A Study Of Electromagnetic Composite Scattering From The Time-Varied Sea Surfaces And Targets

Research on the clutter characteristics of the realistic earth and sea environment is the key point in the target recognition and signal separation technologies for the complex environment, and its applications are widely involved in various fields, such as remote sensing, radar imaging, and early warning. The dissertation presents the research of the electromagnetic scattering from the sea surface and the composite scattering from the electrically large targets and the sea surface, which is with emphasis on the Doppler analysis of the time domain electromagnetic echo from the time-varied sea surface and the moving target. The time domain composite scattering is modeled with inclusion of the breaking waves and the ship bow waves in the low grazing angles. The main works are as follows:1. The geometric modelings of the sea surface are summaried based on the sea spectrums, and the two-dimensional nonlinear sea surfaces are generated with the Creamer model. The electromagnetic scattering of the sea surface is investigated with the modified two-scale method and the iterated Kirchhoff Approximation respectively. Including the curvature term and phase term, the modified two-scale method can be used to calculate the co-polarized scattering field from the sea surface. With correction of the electromagnetic currents on the sea surface, the multi-scattering field between the surface facets is analyzed with the iterated Kirchhoff Approximation. The Z-Buffer technology is utilized for the shadowing effect of the deterministic sea surface model and substitutes the classical shadowing functions. The statistical properties of the sea clutter are investigated with the simulated results.2. The four-path model is modified for the electrically large targets on sea surface by the corrections of the reflection coefficients and the scattering paths. The coupling scattering between the two-dimensional sea surface and the three-dimensional electrically large target is analyzed based on the reciprocity theorem, and some conclusions are obtained. The fundamental solutions to the mesh free method are used to analyze the composite scattering of the one-dimensional rough sea surface and the target. The accuracy of the numerical results is equivalent to the method of moment, and the calculation efficiency is promoted prominently.3. Due to the corner reflector structure formed by the ship and the sea surface, the multiple scattering between the ship hull and the sea surface is evaluated with the iterated physical optics method. The self-shadowing of the complex ship-sea model is treated with the Z-Buffer technology, and the convergence of the itetation is discussed. With the analysis of the bistatic scattering cross sections and the synthetic aperture radar (SAR) imaging of the simulated data, the strong coupling scattering of the dihedral corner reflector structure of the ship-sea model is observed near the backward direction.4. Based on the quasi-stationary approximation, the dynamic radar signatures for ships on time-varied sea surface is modeled with the frequency domain methods. The effects of the nonlinear term and the sea wave drift on the Doppler spectrum of the scattering field are investigated for the realistic sea scattering environment. The numerical results agree with the measured data well with inclusion of the horizontal motion of the water waves. The composite scattering of the moving targets on the time-varied sea surface is calculated with the modified four-path model, and the components of the composite scattering are analyzed with the Doppler spectrum.5. The complex sea surface scattering model is established with inclusion of the spilling breakers to analyze the super events in the filed experiments in the low grazing angles. The three-dimensional geometric models of the breaking waves are approximately simulated with the dihedral wedge with finite size. According to the breaking criteria of the sea waves, the distribution of the breakers is obtained, which is also compared with the measured data and empirical formulas. Combined with the surface scattering and the volume scattering, the scattering coefficients and the Doppler spectrum are investigated for the forming reason of the super events in low grazing angles. The relationship between the scattering of ship bow wave and the ship signature is analyzed based on the Kelvin bow wave model, and the effects of the ship bow waves on the composite scattering are investigated.