# Comparison of the Sentinel-3A and B SLSTR Tandem Phase Data Using Metrological Principles
## 4 Results--Sea Surface Temperature Comparison
### Changes in SST Quality Levels  
While the previous sections have shown the SST differences between SLSTR-A and SLSTR-B for different quality levels, there are also cases where at the same location the quality level changes between the SLSTR-A and SLSTR-B observations. Table 6 shows the number of cases for the different SLSTR-A and SLSTR-B quality level combinations where we have limited it to cases where the quality level has to be greater than or equal to 3. Most pixels have a quality level of 5 for both SLSTR-A and SLSTR-B and overall 94% of pixels have the same quality level. Six percent of pixels, however, change their quality level after 30 s. Uniquely, the tandem data allow us to try and see why, in part, the quality level can change within such a short time interval. A change in quality level can be caused by a number of processes. Most obviously, there may be a change in the local conditions (such as the close proximity of cloud), but the impact of noise on the cloud/aerosol detection probability can also change the quality level. To investigate the impact of nearby clouds, we can look at the location of the nearest SLSTR-A pixel which has the same quality level as the SLSTR-B pixel. Figure 8 shows two plots. The left-hand plot shows the distribution of the distance in pixels of the nearest quality level 3 pixel when both SLSTR-A and SLSTR-B are at a quality level of 5. This can be considered to be analogous to the background distribution. The right-hand plot shows the distribution of the nearest quality level 3 pixel when the SLSTR-B observation has changed the quality level to a value of 3 from an SLSTR-A quality level of 5. Compared with the background distribution, this distribution is much more peaked to smaller distances implying that the change in quality level is due to the proximity of lower quality (likely cloudy) pixels. Figure 8 shows that approximately 80% of the cases where the quality level changes may be caused by the proximity of a lower quality level pixel and 20% may be due to observational uncertainty. What does a change in quality level in 30 s mean? Clearly, the nearby presence of lower quality pixels indicates that the SST quality level may be subject to rapid change over time and thus is not stable. Of course, this does not impact the quality level derived from the instantaneous measurement but may change the interpretation of cloud mask/quality level assignment schemes which use cloud proximity \begin{table}
\begin{tabular}{c c c} \hline
**QL SLSTR-A \& SLSTR-B** & **Number of Matches** & **Fraction of Total** \\ \hline
5 \& 5 & 47,135,591 & 76.1\% \\
4 \& 4 & 7,575,917 & 12.2\% \\
3 \& 3 & 3,684,585 & 6.0\% \\
5 \& 4 & 1,472,581 & 2.4\% \\
5 \& 3 & 1,654,688 & 2.7\% \\
4 \& 3 & 421,671 & 0.7\% \\ \hline \end{tabular}
\end{table} 
Table 6: Fraction of pixels with a 30 s time difference in different quality levels for SLSTR-A and SLSTR-B. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Algorithm**} & \multicolumn{2}{c}{**Quality Level 3**} & \multicolumn{2}{c}{**Quality Level 4**} & \multicolumn{2}{c}{**Quality Level 5**} \\ \cline{2-7}  & **SLSTR-A** & **SLSTR-B** & **SLSTR-A** & **SLSTR-B** & **SLSTR-A** & **SLSTR-B** \\ \hline N2 & \(-\)0.3 & \(-\)0.15 & 0.24 & 0.34 & \(-\)0.01 & 0.01 \\ N3 & 0.0 & \(-\)0.07 & 0.06 & 0.00 & 0.04 & 0.06 \\ D2 & 0.15 & \(-\)0.07 & \(-\)0.5 & \(-\)0.18 & 0.02 & \(-\)0.05 \\ D3 & 0.12 & 0.07 & 0.39 & \(-\)0.13 & 0.00 & 0.01 \\ \hline \end{tabular}
\end{table} 
Table 5: GHRSST SST bias values for SLSTR-A and SLSTR-B for quality levels 3, 4 and 5.  
as one of the tests. This variability may also impact uncertainty components related to SST averaging such as those described in [13]. There is also a population of pixels where the distance to the lower quality level pixel is large. In this case, it is likely that the change in quality level is driven more by measurement uncertainty. This then means that there is a form of uncertainty in the quality level assignment itself.