Earth observation satellites routinely monitor sea-surface temperature. However, they require in situ references for calibration and validation. To support this step, drifting buoys carrying sensors with improved calibration were deployed. This paper finds that sea state and immersion depth are important to better understand the buoy measurements. A new drifting buoy was designed as a result, in the framework of the European Union Copernicus program, with an accuracy found to be within 0.01 °C.