Theoretical And Experimental Study On Spatial Resolution And Stability Of Microwave Radiometer

A microwave radiometer is a highly sensitivity receiver capable of measuring low levels of microwave radiation and an important equipment of microwave passive target detection, which is used widely in remote sensing, concealed danger detection within security field, homing system, precision measurement and environment surveillance et al. With the development of application of microwave radiometer, better performance of microwave radiometer is required than before. Firstly, in order to improve spatial resolution of microwave radiometer, the large real aperture antenna is needed. It is not suitable for airborne and spaceborne microwave radiometer, moreover the sensitivity of microwave radiometer is decreased with reducing integration time, so the conflict between the spatial resolution and size of antenna should be resolved as a stringent key problem of microwave radiometer. Secondly, due to the influence of system gain and receiver noise fluctuation, it is hard to develop a microwave radiometer with long-term stability, so how to improve the long-term stability is another key problem for microwave radiometer. In this dissertation, interferometry imaging technology is studied for improving microwave radiometers spatial resolution. Basic theory, spatial resolution and sensitivity of synthetic aperture microwave radiometers and supersynthesis radiometers are analyzed, and one dimensional imaging experiment of noise point source is performed. Periodic calibration technology that can improve long-term stability of microwave radiometer is also studied in the dissertation. The microwave radiometer with periodic calibration is designed. The sensitivity, accuracy and long-term stability of radiometer with periodic calibration are analyzed and tested.The main research work of this thesis includes the following several respects.(1) The basic theory, spatial resolution and sensitivity of synthetic aperture radiometers are analyzed. The formulae of radiometric sensitivity and the formulae of microwave radiation image sensitivity are compared. Imbalance of phase and amplitude on sensitivity of synthetic aperture radiometers is studied. Correlation radiometer is a basic unit of synthetic aperture radiometers, so zero intermediate frequency two-channel correlation radiometers at 8mm wavelength and dual intermediate frequency super-heterodyne two-channel correlation radiometers at 8mm wavelength are designed respectively. A series of two channel digital correlation experiments are performed. One dimensional noise point source image is obtained by moving the baseline, so high spatial resolution is achieved successfully by little antenna array. Performance of airborne T shape antenna array synthetic aperture radiometers to metal target detection is analyzed, and the method of computing the maximum distance of detecting target is presented.(2) Microwave radiation image is achieved by supersynthesis radiometers using matched filtering to visibility usually. In this section, Fourier inversion imaging theory by supersynthesis radiometers is deduced. Spatial resolution and sensitivity of supersynthesis radiometers are presented. One dimensional noise point source imaging experiment is fulfilled using zero intermediate frequency two-channel correlation radiometer at 8mm wavelength by matched filtering method and Fourier inversion method respectively.(3) Uncertainty equations for all kinds of antenna array microwave radiometers including synthetic aperture radiometers, supersynthesis radiometers, Doppler microwave radiometer, phase array microwave radiometers and focal plane array microwave radiometers are all deduced, and uncertainty equation for all kinds of microwave radiometer are compared.(4) The analysis of sensitivity, accuracy and long-time stability is presented. Radiometer at 8mm wavelength with periodic calibration is designed, which employs direct detection receiver. It has more broad frequency band than traditional super-heterodyne receivers. The high temperature calibration source is fulfilled by opening active noise source, and low temperature calibration source is fulfilled by closing active noise source. Integration time of microwave radiometer can be set easily by RC integration circuit in series with digital integration circuit. Sensitivity and long-time stability are tested and the experimental results show that sensitivity and long-time stability of the radiometer are satisfactory.