Calibration Of Spaceborne Microwave Radiometer With Fixed-beam Pointing

Microwave radiometer is a passive microwave remote sensor, which receives the thermal emission from the observed objects in microwave frequency region, and characteristics of the objects can be retrieved from the measured data. In order to achieve quantitative measurement, it is necessary to setup the quantitative relationship between the output of the microwave radiometer, which is usually the detected voltage or its digitalized record, and the input to the microwave radiometer, which is the defined by the brightness temperature (or noise temperature) of the observed objects.The quantitative input-output relationship can be built by calibration of the radiometer system, which is implemented by analysis of the output of the system with known or controllable input brightness temperature. For spaceborne microwave radiometers with scanned antenna beam and fixed antenna beam, the calibration should be implemented with different methods, where the latter has more complicated requirements due to different transmission channels for power from the observed target and power from the calibration reference sources.In this thesis, calibration methods for spaceborne microwave radiometers with fixed-beam pointing will be investigated. Analysis of calibration model and model parameters, design of ground based calibration experiment, analysis of calibration error and evaluation of instrument performances will be provided. Based on the modeling and analysis, difficulties and key issues of the calibration method will be presented. The results of this thesis can provide meaningful references to the design of future spaceborne microwave radiometers and the calibration scheme with fixed-beam pointing.Firstly, an introduction to the calibration model, analysis of error of the"two-point"calibration method and the related nonlinear error model are presented. From the nonlinear model of the square law detector, which is usually used by the microwave radiometer, the relationship between residual error of calibration and nonlinear coefficientμafter"two-point"calibration is derived. The feasibility of modifying residual error of on-orbit calibration using nonlinear coefficient is verified. For microwave radiometers with fixed-beam pointing, noise temperature contributions of microwave network of the observation channel and the calibration channel should be evaluated and corrected separately in the calibrations. After analysis of several transfer models of network components, the analysis method with microwave network theory is adopted. Based on microwave network scattering parameters, equivalent transfer coefficient radiation model is built, calibration equations of microwave radiometer are derived. In the modeling, the impacts of network temperature gradient and scattering coefficients of the calibration switch to calibration model of microwave radiometer are analyzed.Secondly, a statistical analysis of the impacts of system parameters on the calibration accuracy is presented. The calibration model of microwave radiometer with fixed-beam pointing is decided by a large number of scattering parameters of the transmission network of both the observation channel and calibration channel, including but not limited to the transmission coefficient, reflection coefficient and their phases. Orthogonal experiment design method (orthogonal) is adopted to analyze the influence of the scattering parameters to the calibration of antenna temperature. The statistical distribution of calibration coefficients is obtained with defined measuring error range of scattering parameters. The statistical results can provide a meaningful reference to the design of the ground based calibration experiment to determine calibration coefficient.Thirdly, the ground based calibration experiment and result of calibration data analysis are provided. Ground based calibration experiment of fixed-beam microwave radiometers is more complicated than that of the scanned-beam microwave radiometers, where not only the calibration coefficient of calibration equation, but also nonlinear coefficient of microwave radiometer system should be determined. Retrieval of these nonlinear coefficients requires that the equivalent transmission coefficients of the observation channel and the calibration channel should be determined. However, due to the dependence between the temperatures of the switch of the observation channel and the calibration channel, only the ratio of the equivalent transmission coefficients of observation channel and the calibration channel can be derived, while the exact value of these two coefficients are difficult to obtain. As a result, referring to the analysis of model parameters in chapter 4, a reasonable solution can be found with model parameters located within the measurement error range, and then the equivalent transmission coefficients of observation channel and calibration channel can be determined. The nonlinear coefficient can be retrieved from the residual error by numerical regression algorithm.Finally, several issues of on-orbit calibration of microwave radiometer are addressed. The impacts of other emission sources in addition to the cosmic background emission, which can also be received by the calibration antenna, and the ohmic loss of antenna reflector is also considered.