Investigation On Composite Electromagnetic Scattering From Electrical Large Scale Rough Sea Surface And Target

In this paper, the author has discussed the characteristic of electromagnetic scattering from rough sea or soil surface with or without a target. This paper aims at solving two principal problems in this research area:how to reduce the computational cost and how to apply the computational electromagnetic methods into complex realistic marine scene. The calculation of EM scattering from a rough surface is an important subject in many fields such as radar surveillance, target tracking and ocean remote sensing. The principal achievements are listed in the following:1. An efficient multiregion model is extended with the single integral equation (SIE) method to calculate the EM scattering from a dielectric rough surface with or without a perfectly electric conducting (PEC) target above it. In the multiregion model, the rough surface is divided into multiple regions depending on their positions along the rough surface as well as the energy distribution of the incident wave. Two intermediate regions are chosen as the domain region on which the method of moment (MoM) is adopted. The new model can greatly reduce the number of unknowns compared with those of the full SIE analysis. This model can obtain accurate BSC in small and moderate scattering angle regions.2. This thesis has applied the aforementioned multiregion model to the composite EM scattering from a dielectric target and a dielectric rough surface. When the dielectric target, instead of a PEC target, is located above or below the rough surface, the situation will be changed. In this situation, the equivalence principle should be used three times, e.g. there are three equivalent models, one is equivalent to above the rough surface (excluding the target), one is equivalent below the rough surface, and the last one is in the target. This makes the equations more complicate.3. In order to describe the phenomena of sea spikes as well as the’super events’from realistic sea clutter, wedges are posited on the two scale sea surface to give a approximate imitation of the breaking waves. The scattering of the wedge and the multiple reflections occurring from the sea and the wedges are taken into consideration by ray tracing for large HH/VV ratio. With the technic mentioned above, the time sequence of the time varying sea surface is obtained. The spiking events are identified from the background. Then the sea clutter properties from the sea with breaking waves are compared with that of the sea. It turns out that the spiking events are more likely to advent in sea with breaking waves. The Doppler spectrum of sea with breaking wave has a good improvement for both polarizations. The results show that the spike events are more likely to happen under high sea state and high radar resolution. All these simulation results shows that the proposed scattering model is more suitable to simulate the sea clutter of lowing grazing incident angle and available to explain the physical mechanism responsible for the sea spikes.4. The present study focuses on the modulation of the scattering coefficient and of the Doppler spectrum in the presence of time-varying internal waves. The internal wave gradient facilitates the hydrodynamic modulation of rough capillary waves. In addition, this study adopts the wave-current interaction to modify the capillary wave spectrum. Then, the facet model is applied to calculate the scattering characteristics of a large-scope 2D sea surface, beneath which an internal wave travels. This model simplifies the sea surface into a two-scale profile. Long waves are locally approximated by a tilted facet that is considerably longer than the electromagnetic wavelength. Capillary waves are considered to be a set of sinusoidal ripple patches that superimpose on these long waves. The wavenumber of the sinusoidal waves must satisfy the resonant condition of Bragg waves that dominate the scattered wave component.5. After a careful examination on the traditional multi-path model, the author propose a new approach for dealing with the coupling scattering interaction according to the distribution characteristic of the specular-reflection scattering elements on the deterministic rough profiles. The reflect coefficient of rough surface in the traditional multi-path model is modified according the scattering of each facet of the sea. This proposed method has consided the coupling between target and each facet in a more reasonable way. Thus, the proposed method is more suitable for the calculation of the composite scattering from a target at a deterministic rough sea surface.