Calibration And Error Analysis Of Microwave Radiometer

Calibration of microwave radiometer is to determine accurate quantitativerelationship between output voltage of radiometer and the radiation received by antenna. Itis important to calibrate precisely for the retrieve accuracy from measured brighttemperature. The liquid nitrogen-LN calibration method is often used to calibrate. Theaccuracy of LN calibration is relatively high. However, it is not practical to be applied andautomated in long-term, routine operations. It is easy to apply tipping curve method whichrequires normal criterion. The tipping curve method has been applied for the ground-basedmicrowave radiometer abroad.Evaluation of antenna temperature is a key issue for LN method with varyingtemperature calibration load. It involves with three aspects: the measurement ofreflectivity of radiation absorbing material (RAM), the theoretical computation of thebrightness temperature radiated from calibration load and research on the radiation patternof antenna. Composition and evaluation of the antenna temperature are analyzed and flowof evaluation is presented. Errors due to impedance mismatch and accuracy of thecalibration equation are analyzed. Errors due to the nonlinearity of radiometer areanalyzed in this paper too.The tipping method couples a radiometer equation and the theory of the radiativetransfer in the atmosphere. The principle of radiative transfer and radiation model ofatmosphere are introduced, and evaluation of sky brightness temperature is presented. Theiteration of tipping curve method is derived and implemented with matlab. The speed ofconvergence and coincidence are investigated, difference of antenna temperatureevaluated based on the tipping curve method and inputted are given. The effect of themean radiative temperature of the atmosphere is analyzed too.In order to validate the correctness and performance, analysis on the performance arecarried through based on the measured sky brightness temperature. The calibrationparameters evaluated based on the iteration, are compared with that achieved with LNmethod. The error were analyzed, and the brightness temperature computed with tippingcurve method and measured by radiometer viewed the sky were compared. The resultsshow that the errors are small if the zenith angle is relatively accurate. The impact of thevarying of zenith on sky brightness temperature and iteration are analyzed, and the reasonof varying of calibration parameters evaluated with different zenith angle is analyzedfurther. The accuracy of a radiometer pointed to the zenith is vital to accuracy ofcalibration with the tipping curve method and the influence of the zenith angle is great onthe iteration when it is little, even small error could lead to big error of calibration parameter through the research. Effect of Earth Curvature and Atmospheric RefractiveIndex were analyzed too.