Abstract
Measurements of the disk-integrated brightness temperature of the Moon at 89, 157, 183, and 190 GHz are presented for phase angles between -80° and 50° relative to full Moon. They were obtained with the Microwave Humidity Sounder (MHS) on NOAA-18 from 39 instances when the Moon appeared in the deep space view of the instrument. Polynomials were fitted to the measured values and the maximum temperature and the phase angle of its occurrence were determined. A comparison of these results with the predictions from three different models or rather parametrical expressions by Keihm, Mo & Kigawa, and Yang et al. revealed significantly larger phase lags for the lower frequencies in the measurements with MHS. As the Moon has appeared thousands of times in the field of view of all microwave sounders combined, this investigation demonstrates the potential of weather satellites for fine tuning models and establishing the Moon as extremely accurate calibration reference.
Highlights
In the year 1998 the first member of the fifth generation of satellites for Earth observation on polar orbits was launched by the National Oceanic and Atmospheric Administration (NOAA): NOAA-15
They were obtained with the Microwave Humidity Sounder (MHS) on NOAA-18 from 39 instances when the Moon appeared in the deep space view of the instrument
Our investigation has produced two major, new findings: (1) The difference in beam size between the sounding channels and H2 is almost ten times as large as the value measured with the ground tests that were supposed to demonstrate the compliance with the requirements for MHS on NOAA-18
Summary
In the year 1998 the first member of the fifth generation of satellites for Earth observation on polar orbits was launched by the National Oceanic and Atmospheric Administration (NOAA): NOAA-15. As the Moon can only appear in the DSV when it is positioned close to the orbital axis of the artificial satellite and more than 90∘ away from the Sun, the equator crossing time puts a tight constraint on the phase angles suitable for observation. NOAA-18 changed its local equator crossing time between the years 2008 and 2018 by more than six hours This peculiarity makes it possible to calculate the brightness temperature of the Moon over a range of phase angles larger than 125∘ by analyzing its intrusions in the DSV of MHS (Microwave Humidity Sounder) on this satellite. As these measurements were carried out with both waxing and waning Moon, they allowed determining the phase angle of maximum brightness temperature with high accuracy for all channels of MHS, i.e., 89, 157, 183, and 190 GHz
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