Research On The Key Technology For Aperture Synthesis Microwave Radiometric Imaging System

As a new array interferometric microwave radiometric imaging systems, aperture synthesis radiometer (ASR) formed a large physical observation aperture by a thinned array of single small-aperture antennas, which reduced the antenna size and weight, and can instantaneously image for the entire field of view (FOV) without scanning, so it can better meet the needs of practical application and has a strong application prospects.However, more power associated with more restrictions. To achieve good imaging performance in practice, ASR need to address the following three key problems:â‘ How to calibrate systematic errors introduced by various non-ideal factors of the hardware system?â‘¡How to realize image inversion numerical algorithm of high performance that solve the problem of aperture synthesis brightness temperature image reconstruction?â‘¢How to quantitative evaluation of the performance of image inversion algorithm as well as overall system imaging performance? These three problems are the progressive levels. The high-quality brightness temperature images is the final destination of aperture synthesis microwave radiometric imaging system, so it is necessary to realize quantitative evaluation of the inversion brightness temperature image for estimation of the performance of overall system imaging performance; and it is necessary to realize image inversion numerical algorithm of high performance for high-quality brightness temperature images; finally the image inversion algorithm requires the measured data of as accuracy as possible, which must be based on in-depth understanding of systematic errors characteristics and proper calibration of systematic errors. The research on the solutions and theory of three key problems of aperture synthesis microwave radiometric imaging system is the subject of this paper, the main contents are as follows:Firstly from the point of visibility function, in the paper the systematic errors were classified into three types by the roles in the visibility function:additive error,multiplicative errors of being independent of orientation,multiplicative errors of being dependent on orientation. The overall system calibration method for the first two errors is presented, which respectively made use of external reference scene and external single noise source, and has be verified by experiments. The most prominent feature of the error analysis and calibration is the idea of overall system that simplified the complicated problem, which achieve overall errors calibration without increasing system complexity.The inverse problem of aperture synthesis image reconstruction is ill-posed, which caused by the multiplicative errors of being dependent on orientation. Two major elements required for its solution are:â‘ proper image inversion numerical algorithm;â‘¡system response G matrix. In the low SNR condition of radiometer, the usual generalized solution of the inverse problem may be completely corrupted by noise and therefore completely deprived of the exact solution. So the mathematical regularization methods were proposed to solve this problem, which made use of regularization parameter to achieve a balance between the noise and the true solution. In this paper, firstly the mathematical model of the inverse problem of aperture synthesis image reconstruction was establishment, secondly the mathematical regularization methods were applied to the solution of the problem of aperture synthesis image reconstruction and regularization image inversion numerical algorithm was realized, then system response G matrix was measured by using an external point source, finally the combination of both was successfully applied to ASR brightness temperature images reconstruction to get the high-quality inversion brightness temperature images.Traditionally, sensitivity is major performance parameter of microwave radiometer measurement precision, but it is not appropriate to quality assessment of single brightness temperature image. In the paper, the image signal to noise ratio (SNR) was present as a performance parameter of image assessment. A scene of single target against the uniform background was set the reference one, the difference of brightness temperature between the target and the background was defined as the signal intensity of image, the fluctuation in the background was defined as the noise intensity of image which was calculated by the standard deviation of the background. The image SNR defined a better performance parameter of quality assessment of single image based on its spatial statistical properties. In the paper, the image SNR was successfully applied to the assessment of the ASR images of imaging experiment, the performance of various image inversion algorithms were compared; contrariwise the assessment of images was also applied to the choice of regularization parameter and promoted the research of regularized inversion numerical algorithm.In a word, aperture synthesis microwave radiometric imaging system and its key technologies were comprehensively described, and the unique solutions of three key problems of ASR were present in the paper. One of the features of the paper is that experiment results were given for all the main theory and solution methods in the paper. The paper was expected to be a bridge between the theory and the practical application of aperture synthesis microwave radiometric imaging system, and can promote the wide application of the technology in various fields.