Microwave Radiometric Imaging With Mirrored Interferometric Aperture Synthesis

An aperture synthesis radiometer has the advantage of reducing antenna volume and weight needed for a given spatial resolution and the advantage of covering a large field of view without mechanical scanning. It provides a means for the radiometers operated at low frequencies to obtain high spatial resolution for the earth observation from space. But the superiority in spatial resolution of the technology is at the price of the complexity of system and signal processing. For a spaceborne aperture synthesis radiometer with hundrends of antennas and thousands of corrleators, the system and the signal processing are very complex, which are obstacles to achieve the required system performance. In this paper, mirrored interferometric aperture synthesis (MIAS) is proposed, which can achieve the same spatial resolution as a large traditional aperture synthesis system but needs fewer antennas and simple receiver structure. The fundamental principle and key problems of MIAS imaging are presented. The main aspects are as follows:(1) The fundamental principle and the performances of one-dimansional MIAS. Firstly the pricinple of MIAS imaging is presented. The correlation between the signals collected by a pair of antennas can be represented by the subtraction of the cosine visibility at different two baselines. The cosine visibility can be obtained by solving the linear equations, and the brightness temperature image can be reconstructed from the cosine visibility by the inverse cosine transform. Then the spatial resolution and the sensitivity of MIAS imaging are analyzed, and the method, combing the linear equations at different distances from the array to the reflector, is proposed to guarantee the baseline integrity. Moreover, simulation experiments are conducted to demonstrate the principle of MIAS, the spatial resolution, and the sensitivity. The results are in good agreement with the theoretical. Finally, comparison between MIAS and traditional interferometric aperture synthesis is made. With the same spatial resolution, theoretical analysis and simulation results show that MIAS can greatly reduce the number of antennas, analog-to-digital converters (ADC), and correlators, and simplify the receiver structure.(2) The fundamental principle and the performances of two-dimansional MIAS. Firstly, the process how to expand the one-dimensional MIAS to two-dimensional MIAS is presented. Then as the one-dimensional MIAS, the imaging principle and the system performances are presented. Simulation experiments for it are also conducted. The theoretical analysis and the simulation results show that two-dimensional MIAS can provide high spatial resolution in the two dimensional.(3) The impacts of system errors on the performances of MIAS The relationship between the system errors and the system performances is established. Based on it and combining the Parseval principle, the root-mean-square rule is proposed to evaluate the impacts, and the upper limit of the impacts on the accuracy is given according to the matrix nrom. Then the model for the reflector imperfection, antenna errors, and channel errors are established, and the impacts of these errors are quantified through simulation.(4) Image reconstruction method of MIAS imaging. Due to the impacts of system errors, the reconstructed image by inverse cosine transform is with bad quality. And reconstruction methods based on the impulse response matrix, the G operator, are introduced. The image reconstruction is an ill-posed inverse problem, and due to the impacts of system errors and noise, the common generalized inverse reconstruction is not robust, which cause the solution deviating from the real one severely. In order to solve the problem, two regularization methods, truncated singular value decomposition and tikhonov regularization, are introduced, and the simulation results demonstrate the validity of the two regularization methods.In this paper, main aspects of MIAS, including imaging principle, imaging performances, error analysis, and image reconstruction are presented, which provide good references for the evolution of MIAS imaging and the design of a MIAS radiometer. MIAS also provides a means for the earth observation with high spatial resolution from space.