The DARA-PROBA3 radiometer: results from the pre-flight calibration campaign

Abstract The Project for On-Board Autonomy-3 (PROBA3) is the fourth satellite technology development and demonstration mission within the European Space Agency General Support Technology Program (ESA’s GSTP). The mission has started on the 5th of December 2024 with the launch of two satellites. One of the two PROBA3 satellites includes the digital absolute radiometer (DARA) to acquire total solar irradiance observations. Developed and manufactured in Switzerland by the PMOD/WRC, DARA features three black cavity receivers designed to measure total solar irradiance. The pre-flight calibration campaign has been completed for these three cavities at the facilities in Davos (PMOD/WRC) and the Laboratory for Atmospheric and Space Physics (LASP) in Boulder, USA. We discuss the calibration methodology used to transform the raw measurements into irradiance observations. A system-level calibration of DARA against a NIST (National Institute of Standards and Technology) traceable radiometer at LASP allowed us to estimate the conversion ratio from DARA measurements to the International System of Units (SI) in both power and irradiance modes. The ratios in power mode show better agreement between the observations recorded by the DARA and the NIST traceable radiometer than those in irradiance mode. Additionally, by referencing the World Radiometric Reference (WRR) in irradiance mode, we demonstrate that the calibration factors for irradiance and optical power are compatible within a 95% confidence interval. Cavity B demonstrates the most consistent performance and is designated as the nominal cavity for space operation, with cavity C serving as backup. The results confirm that DARA achieves SI-traceable irradiance measurements supporting the long-term accuracy and stability required for TSI monitoring in space.