Object Information Extraction Based On Polarization Characteristics Of Millimeter Wave Radiation

Millimeter wave(MMW)radiation has good penetration and concealment,and has the ability to work all day.With the above advantages,it has become a new method of target information acquisition.However,the current millimeter wave radiation target detection technology still stays at the level of using the energy difference between the target and the background,and it is urgent to study the material and structure information of the target.The MMW radiation of the target is partially polarized.Some scholars have tried to use the multi-polarized radiation characteristics of the target to obtain the target information,but there are still some problems.First,the target information is incomplete:only the real part of the complex permittivity and the azimuth of the normal vector can be estimated;furthermore,the feature quantity used to describe the material information of the target is not easy to be measured practically and is not a stable quantity.How to obtain more complete,accurate and stable target information based on the polarization characteristics needs to be studied in depth.Based on this,the main work of this dissertation is as follows.(1)The complex permittivity is estimated by MMW radiation polarimetric measurement.In this dissertation,the influence of complex permittivity on MMW radiation characteristics is analyzed.It is found that different substances distributed on the same emissivity contour only show significant differences near Brewster angle under vertical polarization.Based on this phenomenon,this dissertation proposes that the complete complex permittivity can be obtained by measuring the vertical polarization characteristics of the object near Brewster angle and cooperating with the measurements at other incident angles.Based on the above theoretical analysis and experimental results,the concept of equivalent permittivity is further proposed.(2)Aiming at the measurement problem of polarization feature quantity in practical application,according to the fact that radiation energy can be divided into self radiation part and reflection part,this dissertation defines RDo P(Reflecting Degree of Polarization)and EDo P(Emissive Degree of Polarization),and gives the measurement method of RDo P by using the uncorrelation between emission part and reflection part.This method has the characteristics of high precision,low cost and easy measurement.Furthermore,aiming at the instability of the polarization feature quantity to represent the target material information,this dissertation proposes to use the equivalent permittivity derived from RDo P to represent the target material information.The equivalent permittivity hardly changes with the change of incident angle,which is suitable for object classification and recognition.Simulation and experiment results show the effectiveness of this idea.(3)The normal vector of object surface is estimated by MMW radiation polarimetric measurement.This dissertation uses the modulation effect of surface orientation on the phase angle of radiation,derives the phase angle through multi polarization measurement in each observation direction,and then reconstructs the surface normal vector.This method has less restrictions on the measurement environment and objects.However,this method needs to be measured from at least two observation directions,which limits its applications.This dissertation further proposes a surface normal vector reconstruction method which can be realized in a single observation direction.In this method,the phase angle is still derived by multi polarization measurement,and the incident angle is derived based on the radiation model simplified by equivalent permittivity.This method is only suitable for targets with smooth surface.The main innovation of this study is to establish a mathematical model between the interest parameters and polarization radiation,and reduce the dimension of the mathematical model by constructing the characterization parameters of the target material information(i.e.equivalent permittivity).Based on the equivalent permittivity,this dissertation solves the problem of equation under determination in the inversion of the target information,so as to achieve the acquisition of the target material information and structure information in the close-range application of MMW radiation.This research will help promote the application of MMW radiation multipolarization measurement in human security inspection,ground object monitoring,military target detection and so on,and provide theoretical and methodological support for the acquisition of target information.