*5.1. Aqua MODIS, Sentinel-3A OLCI and SNPP VIIRS Comparisons*

Figure 20a shows the three comparison time series using Aqua MODIS B8 (blue squares), Terra MODIS B8 (green triangles), and Sentinel-3A OLCI Oa02 (red crosses) against SNPP VIIRS M1. It is seen that OLCI Oa02-based result is stable within 1% without strong seasonal modulation and short-term drift. On the other hand, the Aqua MODIS B8-based result reveals a 3% peak-to-trough seasonal variation beyond the 1% error bar, while the Terra MODIS B8-based result is of 2% seasonal variability along with a 2% difference with that of Aqua MODIS B8. Certainly, spectral mismatch between MODIS B8 and VIIRS M1 can induce seasonal pattern in both time series, but it remains possible that some physical or optical effect is impacting both Terra and Aqua MODIS B8. In addition, the discrepancy between the two MODIS-based results points to some inconsistency in the on-orbit calibration of MODIS B8.

**Figure 20.** The radiometric comparison time series, from May 2017 to September 2017, of (**a**) MODIS B8 and Sentinel-3A OLCI Oa02 versus SNPP VIIRS M1 and (**b**) MODIS B2 and Sentinel-3A OLCI Oa17 versus SNPP VIIRS M7.

Chu et al. [7] have previously concluded that IDPS-generated radiance for SNPP VIIRS M1 contains a long-term drift of approximately 0.4% over the four-year period from February 2012 to February 2016. For this recent 16-month period, the drift in IDPS-generated SNPP VIIRS M1 radiance is estimated to only ~0.15%, which is too small to be seen in these time series. The result suggests that OLCI Oa02 is not likely to have any significant short-term drift over the 16-month period.

Figure 20b shows the three comparison time series against SNPP VIIRS M7 using Aqua MODIS B2 (blue squares), Terra MODIS B2 (green triangles), and Sentinel-3A OLCI Oa17 (red stars). This set of bands is a clean case study due to well-matched RSRs, thus providing a good example of multisensor cross-check that can identify radiometric deviations. The precision is 1.15% for Aqua MODIS B2 result, 1.55% for Terra MODIS result, and 1.74% for OLCI Oa17 result. It is seen that both Terra MODIS and OLCI agree well with SNPP VIIRS M7, with time series consistent at ~1.0, as expected. However, Aqua

MODIS B2 shows a clear upward shift of ~2% against SNPP VIIRS M7 starting sometime between May and October 2017. This discontinuity had been documented in a preliminary study [30] but the times series is extended here to make clearer the upward discontinuity. Based on the overall result, it is concluded that Aqua MODIS B2 went through the 2% radiometric jump just before October 2016.
