###### Abstract  
The Visible Infrared Imaging Radiometer Suite (VIIRS) Day/Night Band (DNB) is capable of observing reflected lunar radiances at night with its high gain stage (HGS), and the radiometric calibration is traceable to the sun through gain transfer from the low gain stage (LGS) calibrated near the terminator with the solar diffuser. Meanwhile, deep convective clouds (DCC) are known to have a stable reflectance in the visible spectral range. Therefore, the reflected lunar radiance at night from the DCC provides a unique dataset for the inter-calibration of VIIRS DNB on different satellites such as Suomi National Polar-orbiting Partnership (NPP) and NOAA-20, as well as quantifying the lunar radiance as a function of lunar phase angle. This study demonstrates a methodology for comparing nighttime Suomi NPP and NOAA-20 VIIRS DNB measured DCC reflected lunar radiance at various phase angles using data from July 2018 to March 2019 with an 86 second sampling interval and comparing Suomi NPP VIIRS DNB measured lunar radiances with those from lunar model predictions. The result shows good consistency between these two instruments on the two satellites, although a low bias in the NOAA-20 VIIRS DNB of ~5% is found. Also, observed lunar radiance from VIIRS DNB on Suomi NPP is found to be consistent with model predictions within \(3\%\pm 5\%\) (\(1\sigma\)) for a large range of lunar phase angles. However, discrepancies are significant near full moon, due to lunar opposition effects, and limitations of the lunar models. This study is useful not only for monitoring the DNB calibration stability and consistency across satellites, but also may help validate lunar models independently. VIIRS DNB; nighttime low light remote sensing; reflected lunar light; deep convective clouds; suomi NPP; NOAA-20; radiometric measurement consistency +
Footnote †: journal: _Journal of the Royal Society B: Biological Society B:  
significant complement to the solar diffuser-based calibration as demonstrated by previous studies of the MODIS and VIIIRS [1; 2; 3]. On the other hand, despite the irrefutable stability of the lunar surface reflectance, the lunar radiance or irradiance has significant variability, primarily as a function of lunar phase angle, and to a smaller extent due to the libration, spectral variation, and distances between the earth and moon, as well as the moon and sun. As a result, absolute lunar calibration still has large uncertainties, although studies have shown that the uncertainties in the stability trending can be considerably smaller, or better than 0.1% [4]. To reduce the uncertainties, extensive efforts have been made by the calibration community to improve the lunar irradiance models which predict the lunar irradiance/radiance given a particular time and observation geometry. The latest efforts include the Global Space-based Inter-calibration System (GSICS) Robotic Lunar Observatory (ROLO) model (also known as GIRO) which is a community effort involving many space and meteorological satellite agencies [5; 6]. However, models rely on observations by well-characterized radiometers over a long period of time at specific angles, which are not widely available. In this study, we present independent measurements of the lunar radiance reflected by the deep convective clouds (DCC) at nighttime by the VIIIRS Day/Night Band (DNB) on Suomi NPP and NOAA-20 over time and as a function of lunar phase and zenith angles with an 86 second sampling interval. The lunar observations by Suomi NPP and NOAA-20 VIIIRS DNB (the latter reached validated maturity in summer 2018) are compared by lunar phase angle to evaluate their consistency, and the data are also compared to the lunar irradiance model predictions by the GIRO and another independent model published in [7], i.e., the MT2009. A time series is generated to compare the observations with model predictions, and the differences in the predicted values between the two lunar models are also evaluated.