Research On Synthetic Aperture Radar Imaging And Moving Target Indication At Terahertz Frequencies

Terahertz Synthetic Aperture Radar(THz-SAR)is the combination of THz and radar technology,which is widely adopted in military and civil applications due to its small size,high resolution,real time signal processing and high Doppler sensitivity.Especially,THz SAR has significant potential inground target detection from air board with high rate and perfectresolution.However,lack of target scattering mechanism leads to the difficulty for echo simulation,target character analysis,and target recognition.In addition,it is more difficult to get high quality of radar image with THz band because of the severe impact of moving platform and motion compensation.And the parameters estimation of moving target in THz band is influenced by the Doppler mixture of moving target echoes.Focusing the problems mentioned above,this dissertation investigates the establishment of THz-SAR simulation system,and some key techniques in imaging algorithm and moving target detection.First,system analysis and THz-SAR modelare studied.Based on the application background of THz-SAR,a set of radar parameters that meet the application requirements are designed,including imaging rate,frequency window,effect range limit,pulse repetition frequency,etc,which are the bases of system analysis and algorithm research carried out later.Meanwhile,restrictions between these parameters and effects of the adopted approximations are analyzed.THz-SAR suits for the near field detection,while itis not available to the far filed detection because the approximation of plane electromagnetic waves is not satisfied.For THz-SAR,the traditional stop-move-stop model cannot be used,and the elaboration analysis is requiredin real situation.Therefore,anapproximation of four equivalent transmit-receive phase center is proposed for the one transmit and four receive antenna system.The investigation finds out that the Fresnel Approximation is still tenable in THz band.A whole process model of echo signal simulation includingtarget geometric modeling,target's radar cross section(RCS)calculation,background scattering modeling,background's RCS calculation,fast echo signal generation and signal synthesizing is constructed and became the solid foundation of the following researches.Secondly,stationary target imaging and motion compensation of THz-SAR are studied.A two-dimensional high resolution principle is analyzed.To deal with the high sampling problem,sparse recovery algorithms are applied to THz-SAR imaging.Motion compensation of position and phase error are achieved by using iteration algorithm to reconstruct target scattering coefficients,as THz-SAR imaging is sensitive to the movement of platform.The accurate estimation and compensation of airborne movement and signal phase are essential for high resolution radar image,which is solved by grads optimization method with good convergence.For THz-SAR high frame rate video imaging,the spot target under strip mode is analyzed,and vehicle borne imaging experiment is verified.Thirdly,motion detection and imaging of THz-SAR are studied.The model of constant acceleration moving target and vibration target are established,and the influence of target motion to echo phase and imaging is then analyzed.For moving targets,the influence to the item in the expanded form of target range formula varies with the Doppler effect caused by different directionmovement as well as radial velocity and acceleration in the range Doppler and azimuth Doppler,which leads to mismatch and vague for imaging.For vibration targets,the mixture of Doppler frequency makes it difficult to extract targets' information.In experiments,different vibration parameters of tanks are investigated.Vibration in the radial direction leads to the vagueness of SAR image,while vibration in the azimuth direction does not influence the Doppler frequency significantly.For the method of hybrid DPCA and ATI technique,motion detection and parameter estimation are implemented.Compensation method based on phase focusing is proposed,and the precision of moving targets' radar imaging in THz band is enhanced.Finally,the parameterized THz-SAR moving target imaging method is studied.A uniform detection and imaging model based on sparse recovery technology is established for three movement modes of targets,which are constant acceleration moving mode,mono frequency vibration mode and double frequency vibration mode,respectively.In the experiment,by comparing with several recovery methods,the expansion compression variance component based method could estimate the position of scattering point and moving targets' parameters simultaneously.The radar imaging resolution is enhanced,and the quality of resisting noise is improved.