On the Total Reflectivity Estimation of Microwave Calibration Targets by Backscattering Measurements

Before practical usage in microwave radiometers, the calibration targets must be tested for its emissivity/total reflectivity. In this work, we address the issue that remains in the total reflectivity determination, which is estimating the total reflectivity based on measured backscattering in the monostatic configuration, specifically at high frequencies. Based on the efficient modeling on periodic unit of the array-shaped calibration target, we quantitatively evaluate the relationship between backscattering and the total reflectivity, on the common structure of coated cones and coated pyramids. It is found that the total reflectivity estimation becomes target-specific for the coated cones; meanwhile, stable compensation can be concluded for the pyramidal units, which is important for practical testing in submillimeter applications. Then, the disturbing effects of the geometric defects on the total reflectivity estimation are discussed, and it is inferred that wideband measurement is vital for diagnosing those defects. Furthermore, actual test results are analyzed, showing the complex scenario in estimating the total reflectivity of a prototype with shape defects. Also, the numerical concluded $C_{g}$ compensation is verified in the clear case of a fine prototype. The findings in this work offer a direct reference for the application of microwave radiometer calibration, especially in the region of submillimeter waves.