Calibration and Characterization of Satellite‐Borne Microwave Sounders With the Moon

Abstract

AbstractA major problem with calculating the uncertainties of measurements with weather satellites is the fact that a full characterization and calibration of their instruments can only be carried out before launch. The Moon, however, makes at least some of these activities possible in flight as well by providing a reliable flux reference at a well‐defined position. We used serendipitous observations of the Moon with AMSU‐B and MHS on eight different satellites to measure pointing accuracy, spectral channels coregistration, and beamwidth with unprecedented accuracy in flight. In addition, we compared these findings with the corresponding values obtained on ground. By analyzing more than a hundred Moon intrusions in the deep space view, we could determine the radiance of the Moon as a function of its phase angle and distance from the Sun. The difference in average brightness temperature of the lunar disk between perihelion and aphelion amounts to 4.4 ± 2.3 K at 183 GHz. We compare the measured brightness temperature of the Moon as a function of phase angle between −85° (waxing) and +76° (waning) with the predictions from two models and find that one of them reproduces the shape of this function very well.