Quantitative Modeling Method Of Brightness Temperature Transfer From Calibration Target To Antenna Of Microwave Radiometer

The calibration process of the microwave radiometer load calibrates its observed brightness temperature,by transferring the reference brightness temperature provided by the blackbody calibration target to the microwave radiometer receiving antenna in close range.Therefore,the calibration technology plays an indispensable and decisive role in the stable and accurate radiometer observation,and is a key technology in the field of passive microwave remote sensing.In recent years,basic research on the transfer process of brightness temperature in calibration link has become an attention focus domestically and internationally.However,the mismatch between classical far-field concepts and actual near-field physical processes has led to a lack of appropriate and effective analysis methods.Also,the establishment of microwave band brightness temperature benchmarks and its transfer standards lack necessary mechanism support.In response to this issue,this article constructs a modeling method for brightness temperature transfer that can simultaneously describe complex radiation targets and near-field receiving antennas.Firstly,based on the dyadic Green’s function and fluctuation dissipation theory,the forward brightness temperature transfer model from the calibration target radiation to the antenna reception is derived,under thermal equilibrium state.Its quantification and verification are completed based on the classical isothermal blackbody chamber scenario.Furthermore,based on the Rayleigh Carson reciprocity theorem and the single mode transmission condition at the antenna port,the reciprocity relationship under the near-field radiation measurement scenario is constructed.Then,the inverse calculation model of brightness temperature transfer from the antenna near-field irradiation to the calibration target absorption is derived.The forward and backward methods achieve physical unity based on the near-field reciprocity.Theirs comprehensive usage can comprehensively analyze the brightness temperature transfer process.Among them,the forward method can directly analyze the complex near-field effects in the calibration link,while the inverse method can efficiently calculate the transmitted brightness temperature.Based on the forward and inverse modeling methodology,the mechanism of near-field brightness temperature transfer can be preliminarily explored,by utilizing the two-dimensional finite difference time domain algorithm for numerical analysis.The near-field and far-field radiation characteristics of typical calibration targets’ structures are analyzed.The connection between the far-field Lambert characteristics of pyramids array and its near-field radiation enhancement is observed.The disturbance factors to the reception effects of near-field antennas in the calibration link are investigated.Results show the influences on the brightness temperature transfer process of the structural near-field radiation of the calibration target,and the mutual coupling between target and antenna structure.The results of this article provide some inspiration for the close-range application of calibration target and its form matching with antenna,and provide a possible solution to the problem of inaccurate transfer of brightness temperature from calibration target to antenna in engineering.The methodology constructed in this article has full development potential and application value.It is expected that in the future,it can be used as a basis to achieve high-precision quantitative analysis of actual calibration links,and guide the improvement of calibration accuracy,to serve the development needs of China’s passive microwave remote sensing observation refinement.