Validation of Lunar Microwave Radiative Transfer Model by Using the Calibrated Two-Dimension Moon Scan Observations from NOAA-20 and NOAA-21 ATMS Instrument

As a potential external calibration reference for spaceborne microwave sounding instruments, accurate and reliable information of lunar disk-averaged radiance at millimeter band are important and fundamental. Based on study for 2-D lunar scans of the Advanced Technology Microwave Sounder (ATMS) on board the NOAA-20 satellite, a lunar microwave radiative transfer model (RTM) developed by Keihm was investigated. Microwave radiation can penetrate to lunar sub-surface; thus the microwave thermal emission in terms of brightness temperature can be quite different from surface temperature. Specifically, the phase lag in lower frequency channels such as 23 and 34GHz of ATMS instrument need to be investigated and validated in the model. In our previous studies, by taking the NOAA-20 ATMS observations as the reference truth, the surface emissivity in the lunar RTM can be calibrated. The calibrated RTM model was then evaluated by independent satellite observation data sets from AMSU (Advanced Microwave Sounding Unit) and MHS (Microwave Humidity Sounder) instruments on several NOAA satellites. NOAA-21 was successfully launched in November, 2022. During the post-launch ATMS Cal/Val activities, a special satellite back-flip maneuver was carried out after two days of the full moon phase, during which the two-dimension Moon disk scan observations were collected and calibrated. Compare with the Full Moon phase observations from NOAA-20 ATMS, the phase lag angle can be validated against the RTM simulations.