Study On The Application-Oriented EM Scattering Model For A Marine Environment

The active microwave remote sensing technology plays an important role in marine environmental monitoring and target detection and recognition.The further study on the composite scattering of complex ships in a marine environment can benefit the understanding of radar echo and image characteristics,which could provide theory and data support for target detection and recognition.Therefore,this study has always been a research emphasis in the field of microwave remote sensing.Facing on the practical engineering applications,this dissertation has investigated the scattering modelling and characteristic analyses of a composite marine scene with ships.The main contributions are as follows:Based on the idea of the scattering method simplification and the geometry modeling,two different methods are proposed respectively to study the scattering from electrically large sea surfaces at microwave frequency bands namely the facet-based simplified small slope approximation(FBS-SSA)and the geometry modelling based on the spectral decomposition and the matrix decomposition(SD/MD).Regarding the FBS-SSA,the main idea is to simplify the original first order small slope approximation based on the specular reflection mechanism and the Bragg scattering mechanism.This method breaks the limitation that the facet size should not be larger than one-eighth of the wavelength,which makes it able to analyze the scattering from large sea surface efficiently.In addition,the calculation accuracy is verified by the comparisons of numerical results.As for the SD/MD method,in the generation of a sea surface,the whole wave spectrum is divided into the highfrequency part and the low-frequency part.Then when generating the large-scale gravity waves on the base of the low-frequency spectrum,the two-dimensional inverse fast Fourier transformation(IFFT)can be decomposed into two one-dimensional IFFT operations.Due to the smaller size of the matrix corresponding to the low-frequency spectrum,the memory consumptions can be significantly decreased.Besides,the generation of small-scale waves corresponding to the high-frequency spectrum can be carried out in different areas.The surfaces generated with the proposed method have the same spectral characteristics and scattering characteristics with those generated with the original method,and the numerical results show that the SD/MD method not only ensures the calculation accuracy,but also significantly reduces the memory consumption without reducing the computational efficiency.The two methods lay the foundation to the analyses of scattering from electrically large rough sea surface at microwave frequency bands.Compared with the single polarized echoes,the full-polarized radar echoes contain much more abundant information.Thus,the analyses of full-polarized scattering from sea surface are of great value in the remote sensing of a marine environment and the target detection.For this,based on the second order small slope approximation,the two-scale model and the Bragg scattering mechanism,this dissertation has proposed a full-polarized facet based scattering model(FPFSM)to investigate the full-polarized scattering from sea surface with much lower memory consumption and much higher efficiency.Under high sea conditions,the Bragg scattering mechanism is difficult to explain the high HH/VV ratio and sea spikes phenomena at large incidence angle because of the existence of breaking waves.Therefore,this dissertation has built a scattering model,which is able to analyze the scattering from sea surface under high sea states,by considering the effect of breaking waves.For the proposed method,the geometry structure of breaking waves is obtained by simplifying the real structure without losing the main scattering sources.The spatial distribution is obtained with the slope criterion,which is proved to have a good consistence with the measured coverage,to ensure the reliability of the proposed model.Moreover,the effect of wind speed on the distribution of breaking waves are considered,therefore,the proposed method is able to analyze the scattering characteristics and sea clutter in both low and high sea states.To analyze the scattering from sea surface with targets,a model combined with the FBS-SSA and the geometrical physics and optical physics(GO/PO)method is proposed,in which the coupling scattering between the sea surface and the targets are taken into consideration so that the scattering field can be calculated more reliably.In addition,due to the facet-based idea of this method,it can be used to simulate the SAR echo and highresolution images.On this basis,combined with the geometry of breaking waves and the scattering model,the scattering characteristics of a composite ship-ocean scene under high sea states are analyzed by considering the coupling path between breaking waves and the target,and the effects of breaking waves on the coupling field and the total field are explored.Aiming at the scattering problem of a real marine scene with multi-ships,an improved method based on the FBS-SSA and GO/PO hybrid method is proposed.Regarding this method,a more efficient and usable method is proposed based on the FBS-SSA to analyze the scattering field from sea surface.Then for the coupling field,based on the radar parameters and the size of the targets,the sea surface is divided into the coupled region and the non-coupled region to reduce the amount of occlusion tests and improve the calculation efficiency,which lays the foundation for the analysis of the composite scattering characteristics of a very large sea surface with multi-ships.For a moving ship target in a marine environment,the SAR image of the ship is often offset and defocused due to its motion.To eliminate the effects caused by the movement of the target,based on the truth that the ship wake exists behind the ship with long time and large area,and is related to the motion characteristics of the ship,this dissertation has inversed the velocity of ship with the wakes and further put forward a method to correct the SAR image.The corrected SAR images have better focusing effect than the uncorrected images,thus the proposed method could promote the development of the detection,classification and recognition of ship targets in a marine environment.