Study On The Electromagnetic Scattering And Doppler Spectra From Dynamic Sea Surfaces

The value of research on the electromagnetic (EM) scattering from sea surface isvery significant in the field of ocean remote sensing, ocean early warning and thedetection and recognition of maritime targets. This dissertation systematically presentsthe research on the EM scattering from the sea surface, and focuses on the modeling ofEM scattering from the dynamic time-evolving sea surface and the Doppler spectraanalysis of the EM echoes from the dynamic sea surfaces. The main research andacademic contributions of the current work are as follows:Based on the improvement of the classical composite surface model (CSM), therefined composite surface model (RCSM) is presented. The Geometrical Optics (GO)solution of Kirchhoff Approximation (KA) serves as an additional part to improve thecalculation accuracy of the results near normal incidence. At different sea stateconditions, the shares of the contribution to total scattering from large-scale andsmall-scale sea surface should change, thus based on the validity conditions ofcomponent models in the CSM, the cutoff wavenumber with wind speed dependenceand incident wave frequency dependence is introduced. The non-Gaussian effect of seasurface slope distribution, the effects of the shadow function of surface facets and thecurvature of the surface are taken into account comprehensively, which improves thevalidity and accuracy of the model.Due to the uncertainty for dividing the wave scale, an angular cutoff compositesurface model (ACCSM) is proposed. This model uses the local scattering angle ofspecific facet on the sea surface to choose corresponding EM scattering model, then therapid EM scattering modeling for the specific two-dimensional (2-D) composite scalesurface sample in different sea states is presented. Compared with the CSM, thecontribution to total scattering field from each specific facet can be handled with greatflexibility by the ACCSM. Numerical calculations are carried out for co-polarized andcross-polarized scattering coefficients of2-D dielectric sea surface versus different windspeeds in the L to Ku band. The comparison of numerical results of the ACCSM andexperimental measured data shows they agree well. The ACCSM has a highercomputational efficiency compared with RCSM under the same operating conditions.The linear and nonlinear sea surface geometrical models combined with the EMscattering calculation methods are used to comparatively study the geometricalstatistical properties and the EM scattering characteristics of both types of sea surfaces.The backscattering coefficient and bistatic coefficient of the linear and nonlinear sea surface are numerically calculated, the influence of the radar parameters and marineenvironmental parameters on scattering coefficient are analyzed. It is found that theimpact of the nonlinear interaction between waves mainly reflected in the non-specularscattering directions, and the results for nonlinear sea surface are slightly larger thancorresponding results for linear sea surface.The scattering mechanism for dynamic sea surface is much more complicatedcompared with the counterpart for the static sea surface, thus the EM scattering fromdynamic sea surfaces needs to be accurately processed and analyzed in detail.Combined with the introduction of the tapered incident field, the Small SlopeApproximation (SSA) is utilized to study the Doppler spectra characteristics ofbackscattered echoes from the dynamic sea surface. The influence of nonlinear effect,radar and marine environmental parameters such as incident angle, frequency of theincident wave, polarization, electrical characteristics of the sea, wind speed, winddirection et al. on the Doppler shift and spectral width is analyzed in detail. Simulationresults show that nonlinear interactions between waves cannot be neglected if one aimsat interpreting the radar Doppler spectra. It is also found that the Doppler spectra forvertical polarization and horizontal polarization calculated by the first-order SSA(SSA-1) are the same, while the second-order SSA (SSA-2) is able to reflect thepolarimetric difference of the Doppler spectra. Based on the comparison of the resultsfrom numerical methods, it is found that the SSA-2is capable of elaborating thecharacteristics of the Doppler shift and spectral width of echoes backscattered fromdynamic sea surface for different polarizations, which show its feasibility in the analysisof Doppler spectral characteristics.Combining the hydrodynamic theory with the finite-depth sea spectrum, wesimulate the finte-depth sea waves based on the infinite-depth sea surface modeling.The related statistical parameters are calculated, and the statistical differences betweenthese two types of surfaces are compared and analyzed successively. Based on theSSA-2, the EM scattering and Doppler spectra from dynamic surfaces of finite-depthsea are studied, the influences of the water depth and other radar and marineenvironmental parameters on the scattering coefficient and Doppler spectra areevaluated and analyzed in detail, which reflect that the nonlinear effect in thefinite-depth sea is much more intense.