Real-aperture Forward-looking Fast Imaging Based On Digital Coded Metamaterial

According to the relative relationship between radar and target,radar imaging can be divided into two categories : one is real aperture imaging,and the other is synthetic aperture imaging.The imaging performance of traditional real aperture is limited by antenna aperture,and the imaging performance of synthetic aperture is limited by the relative motion between radar and target,so it cannot be imaging under forward-looking and staring conditions.With the development of microwave correlated imaging system,digital coded metamaterial as a new material has obvious advantages in radar imaging.Digital coded metamaterials have the advantages of low cost,simple design,portability and flexibility.By changing the state of phase coding,the space-time two-dimensional random radiation field can be constructed.The echo signal and the emission signal after the interaction between the target and the electromagnetic wave in the radiation field are correlated to realize super-resolution imaging.This paper focuses on the cutting-edge topic of digital coded metamaterials,and conducts in-depth research on digital coded metamaterial imaging system,digital coded metamaterial imaging algorithm and digital coded metamaterial imaging performance.The main research work is summarized as follows :(1)For the digital coded metamaterial imaging system,on the one hand,the imaging range resolution of the digital coded metamaterial array is positively correlated with the bandwidth.In this paper,the non-uniform transmission line and multi-branch transmission line are used to optimize the structure of the digital coded metamaterial,and the phase bandwidth is expanded.On the other hand,the imaging azimuth and pitch resolution of the digital coded metamaterial array are positively correlated with the degree of freedom of the radiation field.In this paper,the genetic algorithm is used to optimize the phase encoding,and the degree of freedom of the radiation field is improved.(2)For the digital coding metamaterial imaging algorithm,this paper proposes a dynamic mesh fast imaging method.Firstly,the imaging preprocessing is carried out,including the following parts : mathematical modeling of spatial-temporal two-dimensional random field,coarse grid division of imaging plane,and coarse imaging using sparse reconstruction algorithm of compressed sensing.Due to the coarse imaging in the case of large field of view,the beam energy is required to cover the target scene as much as possible.However,the divergence of energy inevitably reduces the echo signal-to-noise ratio or there are inevitably some interference targets in the target scene,which leads to some noise points in the coarse imaging process using sparse reconstruction algorithm.Therefore,it is necessary to remove the interference noise points.This paper draws on the threshold segmentation algorithm used in the field of image processing,and then obtains the approximate area of the target.Then,the local radiation field is optimized based on the approximate area of the target.The energy emitted by the digital coded metamaterial array is concentrated in the local area of the target scene while maintaining a good degree of freedom of the radiation field.Finally,the imaging plane is further meshed,and the sparse reconstruction algorithm of compressed sensing is used to reconstruct the echo signal to realize super-resolution fast imaging.(3)For the imaging performance of digital coded metamaterials,the unavoidable coupling effect between the array elements of digital coded metamaterials and the influence of phase error on the imaging performance are considered in the actual process;In order to analyze the performance of the imaging algorithm,the imaging performance is evaluated,including time complexity,anti-noise robustness,super-resolution performance and so on.