Research On Target Imaging Techniques Of Electromagnetic Vortex Radar

Vortex electromagnetic(EM)wave is a kind of EM wave with unique time-space distribution,which has a twisted helical wavefront.The radiation fields at different positions in the beam have spatical diversity,which provides the physical basis for target detection and resolution.Based on this characteristic,the newly developed EM vortex radar imaging is capable of acquiring target azimuth resolution,which is expected to overcome the challenge of high resolution imaging in forward-looking or staring observation geometry.Based on EM vortex imaging principle,this thesis focuses on investigating the scattering characteristics of vortex EM wave,the imaging resolution and some key issues for radar imaging application,expecting to provide theoretical support for the practical application of EM vortex imaging techniques in real scences.Firstly,the background and siginificance of this thesis are introduced and the current research status are presented.Some fundamental problems in EM vortex imaging are summarized on the aspects of the imaging physical basis,the imaging principle,and the imaging method.Radar target scattering characteristics are the foundation of radar imaging.When exploring vortex EM waves for radar detection and imaging application,the scattering mechanism of the vortex EM waves must be investigated.Based on the propagation characteristics of vortex EM waves,the interaction process between vortex EM waves and targets is analzed firstly.A calculation method for target scattering field is proposed,the analytical expressions for the scattering field of typical targets are derived.The scattering characteristics and influence factors are analyzed furtherly,including the intensity characteristics,phase characteristics and orbital angular momentum(OAM)spectrum distributions.The modulation characteristics of the OAM spectrum in the target scattering field are revealed.Subsquently,the analytical expressions of radar cross section(RCS)are derived,and the monostatic and bistatic RCS distributions are studied.An EM simulation method of vortex EM scattering characteristics is proposed,and the simulation conclusions verify the validity of vortex EM scattering theory,which provide theoretical support for OAM-based radar detection and imaging.The resolution is a basic index to evaluate the performance of radar imaging performance.The characterization of EM vortex imaging resolution is a fundamental problem in EM vortex imaging technology.Starting with the basic principle and imaging mathematical models of EM vortex imaging,the intrinsic resolution and practical resolution are studied by using spatial ambiguity function and point spread function(PSF).The nominal resolution limit and practical resolution limit are obtained.The influence factors of azimuth resolution and spatial resolution are further analyzed,which provides theoretical guidance for the design of EM vortex imaging radar.Finally,EM vortex imaging experiments are carried out,and the experimental data processing results verify the performance of EM vortex imaging and resolution theory.EM vortex imaging needs to obtain the target echoes with different OAM modes,which usually requires a certain amount of time observation.In order to solve the problem of low irradiation efficiency of existing methods,an EM vortex imaging method based on OAM multiplexing beams is proposed.Firstly,the generation of OAM multiplexing beams is studied.The mathematical model of dual coupled OAM beam is established and the radiation characteristics and influence factors are analyzed.Then the EM vortex imaging method based on dual coupled OAM beams are studied under two operating modes: “Multiple-In Multiple-Out(MIMO)” and “Multiple-in Single-Out(MISO)”.In the “MIMO” mode,a circular array is used to demodulate the multiplexing OAM mode,and the imaging methods based on azimuth filtering and signal cancellation are proposed.In the “MISO” mode,an imaging method based on the Hilbert transform is proposed.The simulation results show that compared with the existing EM imaging method,the proposed approaches can achieve the same resolution with half irradiation time.Finally,an EM vortex imaging experiment using dual coupled OAM beams under “MISO” mode is carried out in a microwave anechoic chamber,and the experimental results verify the effectiveness of the proposed method.In order to solve the problem of poor imaging quality in noise environment with limited OAM modes,an EM vortex imaging method based on fractional OAM beam is proposed.The generation method of integral order vortex EM wave is extended to fractional OAM beam.An array-based fractional OAM beam generation method is proposed,and the radiation characteristics and influence factors are analyzed.Then the mathematical imaging model is established and the relationship between the number of fractional OAM modes and the improvement of imaging quality is derived.The ideal point target imaging and the extended target imaging simulations are carried out to demonstrate the improvement of the imaging performance quantitatively.The relationship among the imaging quality,signal-to-noise ratio(SNR)and fractional OAM mode interval is acquired,which benefits the parameter design of EM vortex imaging radar.At the end,the work of this thesis is summarized,the development trend and research directions of EM vortex imaging techiques are pointed out.