First Light Measurements Recorded by the Digital Absolute Radiometer Onboard the ESA PROBA-3 Mission

Abstract We present the first light results from the Digital Absolute Radiometer(DARA) onboard the European Space Agency’s PROBA-3 mission, launched on5 December 2024. DARA-PROBA3 features three cavities that perform TotalSolar Irradiance (TSI) measurements based on their component-level calibra-tion. The uncertainty budget established before launch is revised in flight andextended with a dark correction derived from deep-space measurements, givingcombined standard uncertainties of 181, 164 and 169 ppm for cavities A, Band C. First light was established on 25 February 2025, the three cavities record-ing 1362.15, 1362.02 and 1361.97 W m−2 at their respective first-measurementepochs. Averaged over their individual observation windows, DARA-PROBA3gives a mean TSI of 1361.96 ± 0.09 W m−2 across the three cavities, in agreementwith the value recorded by the VIRGO/PMO6 radiometer over the same interval(i.e. 1361.94 W m−2). Extending the analysis to the post-commissioning period,from August to October 2025, we identify two classes of instrumental outlierassociated with the orbital geometry of the spacecraft. DARA-PROBA3 followsthe VIRGO/PMO6 record at both minute and daily resolution, and shows thesame sensitivity to solar fluctuations as DARA-JTSIM and TSIS-1/TIM at dailyresolution, establishing a baseline for the mission