# Comparison of the Sentinel-3A and B SLSTR Tandem Phase Data Using Metrological Principles
## 3 Results--Radiometric Comparison
### Sensor Linearity  
The linearity of the sensors, may be examined by plotting their differences as a function of scene temperature. This gives an insight into the performance and validity of the instrument measurement function, as artefacts of poorly characterised and formulated measurement function will be present in functional form of the comparison results. Figure 4 shows the linearity of the grid cell difference data examined in Section 3.1. Here, the SLSTR-A/B differences have been binned into evenly-spaced brightness temperature bins, with uncertainties propagated following the the same approach as for spatial binning described in Section 2.2.1. The uncertainties shown in Figure 4 are displayed as \"expanded uncertainties\" [11]--in this case with a coverage factor, \(k\), of 3 (often called 3 \(\sigma\)). If the uncertainties are valid, there is therefore a \(\sim\)99.7% probability that the observed difference is covered by these uncertainties (i.e., the uncertainty bar crosses 0). As expected from the results in the previous section, the plots show this is the case for only some channels/views within the linear region of the detectors. Again, the linear region of instrument response, as explained in Section 3.1, is defined as scene temperatures close to those of the two blackbody calibration targets (roughly \(>\)250 K). These blackbody temperatures are plotted as dashed lines in Figure 4 for reference, which define the linear region of detector response. Figure 4: Differences between SLSTR-A and SLSTR-B for thermal infrared channels in the oblique and nadir views plotted as functions of binned scene radiance, error bars indicate propagated instrument calibration uncertainties at \(k=3\), the two dashed vertical lines indicate the temperatures of SLSTR’s blackbody calibration targets (data: 3 September 2018). The best performing channels are again in the nadir view, especially for S8, which shows reasonable consistency all the way to lower end of the scene temperature range. For S9, a small offset (\(\sim\)50 mK) is observed between SLSTR-A and B, which generally exceeds a level justified by the uncertainties, as observed in Section 3.2. One potential cause for this could be detector spectral response sensitivity to temperature, as this is not compensated for in the SLSTR measurement function. This is recommended for future analysis. S7 performs similarly in both nadir and oblique views, with reasonable agreement found in the linear region that severely degrades at colder scene temperatures. As previously remarked, signal at these temperatures is extremely low in this 3.742 \(\upmu\)m channel, but the large differences are not explained by the large uncertainties. Additionally, the differences vary very smoothly as a function of decreasing temperature and so there is scope in future work to model this to, for example, harmonise the calibration of the two SLSTR sensors. Again, poor performance is found for the S8 and S9 channels in the oblique view. For these channels, an offset that trends across the scene temperature range is found, explaining the non-normal distributions found in Figure 3. Such a result indicates there is an error effect in one or both of the instruments that should be corrected for, or an additional component should be added to the uncertainty budget. Such an effect has, in fact, been observed since the pre-flight calibration in both instruments, though primarily SLSTR-A, in channels S8 and S9 in the oblique view [10] and is thought to be the result of an internal stray light effect caused by a non-black coating of an optical stop within the instrument that was modified for SLSTR-B. An empirical correction was proposed for this effect; however, although it is clearly seen in the tandem analysis, initial comparisons between SLSTR-A and IASI-A could not observe this effect on-orbit and so no correction was applied at the time [12].