| Situated between the infrared and the microwave region of the electromagnetic spectrum,Terahertz has attracted recent interset in discipline of imaging within engineering and science.In the past two decades,various THz imaging techniques have been developed.Notable developments include radar synthesis imaging,tomographic imaging and holographic imaging.However,each type of THz imaging is only good for well-defined scenario with several assumptions.These assumptions restrict the applicability of these THz imaging techniques.To extend the applicalility of these THz imaging techniques,it is necessary to study the mechanism of THz radar imaging and propose new THz radar systems and imaging algorithms.Combining the related projects during the period of pursuing the doctor's degree,this dissertation does the deeply researches on the following aspects:(1)The analysis of the relationship between radar,tomographic and holographic imaging is refined.First,the mechanisms of the three imaging disciplines are investigated based on the inverse scattering theory.Moreover,the mathematical similarities of echoes between the three imaging disciplines are researched in the wave-nuber domain.Finally,the differences of targets between the three disciplines are studied.(2)The terahertz synthetic aperture radar is investigated.First,to compensate for the degradation imparted by the nonlinearity in both terahertz power amplifier and the terahertz SHM down converter,a calibration algorithm is proposed to improve the range resolution of terahertz radar.Moreover,the target scattering characteristics at terahertz frequencies are introduced.Almost all the observed targets at terahertz frequences are anisotropic targets thus it is not necessary to carry out the coherent processing within the whole aperture.A sub-aperture uncoherent imaging algorithm which has higher efficiency is proposed.Finally,a summary of a 0.14 THz inverse synthetic aperture radar is presented.And the prediction capabilities of the sub-aperture uncoherent imaging algorithm are proved to be feasible and effective at terahertz frequences.(3)The terahertz doppler tomographic radar is investigated.Doppler tomography radar uses the Doppler profiles of a rotating target to obtain the tomography imaging,which can achieve very high spatial resolution(up to 1/4 wavelength).This is very attractive in terahertz range because potentially millimeter or sub-millimeter resolution can be achieved with single-tone signals.However,after the early work in microwave range,there is no experimental result in terahertz range can be found in public literatures.In this paper,the Doppler radar tomography is investigated and the theoretical spatial resolution is analyzed.Then the algorithm of the Doppler radar tomography based on convolution back-projection method is proposed.Finally,some measurement results of imaging using 662 GHz single-tone signals are presented.(4)The terahertz holographic radar is investigated.Holographic radar is well suited for the detection of concealed weapons or other contraband carried on personnel since radio waves are nonionizing,readily penetrate common clothing material,and are reflected from the human body and any concealed items.Despite the theoretical potential of holographic radar to provide a useful detection technique,this is unlikely to be possible in crowd circumstances.The imaging tends to be blured by the combined effects of multi-targets and target-motion.This paper proposes a algorithm of holographic radar imaging based on range-doppler concepts,which has some advantage in multi-moving target compensation and rapid imaging.According to the symmetry between the time-sampling and the space-sampling,the process of deducing the range-doppler holographic radar imaging algorithm is also proposed in this paper,and simulations and examination results are given to show that this algorithm has the ability to get the multi-targets image rapidly. |
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