**4. Capability at Di**ff**erent Regimes of Spatial Resolution**

An assessment of the intercomparison at finer regimes of spatial resolution provides an understanding of what capability can be achieved in the coming era. For this purpose, the inter-RSB comparisons using SNPP VIIRS M7 (862 nm; 750 m) and I2 (862 nm; 375 m) against Aqua MODIS B2 (859 nm; 250 m), and Sentinel-3A OLCI Oa17 (865 nm; 300 m) are used to test four regimes of spatial resolution. Because the RSRs of SNPP VIIRS M7 and I2 are effectively identical, intercomparisons against them directly shows the impact of different spatial resolutions. In addition, Aqua MODIS B2, at 250-m native spatial resolution, also comes with aggregated data at 500-m and 1-km resolutions (Table 2) and provides direct testing of different spatial resolutions

The four regimes of spatial resolutions to be tested are described as follows. First, SNPP VIIRS M7, at 750-m, can be matched with the aggregate 1-km and 500-m data of Aqua MODIS B2, generating comparisons at the 1-km and 750-m regimes. Second, SNPP VIIRS I2, at 375-m, can be matched with the aggregate 500-m and the native 250-m data of Aqua MODIS B2, generating comparisons at the 500-m and 375-m regimes. For each pairing, the regime of intercomparison is defined by the lower spatial resolution. For Sentinel-3A OLCI at 300 m, the match with SNPP VIIRS M7 will be at the 750-m regime, and the match with SNPP VIIRS I2 will be at the 375-m regime.

Figure 18a shows the time series of Aqua MODIS B2 versus SNPP VIIRS M7 at the 1-km (blue squares) and the 750-m (green stars) regime for the first six years of SNPP VIIRS mission; Figure 18b shows the time series of Aqua MODIS B2 versus SNPP VIIRS I2 at the 500-m (red triangles) regime and the 375-m (cyan crosses). The precision threshold for each SNO event is 3%. The two times series in each plot have been carefully selected and matched to allow unambiguous event-to-event comparison. The time series cleansed and used here for illustration are otherwise slightly different from result strictly from the prescribed constrained procedure. The key and unexpected finding is that the three finer regimes appear only fractionally better than the 1-km regime—this hints at a lower limit of the statistical capability of the inter-RSB comparison methodology, or perhaps an additional physical effect at the level of 750-m scale. This can be an issue worthy of future pursuit.

For a more explicit demonstration, Figure 18c shows the precision result ranked from the tightest to the worst, but using the SNO events of the 375-m regime time series shown in Figure 18b as the reference of ranked events—the purpose is to reveal the statistical quality of individual events at different regimes. The SNO events of the 375-m regime (cyan crosses) are first sorted according to their precision from best to worst, and then results of other three regimes following the same SNO event sequence are plotted accordingly. That is, the 1-km, 750-m and the 500-m regime result are not separately sorted, but follow the same sorting event-by-event as that of the sorted 375-m regime result for comparison.

First, all of the most precise SNO events converge toward the beginning of the plot at about 0.15% to 0.2% precision, and this is because of the excellent homogeneity of clear-scene events. This indicates the comparison analysis has the inherent capability to reach 0.15% level. Second, the ranked result shows different intervals of slightly different pattern—a smooth pattern up to event 200 under 0.6% precision, followed by a stronger increasing pattern with more noise from event 200 to 350 and up to 2% precision, and finally the sharply rising and noisy pattern after event 350 and 2% precision. This even-by-event showing of the precision quality reveals the how precision threshold may be decided for a time series. For these cases, a 1.0% precision threshold seems a good balanced choice between having tight error bars and number of events. Third and most importantly as a focus of this examination, the result of the 750-m (green stars) and 500-m (red triangles) regimes can be seen to evenly straddle around the 375-m (cyan crosses) regime result, showing consistent agreement among the three finer regimes. On the other hand, the 1-km precision result (blue squares) is on the average higher than the result of three other regimes, as already revealed in Figure 18a,b. The capability of the radiometric intercomparison methodology, at least in the context of the constrained procedure, may have reached optimal result at the 750-m regime.

The time series also reveal some deviating features indicative of some basic on-orbit calibration issues. Although it may deceptively appear that the time series exhibits long-term drift, the result is more consistent with a series of radiometric jumps, suggesting numerous calibration adjustments for Aqua MODIS B2 or SNPP VIIRS M7/I2.

**Figure 18.** Results of four different regimes of intersensor comparison demonstrated by (**a**) Aqua MODIS B2 versus SNPP VIIRS M7 time series at the 1-km and 750-m regimes, (**b**) Aqua MODIS B2 versus SNPP VIIRS I2 time series at the 500-m and 375-m regimes, and (**c**) the precision versus ranked SNO events for all four cases.

The Aqua MODIS versus SNPP VIIRS results shown in Figure 18 are statistically dominated by events over the southern polar scenes, easily noticeable for the clustering of events during the Austral summer period from October to March. To demonstrate events over the northern region, Figure 19 shows the two inter-RSB comparisons of Sentinel-3A OLCI Oa17 (865 nm) versus SNPP VIIRS M7 at the 750-m regime (green diamonds), and versus I2 (862 nm) at the 350-m regime (orange crosses). The two OLCI-based times series are also time-matched to ensure event-by-event correspondence. The subset of the Aqua MODIS B2 versus SNPP VIIRS I2 comparison in Figure 18b occurring in the northern polar region is also shown (cyan diagonal crosses) in Figure 19 for comparison. The three precision results illustrate similar statistical performance at the 750-m and the 375-m regimes, at ~1%, with no clear advantage of the 375-m regime over the 750-m regime. The two OLCI-based times series also demonstrate an overall event-by-event consistency of precision between the two regimes, as also shown by the Aqua MODIS-based result in Figure 18. The combined findings of Aqua MODIS-based and OLCI-based results show that precision result for comparison under 1-km regimes in either polar regions is consistently at ~1% and slightly less.

**Figure 19.** Inter-RSB comparisons of OLCI Oa17 versus SNPP VIIRS M7 and I2, occurring exclusively over the northern polar region, demonstrate the 750-m (green diamonds) and 375-m (orange crosses) regimes. The subset of Aqua MODIS B2 versus SNPP VIIRS I2 comparison occurring over the northern polar region (cyan diagonal crosses) is shown for comparison.

### **5. Multi-instrument Cross-Comparison**

Intercomparison becomes even more useful when three or more sensors of comparable performance capability can be cross-checked. The next few figures exemplify the cross-comparisons of Aqua MODIS, Terra MODIS, and Sentinel-3A OLCI against SNPP VIIRS for the year 2017. The MODIS versus SNPP VIIRS comparison is carried out at the 1-km regime while that of OLCI versus SNPP VIIRS is at the 750-m regime. The time series are plotted over a 20% range centering on the time series means of the OLCI versus SNPP VIIRS, with two dashed lines marking the 2% level above and below the series mean. The applied precision threshold is 3%. The final figure shows the comparison result of three OLCI bands overlapping with SNPP VIIRS M5, explicitly demonstrating the impact of various level of mismatching RSRs. The impact of the spectral mismatch on time series remains one fundamental issue not yet adequately explored by the intersensor community.
