An in-flight calibration assembly for the earth observation instrument Sentinel-4 UVN

An in-orbit calibration assembly is designed and tested to ensure the radiometric accuracy of Sentinel-4/UVN. One of the calibration method consists in measuring the Sun’s irradiance through a diffuser. A major challenge was the apparition of so-called spectral features, which come from speckle patterns due to the thin spectral bands of the instrument. A stack of diffuser was built to increase the average optical path difference above coherence length, thus decreasing the speckle contrast. A unique spectral feature test bench has been developed to validate the performance. A second calibration approach consists in illuminating the instrument with a white light source whose drift is characterized and limited. This paper discusses the design of the calibration assembly and the challenge which were encountered in the process. Also, on-ground tests results are presented, including the description of the test setup, in particular the spectral feature test bench.