The Imaging Technology Of Terahertz FMCW-SAR

With the development of radar imaging technology,the demand for radar performance is increasing.In order to achieve more fine high-resolution imaging,to obtain more micro-motion characteristics of the target,in order to obtain higher frame rate of radar imaging,people have focused on terahertz SAR imaging.However,unlike microwave-band SAR imaging,terahertz SAR imaging is mostly used for near-field imaging.During the imaging process,a series of special problems which have not been encountered or have no impact on microwave-band SAR are found.In this paper,special problems found in terahertz FMCW-SAR imaging are studied and corresponding solutions are proposed.In the first chapter,the background and significance of the research are described.The main significance of the research is to provide theoretical basis and solutions for the special problems encountered in terahertz near-field SAR imaging to further promote the development of terahertz SAR.Then,the development of THz SAR imaging theory and radar system experiment at home and abroad is introduced.The background and current situation of this paper are provided.Finally,the content and structure of the paper are briefly introduced.The second chapter firstly introduces the structure and principle of terahertz radar system.The terahertz radar system mostly adopts FMCW system.When receiving the terahertz radar echo,it is impossible to sample the terahertz signal directly because of the limitation of the digital sampling module.Therefore,most of the terahertz radar systems adopt mode of chirping to get the terahertz signal.The intermediate frequency signal obtained by the target echo and the local oscillator reference signal is sampled to obtain the echo data.In this process,THz wideband LFM signal will produce a lot of nonlinearity.In order to obtain the focusing imaging results,the nonlinearity correction must be done in advance.Next,the nonlinear principle existing in the phase term and amplitude term is deduced theoretically,and the nonlinear correction method of the signal has achieved good results in the experimental application.Finally,aiming at the nonlinear problem caused by sub-harmonic coupling,a new multi-nonlinear correction method based on sub-harmonic suppression is proposed,which can correct the non-linearity caused by sub-harmonic and improve the signal-to-noise ratio of imaging results.In the third chapter,the basic principle of SAR imaging is introduced firstly,and the range Doppler imaging algorithm is deduced for the terahertz FMCW radar system imaging work.Finally,a special problem in the near field imaging of terahertz SAR is discussed: azimuthal distance and space variation.Compared with traditional microwave SAR imaging,terahertz SAR is mainly used in near-field imaging due to power limitation.Radar-to-target range is much smaller than the size of target itself in microwave band SAR imaging.With the addition of terahertz SAR,the frequency of azimuth modulation is a physical quantity which varies with the micro-range during azimuth focusing.It is not considered in SAR imaging.The classical range Doppler algorithm can solve the problem of azimuth airborne variation,but in actual imaging,due to the system and measurement errors,accurate estimation of the distance parameters of each range unit will consume a huge amount of calculation.Aiming at the problem of azimuth frequency modulation in close range terahertz SAR imaging,this paper presents an efficient Azimuth Doppler frequency modulation(Azimuth Doppler Frequency Modulation)imaging algorithm based on special display points,which can achieve high precision azimuth focusing without searching range parameters.It is a practical algorithm for close range terahertz SAR imaging.The fourth chapter mainly studies the error correction in terahertz SAR imaging.On the one hand,the correction errors include the errors of imaging parameters,which usually include the velocity of radar platform,the oblique angle of radar beam illumination,the imaging range parameters,etc.Each error will lead to the defocusing of imaging results.In the last chapter,the minimum entropy method,which is commonly used in terahertz SAR imaging,is introduced.In practice,the accurate values of imaging parameters are often searched through the entropy of imaging results.Besides the error of imaging parameters,the jitter of the radar platform will also cause the defocus of the imaging results.Unlike microwave SAR imaging,terahertz SAR imaging usually does not have inertial navigation system,and platform jitter can not be recorded for jitter compensation.And terahertz SAR is also very sensitive to small amplitude jitter,which can be neglected in traditional SAR imaging.It is also found that terahertz jitter is irregular and can not be compensated by traditional analytical modeling methods in SAR field.In this paper,a jitter autofocus imaging algorithm for radar platform based on phase center tracking is proposed from the perspective of ISAR imaging.The main flow of the algorithm is radar echo envelope alignment,echo initial phase correction,azimuth self-focusing,and finally imaging,which solves the special radar platform jitter compensation problem in terahertz SAR imaging.At the end of this paper,the content of this thesis is summarized,and every problem encountered in terahertz SAR imaging is analyzed and concluded.Finally,the work to be done in the next step is put forward.