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

In this paper, we present the design and radiometriccharacterization of a water-based conical blackbody calibrationtarget to be applied as a precise reference source in laboratoriesand for the accurate calibration of ground-based microwave in-struments. The concept of aqueous blackbody targets circumventsthe conflict between the electromagnetic and thermal propertiesof traditional calibration target designs. The temperature control-lable target consists of a conical low loss plastic shell, which hasbeen manufactured using a stereolithography three-dimensionalprinter to define the water–air interface. An advanced exponentialprofile of the shell is used to compensate the comparatively high re-fractive index of water and to obtain a high emissivity. Simulationsand active measurements of the coherent backscattering S11havebeen performed verifying its excellent electromagnetic properties.Furthermore, radiometric measurements at 110 GHz demonstratethe outstanding thermal performance at various target tempera-tures between 10 and 60 °C. The differences between the bright-ness temperature and the water temperature are measured to beless than 40 mK and therefore significantly smaller in comparisonto traditional calibration targets. In addition, the measured base-line ripples in the spectra caused by the water-based calibrationtarget are small compared to established blackbody concepts.Index Terms—Blackbody targets, calibration, electromagneticdesign, microwave absorbers, microwave radiometry, remotesensing, three-dimensional (3-D) printing.