**6. General Discussions**

Intercomparison of radiometric data, as in any statistical sampling, is not entirely useful without a reliable estimate of error bars. The procedure described herein establishes the reliability of the error bars, or precision, of the comparison events, further making error bar a usable discriminator for selecting best-quality SNO events. The overall result shows that an overall 1% precision is reachable at the 1-km resolution. The constructed multiyear time series, "as is" without adjustment, are capable of capturing various features illustrative of some underlying radiometric or calibration issues as listed below.

1. Long-term drift reveals a systematically worsening error in the on-orbit calibration of the sensor data, as exemplified by Aqua MODIS B4 versus SNPP VIIRS B4 in Figure 11.

2. Sudden radiometric shift reveals a likely one-time calibration adjustment or instrument change as exemplified by the jumps in Aqua MODIS B2 versus SNPP VIIRS M7 before Oct 2016 shown in Figure 20b.

3. Noise and variability reveals scene-based effects as exemplified by almost all inter-RSB comparisons of Aqua MODIS and SNPP VIIRS in Figures 11–17.

4. Seasonal modulation reveals impact of RSR or other physical effects as exemplified by Aqua MODIS B8 versus SNPP VIIRS M1 in Figure 20a and most demonstratively the three OLCI-based time series in Figure 21. Multimodality can also manifest from RSR mismatch. Definitively the seasonal modulation is not an issue related to the SZA.

5. Non-seasonal and sporadic shifts reveal possible calibration instability as exemplified by Aqua MODIS B2 versus SNPP VIIRS M7/I2 in Figure 18a,b.

6. Discrepancy between the different intercomparisons, in addition to possibilities listed above, can reveal additional biases and calibration inconsistencies, as exemplified in the cross-comparisons of Figures 19–21.

So far, this study focuses on the on-orbit performance of the multispectral sensor data in the context of standard operational on-orbit RSB calibration. But the complete evaluation must include sensor data over all extent beyond nadir. It is therefore important to continue to distinguish between the issues of on-orbit RSB characterization from those of other additional calibration adjustments. One such important associated issue is the time-dependent RVS effect of the scan mirror that is known for MODIS [20], although not known in VIIRS and not yet addressed in OLCI. The full calibration of the sensor data for MODIS Collection 6 [19,20] involves additional correction necessary to mitigate this angle-dependent effect throughout the entire spatial extent that cannot be analyzed by the standard on-orbit calibration analysis. While the "nadir-only" framework of intercomparison can expose issues of standard operational on-orbit calibration, it is not designed to address any large-extent issues such as RVS. Nevertheless, this study puts forth a spatial scale-dependent analysis possibly extendable to examine off-nadir issues. The result of this study supports a strategy to first isolate and examine on-orbit calibration before studying other effects.

Nevertheless, some built-in limitations are difficult to overcome, including narrow-band dynamic range, lack of spectral counterparts, or simply missing data. Approaches entirely different from intercomparison, such as using stable Earth scenes, even if less reliable, must necessarily be included to build a full-evaluation strategy. This study also does not isolate the impact of geolocational error, but the overall result highly suggests geolocational issue not to be significant. Regardless, the increasing number of high-performing multispectral sensors in and to be in operation definitively expands the overall usability of intercomparison. OLCI is a prime example—given its dense spectral coverage from 400 nm to 900 nm by 21 bands, 300-m spatial resolution and the built-in on-orbit RSB calibration capability—of a new a powerful radiometric reference in the VIS/NIR range.

Lastly, a recent study by Chu and Dodd [31] demonstrates that the radiometric intercomparison of MODIS versus SNPP VIIRS thermal emissive bands (TEBs) can be analyzed under the "nadir-only" framework, along with homogeneity-ranked and sample size constrained procedure. Although an in-depth study of the capability of the radiometric intercomparison has not yet been carried out for TEBs, the general applicability of the prescribed procedure to RSBs and TEBs is expected.
