| In the microwave remote sensing devoted to Earth, the demand of application with high resolution limits the application of real aperture microwave radiometers. To obtain the brightness temperature map with high resolution, a new microwave radiometer that microwave aperture synthesis radiometers (ASRs) is found, which forms a large physical observation aperture by a thinned array with many small-aperture antennas. Compared to the real aperture microwave radiometers, ASRs have the advantages of high resolution, wide field of view (FOV), and instantaneously imaging for entire FOV with no scanning. In the microwave remote sensing devoted to Earth, the geophysical parameters of Earth with large-scale and high resolution can be obtained by on-board ASRs, such as ocean salinity and soil moisture. Thus, ASRs have good application prospect.However, ASRs have the performance of imaging with high resolution at the expense of the complexity of the system. Due to increasing the complexity of the system hardware, it leads to a lot of errors. Therefore, a good error correction is to ensure that the good performance of ASRs. At present, the internal noise injection is usually used in the calibration of on-board ASRs, which has many advantages, such as high accuracy and in real-time. However, it needs a large noise distribution network, which highly increases the volume and mass, and seriously limits the application of on-board ASRs with large array. In addition, since the internal noise is injected after antenna plane, antenna errors can not be corrected by this method. Proposing a new calibration method, which exhibits a good performance and can also greatly reduce the the volume and mass of ASRs, is the one of the main idea in this paper.After error correction (relative calibration), absolute calibration is needed to obtain remote sensing data with high quality for ASRs. Calibration (including the error correction) accuracy is an important specific in the calibration methods. Improving calibration accuracy is very helpful to improve the quality of quantitative remote sensing data, which has a very important significance.Because of increasingly scarce spectrum resources and man-made factors, radio frequency interference (RFI) seriously affects the normal work and the performance of microwave radiometers. It reduces the quality of the remote sensing data, it also leads to remote sensing data completely unavailable in the worse case, especially in low frequency band (L-band, etc.) of ASRs, such as SMOS and Aquarius. At present, many RFI detection and mitigation algorithms have been proposed. However, the low-level RFI is still hard to detected and mitigated, which seriously reduces the the quality of the subsequent data product, such as ocean salinity.To obtain quantitative remote sensing data of ASRs with high quality, the following research works are made in this paper:(1) A compensation method of antenna mutual coupling effects in ASRs based on the receiving mutual impedance method (RMIM). Compared to the the conventional mutual impedance method (CMIM), RMIM is more appropriate to characterize the mutual coupling behavior between all antennas in ASRs. The relationship between the measured visibility function and the ideal visibility function is derived based on using the equivalent circuit model, and the decouple equation is also derived.(2) The impact of the inconsistency between the receiver's physical temperature on the antenna temperature and the visibility function. The impact of antenna mutual coupling effects on ASRs is analyzed based on S-parameter and a thermodynamic approach, which includes the antenna temperature and the visibility function, and the impacts of the inconsistency between the receiver's physical temperature is also analyzed in two specific cases that the cold-sky and an electromagnetic compatibility (EMC) chamber in detail.(3) A hybrid calibration method based on the redundant space and internal noise injection. A well-conditioned system of calibration equations is established based on the redundant space and internal noise injection, to obtain the amplitude and phase errors of all channels, and the out-of-plane distortion errors of antenna arrays. This hybrid method can greatly reduces the mass and volume of ASRs, and it also can deal with the out-of-plane distortion errors of antenna arrays.(4) The research of the calibration (including the error correction) accuracy in ASRs. The model of calibration accuracy is derived, the accuracy of the receivers'system temperature, the fringe-washing function at origin and the quadrature-corrected normalized correlation are analyzed. The factors associated with the calibration accuracy are also analyzed to improve the calibration accuracy.(5) The research of a RFI detection algorithm. The RFIs in China are addressed using the SMOS L1-c data provided by the European Space Agency (ESA), in the terms of the geographic locations of RFI sources, the probability of RFI occurrence, the sources of RFIs, the RFI characteristic of time and orientation.(6) The low-level RFI detection algorithm based on the European Centre for Medium Range Weather Forecasting (ECMWF) is proposed, which take the surface brightness temperature as the reference of the RFI detection threshold. The proposed algorithm can detect and flag more low-level RFI than the RFI detection algorithm with fixed threshold.In this paper, a compensation method of antenna mutual coupling effects based on RMIM, a hybrid calibration method based on redundant space and internal noise injection, and a low-level RFI detection algorithm with dynamic threshold based on ECMWF are proposed though rigorously deduction and tested in simulations and on-ground experiments or the on-board data. The impact of the inconsistency between the receiver's physical temperature and the model of calibration accuracy are rigorously derived. These results can provide the references for the design of system, the error correction and the RFI detection in ASRs. |
PDF Full Text Request