Radiometric Characterization of a Water-Based Conical Blackbody Calibration Target for Millimeter-Wave Remote Sensing

In this paper, we present the design and radiometric characterization of a water-based conical blackbody calibration target to be applied as a precise reference source in laboratories and for the accurate calibration of ground-based microwave instruments. The concept of aqueous blackbody targets circumvents the conflict between the electromagnetic and thermal properties of traditional calibration target designs. The temperature controllable target consists of a conical low loss plastic shell, which has been manufactured using a stereolithography three-dimensional printer to define the water-air interface. An advanced exponential profile of the shell is used to compensate the comparatively high refractive index of water and to obtain a high emissivity. Simulations and active measurements of the coherent backscattering S11 have been performed verifying its excellent electromagnetic properties. Furthermore, radiometric measurements at 110 GHz demonstrate the outstanding thermal performance at various target temperatures between 10 and 60 °C. The differences between the brightness temperature and the water temperature are measured to be less than 40 mK and therefore significantly smaller in comparison to traditional calibration targets. In addition, the measured baseline ripples in the spectra caused by the water-based calibration target are small compared to established blackbody concepts.