# Comparison of the Sentinel-3A and B SLSTR Tandem Phase Data Using Metrological Principles
## 1 Introduction  
The Sentinel-3 (S3) mission contributes to the Copernicus programme space segment, with the objective of making global, long-term observations of atmospheric, ocean and land parameters [1]. There are four main instruments on-board the mission satellites: the Sea and Land Surface Temperature Radiometer (SLSTR), the Ocean and Land Colour Instrument (OLCI); a dual-frequency Synthetic aperture Radar Altimeter (SRAL), and a Microwave Radiometer (MWR). Two S3 satellites are currently on orbit, providing near daily global coverage. Sentinel-3A (S3A) was launched on 16 February 2016 and Sentinel-3B (S3B) on 25 April 2018. Both satellites have a nominal design life of 7.5 years. Two follow-on satellites, models C and D, are in preparation to replace model A and B, so as a whole, the mission is planned to provide 20 years of continuous data. The SLSTR instrument is primarily designed to retrieve global Surface Temperatures (both Sea (SST) and land (LST)). The specification for the SST retrievals are that they should have an uncertainty of <0.3 K, traceable to international standards and need to continue the 21-year dataset of the Along-Track Scanning Radiometer (ATSR) series [2]. In a similar way to the ATSR sensors, SLSTR scans with two views--measuring both a nadir and a 55\({}^{\circ}\) off-nadir oblique view which provides two atmospheric paths for the same scene, the analysis of which helps mitigate the effect of certain atmospheric components such as stratospheric aerosol. The instrument observes in 11 spectral channels from the visible to the thermal infrared (TIR) (see Table 1), with TIR channels S7 (3.742 \(\upmu\)m), S8 (10.85 \(\upmu\)m) and S9 (12.02 \(\upmu\)m) used by the SST retrieval [3]. At the launch of S3B, it entered a so-called tandem phase with its twin, S3A. During this initial six-month period of the S3B mission, it flew separated from S3A by just 30 s. This was intended to facilitate a detailed sensor-to-sensor intercomparison to ensure that SLSTR-A and SLSTR-B datasets are well calibrated for the derivation climate data records, before the satellites move to their nominal separated orbits. Such analysis is crucial to build the robust, interoperable data records required for the study of, for example, climatic change. In this study, intercomparisons are performed of the tandem phase data for the S3A and S3B SLSTR instruments (referred to as SLSTR-A and SLSTR-B, respectively), for both the Level 1 (L1) radiometric products and derived Level 2 (L2) SST observations. In general, such satellite sensor validation activities are capable of monitoring and investigating a wide variety of instrument and retrieval performance characteristics (e.g., [4]). However, a common limitation of such studies is caused by the lack of uncertainty information available for satellite data products. Generally, the outcomes of such comparisons are to compute inter-sensor differences or \"biases\" in given conditions, but, without uncertainty information, it is difficult to form any definitive quantitative conclusion whether these observed differences indicate the sensors are achieving their specified performance. Efforts are therefore on-going to develop the techniques to facilitate the evaluation and dissemination of satellite sensor uncertainty information [5], with some missions now \begin{table}
\begin{tabular}{c c} \hline \hline
**Channel Name** & **Centre Wavelength** \\ \hline S1 & 0.554 \(\upmu\)m \\ S2 & 0.659 \(\upmu\)m \\ S3 & 0.868 \(\upmu\)m \\ S4 & 1.374 \(\upmu\)m \\ S5 & 1.613 \(\upmu\)m \\ S6 & 2.25 \(\upmu\)m \\ S7 & 3.742 \(\upmu\)m \\ S8 & 10.85 \(\upmu\)m \\ S9 & 12.02 \(\upmu\)m \\ F1 & 3.742 \(\upmu\)m \\ F2 & 10.85 \(\upmu\)m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: SLSTR spectral channels. able to provide this (e.g., Sentinel-2 [6; 7]). Sensor uncertainty information is also available for both SLSTR L1 and L2 SST products [3; 8].