Improved BRDF metrology platform for Copernicus CO2I FCU calibration

Copernicus CO2M is a satellite mission designed for high precision and accuracy mapping of atmospheric carbon dioxide emission sources resulting from anthropogenic activities. The Centre Spatial de Liège (CSL) is responsible for designing and implementing the Flight Calibration Unit (FCU) dedicated to in-flight spectral and radiometric calibration of the CO2I spectroscopic instrument, including the calibration of its main optical components, such as reference diffusers (referred to as solar diffuser standards). The ambitious target sensitivity of the CO2I instrument necessitates an unprecedented level of precision and accuracy in solar diffuser standards calibration. This also requires a low level of stray light, achieved through optical baffling. In this paper, we address key issues related to the optical calibration of FCU diffuser standards, including the engineering aspects of its practical implementation. In particular, we present an original instrumental platform developed by our research group for high-precision measurement of the Bidirectional Reflectance Distribution Function (BRDF) of FCU diffuser standards. Additionally, we report and discuss results of experimental investigations of the presented instrumental approach.