## 2 Background and Previous Studies
To verify the stability of the VIIRS DNB calibration for the period of our study, we also examined the DNB long term stability during day time with reflected solar radiance over DCC in its Low Gain Stage (LGS). Figure 2 shows the time series of the Suomi NPP (and NOAA-20) daytime monthly DCC reflectance. First, the Suomi NPP VIIRS DNB observations over DCC has been very stable after mid-2013. Second, the DCC reflectance was increasing from 2012 to mid-2013 (0.83 to 0.92). This was due to the spectral shift of the DNB caused by the Suomi NPP VIIRS RTA mirror degradation which modulated the spectral response function of the DNB, as discussed earlier. Since mid-2013, the RTA mirror degradation has become stabilized, and more importantly, the DNB spectral response function was updated in the operational calibration [32] which corrected the bias and led to a stable trend after mid-2013 shown in Figure 2. The estimated calibration variability after mid 2013 is on the order of 0.5% in the DCC time series which is very stable. We also see that the stability in 2018-2019 is as good as that in earlier periods. Figure 2 also shows that NOAA-20 VIIRS DNB LGS calibration, with a much shorter time series, became stable after the latest on-orbit calibration update on 27 April 2018. In contrast to a solar diffuser typically used for the onboard calibration of satellite radiometers, DCC does not have a fixed location or shape, and has variability in reflectance due to a number of parameters, However, statistically the DCC reflectance is very stable given a large enough samples, for example, each data point in Figure 2 is aggregated from one month of DCC data which consists of millions of individual samples. In statistical analysis of DCC, it is typical to use the mode value of the samples, as opposed to the mean value to filter out extreme values that are out of the range for smaller variability in the trend. The VIIRS DNB calibration is performed using the low gain stage (LGS) with the onboard solar diffuser using incident sunlight, then the calibration is transferred to the medium gain (MGS) and high gain stages (HGS) using observations near the terminator, where all gain stage data are available. Given the fact that the low gain stage data is stable and traceable to solar calibration, it can be assumed that the high gain stage calibration is also relatively stable. It is known that the absolute radiometric accuracy for the high gain stage is significantly reduced due to the calibration transfer, but since we rely on the stability (as opposite to the absolute accuracy) of the calibration in analyzing the lunar phase dependent reflected radiances from the DCC, the most critical factor in this study is the high gain stage radiometric stability. To further assure the stability of the HGS, we also analyzed the gain changes of the DNB HGS over time. We found that the gain change is at the rate of about 1% per year from 2016 to 2018 based on analysis of onboard calibration data and monthly observations of the dark ocean [8]. Therefore, after the operational routine calibration update, we validated that the stability of the HGS radiance data is at a fraction of a percent for the period of study as a residual of gain change correction in the operational calibration. Figure 2: Long-term reflectance time series for Suomi NPP VIIRS DNB observations over DCC shows great stability after mid-2013 (monthly “mode” reflectance value is 91.6%\(\pm\)0.5%, available at: [[https://ncc.nesdis.noaa.gov/NOAA-20/VSTS.php](https://ncc.nesdis.noaa.gov/NOAA-20/VSTS.php)]([https://ncc.nesdis.noaa.gov/NOAA-20/VSTS.php](https://ncc.nesdis.noaa.gov/NOAA-20/VSTS.php))).