Two versions of this radiometer design have already flown, notes Werner Schmutz of PMOD, who oversaw its development: “A compact version called CLARA flew on Noway’s NorSat-1 CubeSat in 2017, remaining operational to this day, while a previous DARA is serving aboard the Chinese FY-3E weather satellite, launched in 2021. So we have high confidence in the design, which can operate whenever the Proba-3 Occulter is pointed at the Sun within half a degree of accuracy.”

The main difference between Proba-3’s DARA and previous radiometers will be its very elongated orbit, which will carry it 60 000 km above Earth’s surface. DARA can automatically adjust to slight changes in the size of the solar disk based on how far it is away – which are also due to Earth’s yearly elliptical orbit around the Sun. All the radiometer needs to know is its position in space and its data gathering compensates for the shift.