*2.1. The Instruments*

The flight and operational parameters of the satellites and instruments preset numerous limiting conditions for intercomparison. Table 1 lists some key parameters for Sentinel-3A, SNPP, Terra, and Aqua satellites and the key specifications for OLCI, VIIRS, and twin MODIS. Notable differences are the two different repeat cycles, at 27-day or 16-day, and the two nodes of flight, either ascending at 1:30 pm local time or descending at 10:00 am local time, that can influence SNO occurrences. The native spatial resolution at subsatellite point (SSP), determining the number of pixels per unit area, is a key parameter affecting the capability and the statistics of intercomparison. For example, at 1-km regime, a small area of 40 × 40 km-square contains 1600 pixels per SNO event, which is sufficient to attain robust statistics.

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**Table 1.** Selected information and parameters for the four satellites and the multispectral instruments.

Figure 1 illustrates the spectral coverage, represented by the range of the relative spectral response (RSRs) or spectral response functions (SRFs), of selected RSBs considered in this study. Every SNPP VIIRS band up to M7 has a spectral counterpart in MODIS or OLCI, although the two latter sensors contain more bands not spectrally matched by SNPP VIIRS.

**Figure 1.** The spectral coverage of the selected reflective solar bands (RSBs) of MODIS, SNPP VIIRS, and Sentinel-3A OLCI considered in this study. The 250-m, 500-m, and 1-km spatial resolution MODIS bands are shown in orange, red, and purple, respectively. The 750-m and 375-m spatial resolution bands of SNPP VIIRS are shown in blue and light green.

Table 2 lists the specifications of the RSBs corresponding to those in Figure 1. The spatial resolutions of SNPP VIIRS moderate- and imagery-RSBs are at 750-m and 375-m. MODIS bands B1 and B2, B3–B7, and B8–B16 operate at 250-m, 500-m, and 1-km spatial resolution, respectively. MODIS bands B1–B7 are also aggregated at 1-km spatial resolution. All Sentinel-3A OLCI bands have approximately 300-m spatial resolution. The band-to-band comparisons between these sensors can be made in four spatial resolution regimes: 1-km, 750-m, 500-m, and 375-m. The maximum at-sensor radiance (*LMAX*), with units watt/m2/sr/μm, represents the maximum end of the band dynamic range. The spectral categories are visible (VIS), near infrared (NIR), and shortwave infrared (SWIR), in increasing order of wavelength.

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 **2.** The specifications of the matching RSBs of Sentinel-3A OLCI, SNPP VIIRS, and MODIS.

**Table**

The study uses SNPP VIIRS as the common reference against other three sensors. In particular, Aqua MODIS versus SNPP VIIRS has been well studied [5–7] and its result establishes many key baselines. The band pair of Aqua MODIS B5 versus SNPP VIIRS M8 is an ideal case study of the comparison capability given the near identical spectral coverage and the very stable time series [6,7]. The four spectrally well-matched bands of Aqua MODIS B2, SNPP VIIRS I2/M7, and OLCI Oa17, centered at ~860 nm, represent a uniquely interesting set to make a consistent study on the impact of spatial resolution at all four possible regimes at 375 m, 500 m, 750 m, and 1 km. Both Terra MODIS and Sentinel-3A OLCI are not as reliably established as Aqua MODIS but still provide additional useful result. The datasets used in this study are the MODIS Collection 6.0 release [21], the operational SDR version generated by the Interface Data Processing Segment (IDPS) for SNPP VIIRS [22], and the current mission release for Sentinel-3A OLCI [23].
