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Research On Imaging Algorithms Of Terahertz Radar

Posted on:2016-02-02Degree:DoctorType:Dissertation
Country:ChinaCandidate:B ZhangFull Text:PDF
GTID:1108330473456120Subject:Signal and Information Processing
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The research on the electromagnetic properties and potential applications of terahertz wave is a frontier in science and technology field. Using the high spectrum resolution capability of terahertz wave, the material compositions can be determined. Due to the low photon energy, terahertz wave can be used in the security checks of human body and detections of biological sample with harmlessness. Because it can penetrate the non-polar and non-metallic materials, terahertz wave has promising applications in perspective imaging. In these applications, the shape recognition and composition identification based on the terahertz imaging technology are research focuses in terahertz wave field. Due to the penetrativity and large bandwidth of terahertz wave, the combination of terahertz wave and synthetic aperture radar imaging technology can achieve a high resolution imaging of target, and building a real time, high resolution and penetrating imaging system is an our ultimate goal. This system can be used in the applications of battle reconnaissance, disease diagnosis, security checks of key area, nondestructive test and so on. Thus, this dissertation researches the synthetic aperture radar imaging techniques using the terahertz wave radar. This research can improve the home-developed terahertz radar capability in China, and promote the development of terahertz radar high resolution imaging technology.This dissertation studies key issues of terahertz imaging in conditions of far field and near field. Imaging principles, echoes models and imaging algorithms of the inverse synthetic aperture and circular synthetic aperture modes are focused to solve problems of high resolution two imaging and three imaging of terahertz wave radar.The main contents of this dissertation are summarized as follows.1. After analyzing the traditional radar system structure and investigating the development level of electronic device operating in terahertz wave, two schemes for terahertz radar system structure are proposed. One scheme is based on the single frequency source and the other uses two frequency sources to drive transmit chain and receive chain respectively. The structural differences among three radar systems are compared. The coherent reception of the two sources driving scheme is analyzed and proved in theory. Because the non-linearity of linear frequency modulation signal in terahertz radar system results in the resolution degradation, compensation for non-linearity signal is necessary. In this dissertation, we propose a method to solve this problem. The results of simulations and experiments show its validity. Besides, the capability of phase noise cancellation of the radar system is expounded and the suppression level of phase noise is also given. Combined with characteristic of dechirping reception, the theoretical resolution of terahertz radar system in range is illuminated. The range resolution of the terahertz radar system is validated by experimental results.2. For analyzing the imaging methods in different condition conveniently, the theory of radiated fields of radar antenna is introduced. The radiated region of an antenna is separated the reactive near-field, radiating near-field and the far-field. The distance of each region to the antenna is discussed. The boundary of near-filed region and far-field region in synthetic aperture radar imaging is given. The characteristics of near-field region and far-field region of synthetic aperture radar imaging are listed and compared with each other.3. The 2D terahertz imaging methods based on the inverse synthetic aperture in far-field condition are proposed. By means of the approximation of range which is the distance between radar and target, the echoes model can be simplified into a simple expression. Three algorithms for terahertz inverse synthetic aperture radar imaging are proposed based on the simplied radar echo. The feasibility and validity of these algorithms are validated by simulations. And the capabilities of these methods are also compared and analyzed. To achieve 3D high resolution imaging, the approximation methods for range from radar to target are studied. Through analyzing the range approximate expression, we found the change law of compressed data. The algorithm based on the Radon transform and CLEAN technique is proposed. The simulation results validate the validity of the algorithm.4. The two dimension and three dimension imaging algorithms in near-field condition using terahertz radar system are researched. Through analyzing the characteristics of inverse synthetic aperture radar near-filed echoes in time domain and frequency domain, derivation of back-projection method is given and the near-filed imaging method based on Green function decomposition is proposed in detail. The simulation and experimental results validate the feasibility of the proposed algorithm. Compared with the back-projection method and double integral algorithm, the efficiency of the method based on Green function decomposition is higher than others. Based on the analyzing results for circular synthetic aperture radar echo, the algorithm based on spherical wave decomposition theory is proposed. Using the theory of spherical wave, the wave can be decomposed the superposition of plane-wave components. This step decreases the reconstruction difficulty of algorithm. The differences of reconstruction procession among back-projection algorithm, focusing factor algorithm and the method based on spheric wave decomposition theory are conpared. The validity and feasibility of three algorithms are validated by simulation results.
Keywords/Search Tags:Terahertz(THz) radar, Near-field imaging, Far-field imaging, Inverse synthetic aperture radar(ISAR), Circular synthetic aperture radar(CSAR)
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