Investigation Of THz Scattering Features Of Electrically Large Targets With Rough Surfaces

Terahertz region(THz)locates between microwave and far infrared bands,which has not been investigated completely.Since the vibrational and rotational energy levels of many materials locate at THz bands,the terahertz spectrums contain abundant of physical and chemical information.At the same time,most of the non-polar materials are transparent at THz band,while polar materials have strong absorbance to THz electromagnetic radiation.Thus,as a newly developed nondestructive detection,THz detection is also a complementation technology to the Fourier transform spectrums and X-ray.In recent years,with the development of ultrafast laser technology,THz radiation and detection are greatly developed and extensively applied in the realms of industries,military,biological and medical investigation.At present,THz technology can be divided into two fundamental items: the study of material properties by using THz pulses detection,and THz radiation imaging.The requirements also promote the development of THz scattering characteristics of different materials.The THz dispersion properties of different metals and dielectrics are analyzed through the measurements of THz transmissivity and reflective spectra,and the dispersion models are established in this paper.The THz bid-directional reflection distribution functions of random rough surfaces are obtained by using the optimization algorithms.According to the optical methods,the THz scattering models of electrically large targets with rough surfaces are developed to illustrate the THz coherent and incoherent scattering mechanisms.In the meantime,the THz induced contraction relationship of rough non-metal targets is also investigated.Then the THz one-dimensional range profiles and intensity imaging are also proposed in this paper.Based on STK platform,the THz-RCS computation software for active targets in the space is developed.The backscattering RCS and intensity images of active targets are real-time monitored.Meanwhile,the ISAR images of simple targets are preliminary obtained based on optical methods.The research provides theoretical foundation for the THz scattering detection,extraction and recognition of space targets,THz compact experiments of scaled targets basically.The main achievements are as follows:By using the Vertex 80 V Fourier spectrometer,the transmissivity and reflective spectra of metal/dielectric surfaces with different roughness are measured.The effects of surface roughness,thickness and material type are analyzed.According to the Kramers-Kronig relationship of the amplitude and phase of THz electrical field,the terahertz complex dielectric parameters are inversed by using the experiments of far infrared transform spectrometer and ellipsometer.Combined with Drude model and Huangkun-Boen model,the THz refraction spectrums of metals and dielectrics are also analyzed.In terahertz band,most of the targets can be viewed as dielectric targets,which have different scattering and imaging features compared with smooth conductors.In the meantime,the real targets have larger size than incident wavelength,then the electrically large targets will bring great difficulty to the traditional EM algorithm.Thus the THz scattering investigation of electrically large targets with rough surfaces is important.The scattering features of rough targets are composed by coherent and incoherent components.In the optical realm,the coherent scattering components come from the neighborhood of its specular points,and the incoherent scattering is aroused by the rough surfaces.To the plane wave of incidence,if the radius of curvature at each point is larger than the wavelength,and the root-mean-square height of rough surface is a few nanometers or microns,or the rms of surface slope is much smaller than 1,the coherent and incoherent scattering features of large rough convex targets can be observed by using Physical Optics,including Kirchhoff approximation and method of stationary phase.The bistatic and backscattering RCS of simple targets such as sphere,cylinders,and cones are numerically simulated.The effects of the geometrical size,surface roughness,dispersion parameters,frequencies and polarization on the coherent and incoherent scattering features are analyzed.Based on the scaler approximation,the target is discretized using triangular patches,the coherent and incoherent scattering features of each rough patch can be investigated,and the information of amplitude and phase of scattering field is simulated,then the incoherent RCS is numerically obtained.Combined with the analytical RCS solutions of smooth targets from Mie theory and lambert spheres,the coherent and incoherent scattering component can be calibrated.Bid-directional reflection distribution function(BRDF)can present the scattering and radiation properties of rough surfaces.BRDF illustrates the scattering energy distribution of the upper space when the radiation wave incidents on the rough surface in a certain direction,and it depends on the surface roughness,dielectric parameters,wavelengths,polarization,and incident and scattering directions.Based on the surface roughness and inversed complex refractive index of metal and non-metal surfaces,the backscattered scattering distribution are obtained according to the scattering theory of random rough surface,and the BRDF in different incident directions are achieved theoretically.Combined with the artificial immune network algorithm,the optimized THz-BRDF models are established,which will provide great support for the simulation of THz RCS of electrically large targets with rough surfaces.In this paper,the THz-RCS computation of complex rough targets based on the BRDF model are finished.The THz-RCS of spheres,cylinders,cones,cubic and almonds are computed,the surface materials contain metals and nonmetals.The effects of surface roughness,dielectric properties,shape and incidence direction on THz-RCS are analyzed.Combined with the established THz-BRDF models of rough surfaces,the one-dimensional range profiles(RPs)can be obtained when the incidence impulses irradiate on arbitrary rough target.In this paper,the one-dimensional range profiles of different targets with different surface materials and roughness are simulated,including sphere,ellipsoid,cone,cylinder,ogive cylinder and cuboid assemblage.The effects of surface roughness,materials and incident pulse-width are analyzed.And the differences of THz-RPs and Laser RPs are compared.To the sphere and cones,the surface is analytical and the range profiles can be numerically deduced.While to the arbitrary targets,the panel method can be invited.In this paper,set the cylinder as calibrator,the RPs of cylinder based on panel methods are simulated,which coincide with its analytical simulation.Then the RPs of ogive cylinder and cuboid assemblage are obtained,and the effect of incident direction is analyzed at the same time.Based on the 1-D range profiles of cones,the half cone angle and cone height can be inversed in great precision.In this inversion method,a radar array should be established to detect the RPs at certain times;then according to the coordinates of the peaks,the convergent function are established to inverse the cone height and half cone angle.The method can improve the recognition efficiency without any matching database.According to the relationship of BRDF and RCS,the three-dimensional THz intensity images of almond and ogive cylinder are obtained.Combined with STK software and the scattering model of active targets,the visual computing software for THz-RCS and 3D images are developed.The procedure finishes the transformation between target coordinate system,launching coordinate system and geocentric coordinate system.According to Kirchhoff approximation,the root-mean-square(rms)slope can be deduced based on rms height and correlation length,the scattering field can be illustrated by the integration of polarization factor.Then the scattering field distribution at certain incidence angles can be achieved by utilizing Fourier transform,and the further investigation on the amplitude and phase of scattering field can be made theoretically.In this paper,the ISAR images of rough square and circular plates are simulated,the results are in coincidence with the electromagnetic method.Since the practical region in THz experiment is limited,the scattering features in the total THz region are difficult to be investigated.The compact theory in THz region of typical targets is analyzed.In the theory,the frequency,surface roughness and dielectric parameters are kept in constants,the relationship of backscattering RCS and geometrical size are analyzed.Combined with the Physic Optics and stationary phase method,the geometric contraction relationship of spheres,cylinders,cones,cubic and almonds are numerically investigated,and the feasibility of the compact theory is verified.In the meantime,the effects of surface roughness and frequencies are explained,which will provide theoretical support to the compact scaled measurements of THz scattering.