The Study On The Time Domain Near-field Composite Scattering And Imaging From Large Target And Sea Surface

At present,with the increasing working frequency of radar and the wide application of wideband radar,the study of near field scattering is not limited to the experimental measurement.In practical application,especially in radar seeker guidance and fuze echo analysis,the time-domain near field scattering characteristics of target and background should be studied deeply.Based on the time-domain electromagnetic scattering theory,the time-domain bouncing ray method under near-field beam irradiation is studied in this paper,and the radar imaging method based on time-domain scattering echo is also studied.The main research contents of this paper are as follows:Based on the time domain Maxwell equations,the surface integral equation of electromagnetic field is derived,and combined with physical optics approximation,the surface integral expression of time domain physical optics(TDPO)is given.Using the system response analysis method,the expression of time domain plane wave under impulse excitation is given.A time-efficient method for calculating TDPO scattering near-fields under plane wave incidence is presented by introducing the local Green function approximation method.The closed expression has the same concise form as Gordon formula,which is applicable to the calculation of the near field scattering of arbitrary polygonal slabs,and is independent of the time domain pulse width,thus achieving the fast calculation of the TDPO scattering near-fields.Based on the antenna pattern,the time-domain radiation field expression of the antenna is derived.Two acceleration methods for TDPO near-field integrals are given: the time domain near field line integral technique and the surface element method based on local far field approximation.For line integral technique,a TDPO line integral expression is proposed for calculating the backscattering near-field from a conductive target radiated by an ideal dipole source.The surface integral is reduced to a line integral by a strict analytical procedure and the calculation time is accelerated from the square of the electric dimension to the linear one.In the surface element method,the TDPO near-field integral representations are reduced to closed-form expressions to improve computing efficiency by applying locally expanded Green function approximation.The closed-form expression can be used to calculate the scattering field at any position under the beam incidence of any antenna.In order to make up the defects of TDPO and improve the simulation accuracy,a time domain shooting and bouncing rays method(TDSBR)based on ray tracing theory is developed.In the process of tracking the reflected field value,we propose an equivalent mirror antenna method to replace the traditional GO.Different from the plane wave incident situation,in the case of antenna radiation,we obtain the incident fields of each individual facet in the ray path by solving the radiation fields of the equivalent mirror antenna.Since the radiation fields naturally contains the time delay term,the complicated processing of the time delay problem can be avoided by solving the radiation fields from equivalent mirror antenna.The scattering fields of each face can be evaluated by using TDPO proposed in last section.In addition,the forward and backward tracing technique is introduced to determine whether the adjacent triangle of the irradiated triangular face element is illuminated,and kd-tree technique is used to improve the ray tracing efficiency.TDSBR method is used to establish the simulation model of composite electromagnetic scattering echo between target and sea surface.A TDPO closed-form expression for calculating the transient scattering field of a medium target is proposed.The ray-tracing technology proposed in chapter 4 is used to track the composite scene composed of target and sea surface.The multiple scattering field between the sea surface and the target is considered as coupling field.The corresponding TDPO method is used to calculate the transient scattering field for different material types(conductors or media)and the total scattering field is obtained by vector addition.Considering the time-domain pulses emitted by radar in real scenarios,we propose an inverse synthetic aperture radar(ISAR)imaging method using time-domain scattering echo simulated by TDSBR method.The demodulation,pulse compression,and azimuth inverse fast Fourier transform(FFT)are performed on the echo data matrix to obtain the ISAR image under small bandwidth small angle.In addition,for simulation calculation,the wide time width of LFM pulse will lead to too much computation time.In order to improve efficiency,we improve the matching filter to use the modulating gaussian pulse as excitation.Simulation results show that this method can improve the efficiency while ensuring the image quality.