Research On Key Technology Of Target Scattering Characteristics At Terahertz Frequencies

In recent years, the continuous development of terahertz science and technology has promised that terahertz radar has significant meaning in many application fields. As a common basis for the issue, scattering characteristics of targets are not only the foundation of demonstration, design and practical application of terahertz radar systems, but also the premise of giving birth to the the new terahertz radar system. Presently, it is an urgent and important task for mastering target’s scattering characteristics at terahertz frequencies. However, because the terahertz band is just between the microwave and the optical frequency, the material response mechanism of targets is unknown at terahertz frequencies, the influence of target’s surface fine structures which can be compared with the wavelength on terahertz scattering is unclear, the effectively scattering predictions and experimental measurements methods are poverty. The existence of these problems challenges the understanding of scattering characteristics of targets at terahertz frequencies and hinders the application of terahertz radar system in reality. Learning from the research tools and methods of scattering characteristics at microwave and optical frequencies, it may provide some ideas and inspiration for the research of scattering properties of targets at terahertz frequencies.In this paper, in view of the interaction mechanism between the targets and terahertz waves and the factors which affecting the scattering mechanism, the researches of target’s scattering characteristics are carried out at terahertz frequencies.First, considering the different material response mechanisms and ways at terahertz frequencies, an effective model of dielectric function for the typical metallic materials is established through the analysis and discussion of microscopic mechanism. Based on the model of dielectric function, the influence of frequency dispersion of metals on the terahertz scattering characteristics is investagated according to the electromagnetic calculation method which solved by the impedance boundary condition. Meanwhile, precise scattering solutions for the polished metal sphere are proposed for dealing with the large dielectric constant of metals at terahertz frequencies. Accordingly, radar cross section(RCS) curve of the polished metal sphere is obtained from the microwave band to the optical band. The simulation results show that the frequency dispersion has little effect on the RCS of the metallic targets at terahertz frequencies and the normal incidence reflectivity of metallic materials at terahertz frequencies still approaches to unity. The exact solution of sphere scattering can also be used to analyze the scattering properties of a polished sphere of arbitrary material.Secondly, considering the detailed rough structure on target’s surface which is compared with the wavelength of terahertz waves and its signifacant impact on the scattering, scattering characteristics of targets at terahertz frequencies are reasearched by methods of precise geometric modeling, precise electromagnetic calculations and scattering modeling of rough surface, et al. Research is began from simple rough standard targets and the geometry modeling method for the rough standard target is first proposed. The influence of different roughness of rough surfaces on the scattering of the target is evaluated by the numerical calculation methods and the Monte Carlo simulation method. From the full-wave approach view, the coherent scattering and incoherent scattering modeling for the rough metallic sphere are studied. The RCS of a rough metallic sphere is given from the microwave band to the optical band when both the frequency dispersion of metals and the rough surface structure are considered, the simulation results show the intuitive and quantitative phsical concept for the scattering of metallic sphere and explain the enhancement of laser back-scattering for a rough aluminum sphere.Thirdly, after understanding the influence of materials properties and rough structures on the scattering of targets at terahertz frequencies, the conresponding novel methods of scattering modeling and calculation are proposed for real rough convex targets at terahertz frequencies. For the geometric modeling of rough targets, the usual fine facets split way will generate a large mount of facets and make the solving of scattering calculation problem impossible. Here an efficent method which combines deterministic modeling and statistical modeling is proposed and it paves the way for scattering calculation of rough targets. For the realization of scattering calculation, the Kirchhoff approximation theory and full-wave approach theory are introduced and can give the scattering field of real rough convex targets. The proposed scattering prediction methods are verified by the simulations of standard targets and can become effcient tools for scattering characteristics understanding of targets at terahertz frequencies.Finally, the techniques for scattering characteristics analysis of targets and the terahertz radar imaging method are researched. The high resolution of terahertz radar imaging brings great advantages for scattering characteristics analysis of targets. So aiming at the high cross-range resolution of terahertz radar, the azimuth-elevation imaging method is proposed based on the observation within a two- dimensional(2D) rotating angles. This imaging scheme can give an image of the target which is similar to the target’s photograph in 2D Doppler plane.Then it is very useful for scattering center diagnosis and analysis. Facing the increasing number of scattering centers and the appearance of cluster and block distribution for terahertz imaging results, the compressed sensing is introduced to the terahertz radar imaging and the CS-based terahertz radar azimuth-elevation imaging method is proposed based on block sparse recovery techniques. In order to deal with the uneven distribution of block size in actual radar image, the block sparisity signal and the block coherent with the uneven block partition are defined anew. The simulation results show that the image reconstruction performance is improved obviously when the special block structure of signals is exploited. For the common bistatic configuration implementation of terahertz imaging radar presently, a bistatic terahertz radar azimuth-elevation imaging method is proposed and it can be used to obtain the results of bistatic imaging and analyze bistatic scattering chanracteristics of targets. Meantime, the super-resolution image reconstruction is realized with only small number of measurements through the introduction of compressed sensing theory. The simulation results based on electromagnetic calculations data verify the ability to obtain the super-resolution images of the tank at different bistatic angles. Last, on behalf of short wavelength of terahertz waves, the multi-views real array imaging method for terahertz radar is proposed which combines the concept of real array and synthetic aperture imaging. The simulation results show that this imaging scheme has an advantage on improving cross-range resolution and it implies the application in the area of security and stand-off imaging for terahertz radar.