# Benefits and Lessons Learned from the Sentinel-3 Tandem Phase
## 4 Methodology for Inter-Comparisons  
The tandem configuration provides an opportunity to assess the measurement uncertainties. To formalize this point, let us denote by \(L_{\text{A}}\) and \(L_{\text{B}}\) the measurements of the A and B instruments, and define the difference, \(\Delta L=L_{\text{A}}-L_{\text{B}}\). Then, the uncertainty \(u\) associated to the difference \(\Delta L\) satisfies  
\[u^{2}\big{(}\Delta L\big{)}=u^{2}(L_{\text{A}})+u^{2}(L_{\text{B}})-2\ u(L_{ \text{A}},L_{\text{B}})+u^{2}{}_{\text{match-up}}(\Delta L), \tag{1}\]  
where \(u(L_{\text{A}},L_{\text{B}})\) is the covariance of the measurements, and \(u_{\text{match-up}}\) is the uncertainty associated with the match-up. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{SRAL} & \multirow{2}{*}{SWH, WS} & Triple collocations with buoys and models & \multirow{2}{*}{Very good agreement between S3A and S3B} & [18,19] and Section 5.2 \\  & & & Statistics per mode and region & & S3A and S3B \\ \hline MWR & BT & Alternative & \multirow{2}{*}{Harmonization accuracy} & \multirow{2}{*}{[20]} \\  & & & harmonization method & better than 0.5 K obtained \\ \hline \multirow{6}{*}{OLCI} & \multirow{6}{*}{L1} & \multirow{6}{*}{L1} & OLCI-A brighter than OLCI-B & \multirow{6}{*}{} \\  & & & After empirical flat-fielding, & \multirow{6}{*}{} \\  & & & differences are linearly & \\  & & & decreasing with a spectral & \\  & & \multirow{2}{*}{Across-track profiles} & wavelength & \multirow{2}{*}{} \\  & & & Statistics per target classes & & Possible non-linearity effects \\  & & & Temporal stability & and impact of stray-light & \\  & & & correction in camera & \\  & & & Homogenization and & \\  & & & harmonization prototyped & \\  & & & and validated & \\ \hline \multirow{6}{*}{OLCI} & \multirow{6}{*}{L1 over deep convective clouds} & \multirow{6}{*}{Across-track uniformity cross-calibration temporal stability out of tandem} & Although not as precise as tandem analyses, independent analysis over DCCS (monthly statistics) allows quantifying (between 0.5\% and 1\% precision) the relative calibration of OLCI-A and OLCI-B out of tandem phase \\ \hline \multirow{6}{*}{OLCI} & \multirow{6}{*}{L2 land and water} & Impact of L1 harmonization Impact of spectral response differences Impact of viewing geometry & L1 harmonization more efficient than L2 vicarious calibration for water products OGVI (land product) better aligned after harmonization & \\  & & Impact of viewing geometry & OTCI mostly sensitive to spectral differences & \\ \hline OLCI & L3 water & Trend retrieval on simulated datasets & Trend detection capabilities of the Sentinel-3 constellation confirmed & [24] and Section 8.2 \\ \hline \multirow{6}{*}{SLSTR} & \multirow{6}{*}{L1} & \multirow{6}{*}{Sensibility studies (WV, signal linearity) Temporal stability} & Low differences between S1 STR-A and B nadir view, slightly higher for oblique view & [25,26] and Section 6.3 \\  & & & Temporal stability & Non-linearity effects observed, most likely due to internal background signal Uncertainty under-estimated for oblique view & \\ \hline \multirow{6}{*}{SLSTR} & \multirow{6}{*}{L2 SST} & \multirow{6}{*}{L2 SST} & The agreement is generally good, with notable differences and quality level & \\  & & & & \\ \hline SST: sea surface temperature; SWH: significant wave height; WS: wind speed; BT: brightness temperature; OGVI: OLCI green vegetation index; OTCI: OLCI total chlorophyll index; WV: water vapour. & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._The covariance between measurement errors can arise from, e.g., characterizing both instruments with the same set-up. This quantity is generally difficult to estimate. In this, the study has been neglected because we expect it generally to be small and because it has the effect of reducing the uncertainty associated with the difference; therefore, neglecting it provides a more cautious estimate of the uncertainty.