Microwave Calibration Target Design For Improved Brightness Temperature And Research On Reflectivity Measurement Technology

The radiometer calibration is the basis for quantitative observation of passive remote sensing.As the starting point of the whole calibration link,microwave calibration target provides the referencing brightness temperature,which its accuracy directly determines the radiometer calibration precision.For improving the brightness temperature accuracy of space-borne calibration target,the paper proposed the optimized coated pyramid unit with a curved inner kernel.Compared with former researches,this design not only solves the long-standing problem of temperature gradient,but also realizes the optimization of wide-band low reflectivity,which significantly improves the brightness temperature accuracy and provides the accuracy of brightness temperature up to 99.9% for practical applications.The curved kernel outline is a joint of two parabolas,with a thinner coating at the tip and thicker coating at the bottom.It’s an innovative design of simultaneously optimizing electromagnetic and thermal performance and significantly improving the radiation brightness temperature accuracy(brightness temperature offset optimized from 0.3K to about 0.1K in a 30 K temperature difference environment).In the whole process,the new design doesn’t introduce further difficulties in manufacturing.Based on above optimized design,the paper investigated the small-sized calibration targets to further improve the brightness temperature accuracy for sub-millimeter applications,including geometry design and coating materials.Within the typical frequency range of 50-500 GHz,the target miniaturization can lead to clearly small temperature gradient and high-accuracy radiated brightness temperature,only in case that period and coating thickness are both reduced.On the contrary,keeping the coating thickness unchanged will result in the notable decrease of brightness temperature due to temperature deterioration.Additionally,it is advised to use highly-absorbing material for the miniaturized target,such as MF117,due to the reduced coating surface reflection and insufficient absorption.In terms of fabricating,it is noticeable to control the defect of the coated pyramid at the bottom to ensure brightness temperature performance.This study provides a reference how to utilize miniaturization to significantly reduce the brightness temperature offset(brightness temperature offset is about 0.05 K in a 30 K temperature difference environment).Finally,aiming for precise reflectance measurement of calibration targets,the paper studied the influence mechanism of different irradiation condition on the accuracy of mono-static reflectance measurements.Results show that there are significant reflectivity errors in the measurement under small aperture beam irradiation,due to the edge effects and small receiving aperture.In order to obtain accurate and stable reflectivity,the test antenna should have collimated beam and a large aperture(collimated beam focusing antenna or compact antenna test range).In practical,it is feasible to setup mono-static measurement system in the configuration of compact antenna test range,owing to the difficulty in collimation design in X-band and Ku-band.That provides the mechanism reference for the mono-static measurement implementation in practical engineering.