Validation of SLSTR Sea Surface Temperature (SST) with Global Ships4sst L2R Data

SST data derived from multi-spectral infrared measurements made by the SLSTR instruments onboard the Copernicus Sentinel-3 satellites are considered reference measurements based on the design features of the SLSTR instrument. This means that regular long-term validation of the end-to-end performance using independent reference data is essential. This paper presents a sustained SLSTR SST validation activity between 2018 and 2025 based on an extensive set of near-contemporaneous and co-located in situ reference measurements. In situ measurements are derived from high-performance shipborne infrared radiometers that are calibrated regularly against International System of Units (SI) reference standards maintained by a National Metrology Institute (NMI). As such, they are classified as FRMs. The shipborne radiometers are operated by several organizations that co-operate within the International Shipborne FRM Radiometer Network (ISFRN), under the auspices of the European Space Agency (ESA) ships4sst project (a quasi-operational SSTskin measurement program with a common data format and traceable measurements). The mean measured differences between the SLSTR on Sentinel-3A (S3A) and the ships4sst FRM data are −0.01 K with a RSD of 0.20 K for nighttime retrievals and 0.07 K with an RSD of 0.27 K for daytime retrievals. For Sentinel-3B (S3B), the mean measured differences between the SLSTR and the ships4sst data are −0.04 K with an RSD of 0.19 K for nighttime retrievals, and 0.03 K with an RSD of 0.27 K for daytime retrievals. Our comparisons validate the consistency and performance of the Copernicus Sentinel-3 SLSTR SST data and provide the means to link these satellite measurements to NMI reference standards, which is essential for justifying the status of SLSTR SST data as a CDR.