Theoretical And Numerical Studies On HF Scattering Of Targets On Or Above The Sea Surface

This thesis puts emphasis on studying ground wave scattering of targets located near the sea surface. The sea surface can be seen as either a planar interface or a curved interface depending on the distance between the antenna of radar and the target. The effect of rough sea is also taken into account. In this thesis, we study both the theory and computational technique for ground wave propagation and scattering. The Main contributions of author's work are as follows: (1) The application of FDTD method to solve a half-space scattering problem is firstly investigated. A constrain condition for analytically introducing an inhomogeneous or non-plane wave into the FDTD computation is proposed. Both Mur absorbing boundary condition(ABC) and generalized perfectly matched layer(GPML) for half-space lossy media are studied. A formula suitable for extrapolation of near-to-far field is derived based on the Green's function and reciprocity theorem respectively, and the limitation of its application is also discussed. (2) By use of the Barrick's theory for ground wave propagating along the rough sea, a numercial algorithm of evaluating the integral with a normalized effective surface impedance is studied. Also a modification of Rotheram's ground wave formula is introduced. Considering the effect of rough sea, a program called GRWAVEI which may compute the electric field strength and phase has been written. These algorithm are then applied to calculate the ground wave attenuation factor. (3) An approximate scheme named as the scattering-propagation separation method (SPSM) for analyzing the ground wave scattering of targets on or above the sea surafce is presented. This method is used to calculate the monostatic RCS of a ship on the sea, and the results are in good agreement with that obtained from Barnum's semi-empirical formula and the measeured data. A simulation study of interaction between the rough sea and targets is also carried out. (4) To overcome the difficulty of directly using the FDTD method to. analyze the scattering of the low-height target above the sea surface, a simple and effective approach named as the first-order reflection approximation(FRA) is presented in this thesis. The back-scattered fields of a target above the sea surface can be obtained easily if the bistatic scattered fields in free-space can be calculated. Furthermore, by using an equivalent dipole(ED) deduced from an object on a specified height, the reflected wave for an object on different heights can be computed analytically. Numerical results show the feasibility of FRA and ED. Combined with SPSM, FRA, ED and FDTD, the RCS of an airplane at different heights above the sea surface is calculated successfully. (5) With the reciprocity theome and the known ground wave propagation formula of a vertical electric dipole, such as King et aI. formula for a flat earth and Rotheram's residue series for a curved earth, a scheme of FDTD near-to-far zone transformation for ground wave scattering from targets on the sea surface is pesented. Numerical results show excellent agreement between this scheme and analytical results. This scheme provides an effective and strict means for testing the computation precision of SPSM.