary with wind speed,incidence angle and radar frequency.The seawater permittivity and viscosity yield VV/HH polarization dependences of ocean scattering variation.Under neutral condition,the seawater permittivity and air-sea interaction are major factors for L-band while the seawater viscosity and air-sea interaction are major factors for C-and Ku-band.Under non-neutral condition,the air-sea interaction plays the dominant role in SST effects on ocean scattering4.In the numerical simulations of ocean microwave scattering and emission,using method of moments(Mo M)for solving dual surface integral equations faces the problems such as inadequate accuracy and low computational efficiency.In order to deal with this situation,the Nystrom method with high order basis functions is introduced in Mo M.Also,a neighborhood impedance boundary condition(NIBC)technique is proposed to solve the matrix equation efficiently.Thus the Nystrom/NIBC method is developed for the high precision simulations of ocean scattering and emission.We use this method to simulate the ocean emission at L-band and the high accuracy emissivities are obtained.Numerical results show that emissivities using NIBC combined with Nystrom are accurate to 2×10-4 for vertical polarization and 10-4 for the horizontal polarization.5.The calculations of wideband emissivity from the large-scale ocean surfaces with numerical methods are quite computational expensive and memory consumption.Therefore,to address to issues of computational memory and speed,an accurate and fast numerical method is developed by combining the Nystrom/NIBC and the sparse matrix canonical grid(SMCG)method,which is named with Nystrom/NIBC/SMCG method.With this new method,the emissivities of a large-scale ocean surface are obtained and validated at the frequency range from 0.5 to 2 GHz,which are the working bands of the ultra-wideband software-defined microwave radiometer(UWBRAD).Simulation results show that this Nystrom/NIBC/SMCG method can significantly save the memory and improve the computational efficiency.At the meantime,it can also maintain high accuracy of calculation.Through the stuies of mechanism and models of the electromagnetic scattering and emission from randomly rough ocean surfaces,this thesis deepens the understanding of their physical mechanism and properties.These will provide the theoretical basis and technical methods for the applications and developments of ocean microwave remote sensing. |
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