Research On Terahertz Radar Imaging Of Human Hidden Targets And Its Speckle And Polarization Characteristics

The terahertz wave has good penetrability to clothing,no ionizing radiation to the human body,which is one of the ideal working frequencies suitable for human body security imaging.The terahertz radar imaging system for concealed target detection is suitable for rapid detection of potential threats in crowded public places and has great application value in the field of public safety.However,human body and clothing clutter as well as speckle effects in the terahertz human security radar imaging results greatly affect the detection and recognition performance of concealed contraband.The lack of understanding of the imaging results of concealed targets has become one of the important obstacles restricting the terahertz human security radar imaging system from the laboratory to practical.In order to suppress the clutter and speckle effects in the terahertz radar imaging results,and to understand the information in the terahertz radar imaging results of the personnel concealed target,it is urgent to carry out research on terahertz radar imaging and its speckle and polarization characteristics for personnel concealed target detection.Based on the above research purposes,the main work and conclusions of this paper are as follows:(1)Imaging algorithms for terahertz array radar is investigated.A time-frequency domain hybrid method named range migration-back projection(RM-BP)algorithm is proposed for a near-field MIMO-SAR imaging system which uses one-dimensional sparse MIMO linear array scanning.The algorithm can realize the three-dimensional imaging focusing for near-field MIMO-SAR imaging system.In terms of computational complexity,it is one order of magnitude lesser than the traditional BP algorithm;in terms of imaging quality,the algorithm has almost the same imaging performance in distance dimension and MIMO dimension compared withtraditional BP algorithm.Due to the influence of frequency domain approximation,the algorithm has certain sidelobe performance loss than BP algorithmin the SAR dimension.The proposed imaging algorithm has certain advantages in practical applications for near-field MIMO-SAR imaging.(2)Terahertz radar imaging simulation is investigated.A near-field imaging simulation framework for complex array configuration is constructed.A near-field imaging simulation method based on near-field physical optics is proposed.The influence of the illumination characteristics of the array on the imaging results is analyzed,and 0.14THz based near-field MIMO imaging experiments is carried out to verify the effectiveness of the imaging simulation method.Based on the SIMO near-field imaging simulation method based on Multi-layer fast multipole algorithm,the imaging simulation of the target under the occlusion of the rough medium surface is carried out.The influence of the dielectric properties and rough texture of the rough medium surface on the imaging results are analyzed.(3)Terahertz holographic speckle model is investigated.A partially-developed speckle model suitable for the case of a small number of equivalent scattering centers is proposed.The model establishes the quantitative relationship among speckle contrast.Gaussian roughness rms height,correlation length and millimeter wave holographic imaging resolution under specular reflection.The effectiveness of the proposed model is verified by the imaging simulation method based on near-field physical optics.(4)Speckle suppression method in terahertz radar imaging is investigated.An angle-diversity based speckle suppression method for terahertz array radar imaging is proposed.The method is based on the de-correlation property of the angular rotation pattern on the speckle intensity pattern,quickly obtains the single-look imaging results under multiple correlation angles,speckle can be finally suppressed by image rotation correction and multi-view processing.The effectiveness of the proposed method is verified by a 0.34 THz based MIMO array radar imaging system.The method reduces the speckle contrast without deteriorating the imaging resolution,and effectively suppresses the speckle noise in the terahertz array radar.(5)Terahertz fully polarizedimaging for concealed target is investigated.A 0.14THz fully polarized radar holographic three-dimensional imaging system is developed,and the terahertz polarization scattering characteristics of the concealed target is analyzed.The low entropy scattering in the imaging results after the sub-aperture averaged illumination at different angles is analyzed.The main source of cross-polarized echoes of concealed targets is analyzed.