Oblique Incidence Reflectivity of Microwave Radiometer Calibration Targets in G-Band

High emissivity temperature-controlled microwave blackbodies, or calibration targets, are often used as brightness temperature reference sources for radiometer calibration. Calibration targets are, in practice, often viewed from a range of angles due to the scanning nature of operational radiometers (e.g. a conical scanning instrument). Ideally, the calibration target has view-angle-independent emissivity of unity, but any deviation from ideal can bias the brightness temperature radiating from the target. Microwave emissivity is not a directly-measurable quantity, and instead must be inferred through measurements of reflectivity. We measure reflections from calibration targets by quantifying the magnitude of the standing wave formed by the target as it is linearly stepped through space at sub-wavelength increments. We present monostatic reflectivity results over a range of incidence angles for two types of calibration targets; a periodic pyramidal absorber array and a conical cavity blackbody. Measurements are presented at 165.5 GHz and 183.3 GHz, two channels of interest for environmental remote sensing. We find that the pyramidal array has higher reflectivity than the conical cavity at both frequencies and across the range of incidence angles. Additionally, we find that the pyramidal array has a larger range of reflectivity across the range of incidence angles. The reflectivity magnitude decreases as a function of incidence angle for both geometries, with local maxima at normal incidence except for the pyramidal array at 165.5 GHz. The considerable angular variation in reflectivity observed for the pyramidal array could potentially cause significant calibration bias as large as 0.12 K.