Extraction of reflectivity from microwave blackbody target with free-space measurements

We report on the characterization of blackbody target reflections as part of the recent progress on the development of brightness temperature standards for microwave remote sensing at the National Institute of Standards and Technology. The very low reflections from the blackbody targets used in airborne or satellite remote-sensing systems present challenges on how to extract reflection coefficients from the measurements. A full calibration technique was developed for this study by use of a flat aluminum plate used as a known standard in combination with measurements of the empty anechoic chamber. The theoretical basis and measurement procedures are presented. Calibration results validate the method by showing its independence from measurement hardware and conditions. A comparison between the theoretical prediction of reflection coefficients of a free-standing dielectric slab with well documented physical parameters and the de-embedded reflection coefficients from experiments confirms good calibration accuracy. The specific blackbody target used in this study shows well matched properties with a power reflectivity below -40 dB over the entire measurement band (18 GHz to 26 GHz).