| The study of the electromagnetic (EM) scattering characteristics about the shipwakes on the sea surfaces plays an important role in the oceanic remote sensing, shipand submarine detection and identification, and the corresponding parameter inversions.The research subject will be much more complicated and challenging especially when aship is moving in the complex sea environments. It mainly displays in three aspects.First, for the sea waves interacting on the ship wakes, their motion always has a certainamount of polytropy, which makes the research on the scattering from time-varying seaa very difficult task. The major difficulties are the computational cost arises from thenonlinearity and real-time simulation of two-dimensional sea waves. At microwavefrequencies, the problem is still beyond a sufficient solution. Second, the ship wakes onthe sea surface are always with large electrical size and have refined structures, whichbring great challenge on the fast calculation on the ship-wake scattering. Third, thehydrodynamic interaction between the ship wake and sea surface is also very complex.In this case, the problem becomes much more cumbersome. Therefore, the paper carriesout a systematic study on the EM scattering features of ship wakes on the sea surfaces.Stress being laid on three kinds of typical ship wakes, fast geometrical modeling of shipwakes, the corresponding EM scattering methods and Doppler spectral characteristicsare developed and analyzed. The main work is as follows:1. The generating mechanisms of sea surfaces and ship wakes are studied in thethesis. The geometrical features are obtained through the simulations of linear andnonlinear sea surfaces, Kelvin wake, turbulent wake and internal wake. Meanwhile, theEM scattering models of the sea backgrounds, two-scale method (TSM) and small slopeapproximation (SSA), are analyzed and veryified in detail.2. The tapered incident EM wave is combined with the SSA method. The keyresearch on the EM scattering characteristics of ship Kelvin wakes is made. Thecomparative analysis on the scattering differences of Kelvin wakes between the perfectconductor (PEC) and general dielectric sea surfaces. Meanwhile, the effects of windvelocity, ship type, ship size, ship speed and nonlinearity on scattering features fromship Kelvin wakes are also discussed.3. The volume scattering models of the turbulent wakes mainly formed by the foamor bubbles are studied, which is performed through vector radiation transfer (VRT)theory and Mento-Carlo (MC) method.4. Based on the features of the internal wakes with the smaller roughness, together with the second-order small slope approximation (SSA-II) method, the EM scatteringcharacteristics from the ship internal wakes with low sea states are mainly studied. Inaddition, due to the fact that the ship internal wakes have the larger coverage, thecorresponding EM scattering researches are carried out through the regional separation.5. The paper carried out the Doppler spectral studies of ship Kelvin wakes ontime-varying linear and nonlinear sea surfaces and comparatively analyzed the effects ofthe oceanic and ship parameters on the Doppler spectra of ship Kelvin wakes. Theobtained results show that, for the SSA-I method, the normalized Doppler spectra arethe same for both the two copolarization, however, for the SSA-II method, the obviousdiscrepancy occurs between HH and VV polarizations. The main reasons are that theSSA-I method is directly proportional to the wave elevation while the SSA-II methodfurther modifies the SSA-I through the slope of the simulated surfaces. Therefore, theSSA-II method can better describe practical physical backgrounds than the SSA-Imethod. However, from the view of direct summation, the SSA-II method reveals thelarge amount of calculation. Therefore, in order to overcome this difficulty, the fastFourier transform (FFT) technique is utilized to accelerate the SSA-II calculation, whichsatisfies the needs of the real-time calculation in actual EM simulated environments. |
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