*5.2. Impact of RSR Mismatch: Sentinel-3A Oa08-Oa10 versus SNPP VIIRS M5*

The impact of the spectral coverage mismatch between two bands is nontrivial to quantify for intercomparison and is so far not well addressed or even well understood. The two primary effects of the mismatch are the offset from 1.0 in radiometric ratio and the emergence of yearly modulation. Here, only a demonstration of the effect is intended through an illustrative example using SNPP VIIRS M5 as a fixed reference and a set of three adjacent bands in Sentinel-3A OLCI. As shown in Table 2, the three OLCI RSBs, Oa08 (660–670 nm), Oa09 (670–677.5 nm), and Oa010 (677.5–685 nm), cover the 660 to 685 nm spectral region in sequence, with each having some spectral overlap with SNPP VIIRS M5 (662–682 nm).

The three inter-RSB comparison time series are shown in Figure 21 for the year 2017, for Oa08 (red stars), Oa09 (blue diagonal crosses), and Oa10 (green triangles). The three time series have nearly identical precision at 1.76%, yet differences among them are clearly shown. First, different radiometric offsets away from 1.0 expectedly show the dependence on the level of RSR mismatch. Second, while Oa08-based time series seems stable, both Oa09- and Oa10-based time series exhibit greater seasonal modulation, in particular, the Oa10-based result has the largest deviation at ~3%, not accounting for the three outliers below 0.95. This is the definitive demonstration of the different responses to the same set of SNO scenes arising only because the effect of mismatching RSRs. As Oa10 result indicates of it having the largest impact of the spectral mismatch with SNPP VIIRS M5, it shows both the largest downward offset and the most variable seasonal modulation in a consistent manner that is expected. However, what is not clear is how certain mismatch has less impact on the time series than others, such as OLCI Oa08-based result having weaker modulation. Nevertheless, the connection between the offset and the seasonal modulation is direct, that both being the manifestation of the spectral mismatch. Specifically, this connection may be useful for quantifying the impact of spectral mismatch.

**Figure 21.** The inter-RSB comparisons of Sentinel Oa08–Oa10 with SNPP VIIRS M5 demonstrating the impact of different level of spectral mismatch. The precision threshold is 3%.

The three outliers are briefly discussed here as an instance of multimodality. The outliers of each time series correspond to the same SNO events of the other two but at different ratios. These cases can arise from some scenes of less stable condition, such as cloudy or ocean scenes, which on occasions can still be stable enough to pass selection criteria. These cases are technically the result of a different mode, arising from the nontrivial effect of mismatching RSRs responding to different scene conditions. As has been pointed out previously, the presence of outliers or additional modes is one scenario where a targeted removal of certain scene condition, such as cloud, can be applied.
