# Comparison of the Sentinel-3A and B SLSTR Tandem Phase Data Using Metrological Principles
## 5 Discussion
### Measurement Analysis  
The comparison of L1 TIR radiometry has shown that at a global level SLSTR-A and B agree well, with mean differences of order 10 mK observed in the S7, S8 and S9 channels for oblique and nadir views. A particular advantage of the tandem configuration for SLSTR at L1 is that it facilitates a direct comparison of the oblique view in its native geometry, which is not possible when comparing to other nadir-viewing sensors. Additionally, as mentioned, match ups are readily available right across the sensors' dynamic ranges. The combination of these factors allowed for a detailed analysis of performance as a function of scene temperature in both the nadir and oblique views. In the oblique view, this has confirmed a suspected nonlinearity effect in one or both of SLSTR-A/B that trends across the dynamic range. This apparent stray-light issue, originally observed in the pre-flight data, is at a level that significantly exceeds the estimated measurement uncertainty and so a correction should be applied. This study investigated a potential empirical correction, which was found to perform well for the tandem data, further analysis of which is recommended. The observed L1 differences were found to be relatively stable in the period following the end of SLSTR-B commissioning activities and the modification of the SLSTR-A detector set-point temperature. This apparent consistency, combined with the low overall biases described above, make the constellation a good candidate for the retrieval of accurate Climate Data Records. The time series results also highlight the importance of tandem phase planning to its success as a validation exercise. Necessary commissioning phase activities should be scheduled so that they occur prior to a sufficiently long tandem phase. Clear temporal separation of significant events would ease the disentangling of the effects of the S3A and B mission activities. Furthermore, the duration of the tandem phase must ensure that there is a chance for analysis after the instruments have stabilised. In terms of the SST retrievals, the tandem phase data have shown that using two independent, temporally-coincident observations allows a study of the detail of the retrieval process to a level that would be very difficult using more standard validation techniques. With the tandem data, it is possible \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Algorithm**} & **Quality Level 3** & **Quality Level 4** & **Quality Level 5** \\ \cline{2-4}  & \(\sigma\)/K & \(\sigma\)/K & \(\sigma\)/K \\ \hline N2 & 0.14 & 0.15 & 0.16 \\ N3 & 0.77 & 0.62 & 0.46 \\ D2 & 0.67 & 0.83 & 0.51 \\ D3 & 2.36 & 1.93 & 1.72 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Standard deviations (\(\sigma\)) of the uncertainty-normalised distributions for different quality levels and algorithms. to see small biases between the sensors at the hundredth of degree level even for the lower quality level data. This comparison has shown that the use of the SLSTR-A retrieval algorithm on SLSTR-B data (as is currently the case) introduces small but real biases in the highest quality level data. There are two conclusions to be drawn from this. Not only does the tandem phase allow a detailed study of very small differences in the SST retrieval process, but the fact that using the wrong set of retrieval coefficients led to such small differences shows that the two SLSTR instruments are indeed very closely matched (as described in the L1 analysis above) and also means that the Sentinel-3 series will be a good benchmark for climate-related SST studies. The tandem data have also allowed a detailed study of the SST retrievals as a function of GHRSST quality level. This has shown that work needs to be done to harmonise the different retrieval algorithms (N2, N3, D2, D3) across the different quality levels and has shown that and the N2 retrieval is an outlier. The tandem data are also highlighting questions related to the meaning of the GHRSST quality levels.