Research On Near-field Imaging Method And Applications Of The Synthetic Aperture Radiometer System

The theory of synthetic aperture originated in radio astronomy,which can combine multiple small aperture antennas through interferometric measurement to synthesize a large aperture antenna for receiving target radiation signals,achieving instantaneous highresolution imaging,and has high research value in multiple application fields.Currently,the main application scenarios of synthetic aperture radiometers are concentrated in the far-field region,and there has not been systematic research on near-field application.The essential characteristics of near-field radiation signals have not been thoroughly understood,and corresponding near-field imaging models and methods have not been studied.Under near-field conditions,target radiation signals exhibit spherical wavefront characteristics,rather than the plane wavefronts of far-field conditions.Directly using traditional methods for imaging will lead to distortion of the inverted image and incorrect distribution of target brightness temperature.In order to expand the application scenarios of synthetic aperture radiometers and obtain high-quality near-field imaging results,this thesis will focus on the research of the near-field imaging model and method of the aperture synthesis radiometer system,including:(1)Research on near-field target’s characterization of synthetic aperture radiometer.The correlation of the target radiation signal in near-field situations is first investigated in this thesis,which leads to the transformation relationship between the visibility function and the target bright temperature.Furthermore,the distinction between the synthetic aperture near-field imaging model and the far field is clarified,based on the features of the near-field imaging of the synthetic aperture radiometer system and the spherical wave characteristics of the target radiation signal,and on the foundation of in-depth knowledge and understanding of the imaging principle of the synthetic aperture radiometer system.(2)Research on the near-field imaging method of synthetic aperture radiometer.This thesis analyzes the intrinsic connection between the near-field visibility function and the target bright temperature distribution according to the performance properties of the model,explores the role of the target imaging distance on the forward model,and derives the accurate expression of the visibility function containing the near-field error term.For the first time,the fractional Fourier transform is used to re-establish the second mapping relationship between the spatial frequency domain and the spatial position domain in the aperture synthesis near-field imaging model,and the imaging method for synthetic aperture radiometers under near-field conditions is researched.(3)Research on the near-field imaging performance of synthetic aperture radiometer.This paper will theoretically and numerically analyze the performance parameters of resolution,sensitivity,and field of view of the synthetic aperture radiometer near-field imaging system.By fitting the fractional Fourier transform,the near-field point spread function is obtained.Based on the original definitions of corresponding performance parameters and the numerical analysis of simulation experiments,specific mathematical expressions of each performance parameter are obtained.(4)Research on the distance estimation-based optimization method for near-field target imaging quality.After in-depth study of the specific expressions of the visibility function under different conditions of far-field and near-field,this thesis directly constructs the relationship between the near-field visibility function and the target distance variable and demonstrates the additivity of the near-field visibility.Then,a distance estimation iterative strategy based on the simulated annealing algorithm is proposed,and the modified average gradient of the reconstructed image is defined as the evaluation parameter of the reconstructed image quality.Finally,the optimal distance estimation of the target is obtained by using the Metropolis criterion,and the optimal imaging result of the target is further obtained.The present study undertook a comprehensive synthetic aperture radiometer near-field imaging method and application research,which provides methodological support for the future application and development.Exploratory research has been conducted on this research topic,while many complex issues remain to be further resolved.It is hoped that future research will be able to build upon this foundation,promoting the development and maturity of in the synthetic aperture radiometer near-field imaging field.