The measurement model of Copernicus TRUSTED Fiducial Reference Measurement drifting buoys for Sea-Surface Temperature

The European Union’s Copernicus-funded project Toward Fiducial Reference Measurements of Sea Surface Temperature by European Drifters (TRUSTED) has made progress toward fiducial reference measurements (FRMs) from drifting buoys for sea surface temperature (SST). These are a subset of surface drifting buoy measurements commissioned to ensure the quality of very high-accuracy (low uncertainty), climate-quality satellite SST observations. These buoys differ from the standard drifter platform in that they carry an additional TRUSTED Reference Sensor for Temperature (TRST) to comply with the World Meteorological Organization (WMO) agreed specification of High Resolution SST, version 2 (HRSST-2). Among the mandatory characteristics for an FRM, the traceability of measurements and the evaluation of measurement uncertainties are the most important ones, particularly for the validation of satellite data. A measurement uncertainty traceability diagram following metrological best practices has been defined as a first step in establishing a new standard for surface FRM drifting buoy measurements within the TRUSTED project. The derived measurements can be qualified as FRM. The diagram defines all sources of uncertainties, ranging from the calibration of the SST sensor through to the measurements at sea. It then allows a model to be developed that describes and quantifies the detected sources of uncertainties on the measured temperature value and to link this temperature measurement to the International System of Units (SI) with small uncertainties: between 2.5 and 3.5 mK for a TRST and between 5.5 and 6.5 mK for a buoy in the range 2°–35°C.