| Microwave radiometer is the core equipment of passive microwave remote sensing and a powerful tool for ground objects radiation detection, imaging and characteristics recognition. With the widely use of the microwave remote sensing and the development of precision-guided and passive detection technology, it requires higher microwave radiation measurement technology. Due to the restriction of the calibration source and the influence of system gain and receiver noise fluctuation, the long-term stability, high sensitivity and high absolute accuracy of radiometer are not well solved. In this dissertation, a new method of using software to compensate system gain and receiver noise fluctuation is introduced. Then by using this compensatory algorithm, a full automatic digital compensatory millimeter wave (MMW) radiometer is designed. The hardware configurations, software structures, experimental methods and results, and the performance analysis of this new type MMW radiometer are also described. At the same time, in order to meet the needs of metal goal detection and spaceborne observation, some researches on 3mm&8mm sky brightness temperature, smooth water surface brightness temperature, and the antenna radiation efficiency of spaceborne radiometer are carried out. The main research work of this thesis includes the following several respects: 1) According to the two main impact factors of microwave radiometer output: systemic gain and receiver noise, a new method of using software to compensate gain and receiver noise fluctuation is introduced and derived in detail, and then the digital compensatory operator is obtained. After the analysis of the working principle of radiometer, the impact of systemic gain and receiver noise on radiometer performance is pointed out. After compared it with other methods which have been adopted, the digital compensatory method can greatly improve the long-term stability and sensitivity of radiometer. And only one matching load is used, which can prevent the trouble caused by using two cold and hot calibration sources in ground-based or airborne radiometer. 2) The full automatic digital compensatory 8mm radiometer, based on the whole power radiometer and combined with the digital compensatory algorithm which has been put forward, is designed. This radiometer includes the choosing of microwave switch and reference noise source, the design of pre-detection section, low frequency amplifier and full automatic digital compensatory circuit. The schematic circuit diagrams of each part and corresponding performance index are also provided. According to the designed digital compensatory 8mm radiometric measurement and control system model, the compensatory algorithm software and the communication software structure between upper computer and lower computer are provided. The whole software system is divided into two parts: measurement and control software and communication software. The former is written by C51 language, and finishes data acquisition, gain and receiver noise compensation, calibration temperature setting and saving. The latter performs the communication between super computer and under computer as the established communication protocol through RS485 bus. C51 language is used to write lower computer software, and Visual Basic 6.0 is used for upper computer software. In order to verify the dependability and long-term stability of the designed digital compensatory MMW radiometer, a long-time experimental measurement is carried out. The experiments indicate this software compensatory method is very efficient. Through this method, the system gain and receiver noise have a small effect on the output of the radiometer. So the long-term stability and accuracy of the radiometer can be preserved. 3) The performance indexes of the designed full automatic digital compensatory 8mm radiometer are analysed detailedly. Firstly, the equivalent integration time of the cascade system, which is made up of RC integrator and digital integrator, is derived. According to the equivalent integration time, the calculation formula of the new type MMW radiometer sensitivity is provided. And then the radiometer optimal sensitivity, which is obtained by adjusting the sampling average points of the digital integrator, is discussed. After the analysis of experimental output, the long-term stability of this MMW radiometer is also obtained. Finally, the accuracy analytical method of the designed MMW radiometer system is put forward. After a detailed analysis of different error sources under the worst situation, the system accuracy of this MMW radiometer is known. 4) The spaceborne application of the designed MMW radiometer, based on the analysis of the calibration methods and the error sources of spaceborne Microwave radiometer, is discussed. At the same time, the measurement of large aperture antenna radiant efficiency is still a key problem to be solved in spaceborne radiometer calibration. According to the discussed spaceborne radiometer calibration mode, a measuring method of antenna radiant efficiency is put forward. The direct expression of the antenna temperature and the radiometer output voltage is also derived. 5)The 8mm&3mm radiation characteristic measurement of several kinds of typical goals is carried out in our radiometric measurement laboratory. According to the built theoretical model of clear sky and smooth water surface brightness temperature, the theoretical and experimental research on 8mm&3mm sky brightness temperature and water surface brightness temperature is also carried out. The simple and practical method, which is used to obtain sky brightness temperature according to the ground climatic conditions, is put forward. The error analysis shows the simplified theory algorithm can complete meet the needs of engineering calculation. It is significant for metal goal passive detection, meteorologic analysis and forecast. The measurement result also indicates the radiometer must be stable enough to distinguish lawn, loess, cement way and asphalt road. This further proves it is very important to improve the radiometer stability.
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