# Comparison of the Sentinel-3A and B SLSTR Tandem Phase Data Using Metrological Principles
## 3 Results--Radiometric Comparison
### Uncertainty Validation  
For the data analysed in Section 3.1, the uncertainty validation methodology described in Section 2.2.2 was applied. The resulting plots of uncertainty-normalised SLSTR-A/B differences for channels S7, S8 and S9 in nadir and oblique views are shown in Figure 3. Within the context of the metrology-based approach this study takes, uncertainty validation provides the key result of the comparison exercise, quantitatively determining if the sensors are meeting their expected performance.  
Since the uncertainty that the data are normalised by is due to calibration errors that are fully correlated within the measurements, the distribution mean, \(\mu\), is the metric of interest. The width of the distribution is caused by unquantified uncertainty due to random errors, for example, remaining match-up errors.  
As found in the analysis of radiometric differences, the nadir channels appear to perform best. In particular, the S8 nadir channel is performing well, where for the uncertainty-normalised distribution \(|\mu|<1\), validating the claimed uncertainty this channel. The S9 nadir performs somewhat worse, where \(|\mu|>2\), indicating the calibration uncertainty for this channel is likely too small. For the S7 channel in both the nadir and oblique view, we find \(|\mu|<2\) indicating the calibration uncertainties seem reasonable given the observed bias.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Channel Name and View** & **Mean Difference/K** & **Standard Deviation/K** \\ \hline S7 Oblique & 0.06 & 0.13 \\ S8 Oblique & \(-\)0.01 & 0.11 \\ S9 Oblique & \(-\)0.05 & 0.11 \\ S7 Nadir & 0.06 & 0.10 \\ S8 Nadir & 0.01 & 0.06 \\ S9 Nadir & \(-\)0.04 & 0.06 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Mean and standard deviation of differences between SLSTR-A and B observations (S3B \(-\) S3A), in the linear detector region (data: 3 September 2018).  
Figure 3: Distribution of uncertainty-normalised differences between SLSTR-A and B gridded data in the linear detector region (data: 3 September 2018). The solid line vertical line indicates the distribution mean, the dashed vertical lines indicate the distribution standard deviation. The black curve indicates a normal distribution with a standard deviation of 1, centred on the mean of the difference distribution.  
For channels S8 and S9 in the oblique view, however, we find non-normal distributions that point to a more significant issue. This indicates there is a significant error effect missing from the radiometric uncertainty budget for these channels, demonstrating the need for either some sort of correction or an update to the uncertainty budget. This is discussed further in Section 3.3, where a potential correction is investigated.  
Figure 3 shows that, in general, the calibration uncertainties for SLSTR TIR channels are likely somewhat underestimated. This is especially the case since any covariance between SLSTR-A and B (omitted from this uncertainty budget) would tend to reduce the difference uncertainties, which the existing uncertainties would have to further compensate for.