Copernicus Sentinel-2C/D Multi Spectral Instrument straylight characterization due to the Earth albedo

摘要

The Sentinel-2 (S-2) mission is part of the Copernicus Space Component (CSC) – the European Commission’s Earth Observation program. It is designed to provide systematic global acquisitions of land and coastal areas at high-spectral resolution and with a five days revisit frequency, generating products feeding a large range of operational applications in domains such as agriculture, ecosystems management, natural disaster monitoring or water quality monitoring. The mission is currently in its operational phase with a constellation of two satellites (S-2A and S-2B) launched in 2015 and 2017 respectively, each designed for a minimum lifetime of 7.25 years with consumables sized for 12 years. In order to provide a long-term service (up to 20-year of overall mission duration), two additional satellites S-2C and S-2D were funded by the European Commission in order to take over on the constellation. The first one is expected to be launched at the end of 2023 and the second one will be stored until the replacement of S-2B. The main S-2 payload, the Multi Spectral Instrument (MSI), is a push broom instrument with 13 spectral bands covering from the visible and the near infrared (VNIR) to the short wave infrared (SWIR). The optical architecture is based on a Three Mirror Anastigmat (TMA) with a pupil size of 150 mm and an across-track field of view of 20°. As for all Earth observation instruments and specifically for MSI instrument with a design without field stop and an entrance pupil smaller than the first mirror, it is crucial and valuable to characterize the straylight due to the Earth albedo with a good accuracy. In the frame of S-2, an innovative straylight characterization method has been developed in order to simulate the illumination of the Earth albedo seen by the MSI instrument in orbit at 800km altitude leading to a field cone of ±63°. As it is difficult to simulate in one step this configuration, a small source is moved all around the instrument in order to cover the whole Earth field cone. This paper describes this innovative approach as well as the test setup using a robot to quickly place the source for the several required positions to cover the Earth field cone. The last results of the straylight test obtained during the MSI-D test campaign executed in 2022 and their associated accuracy are presented, showing a successful AIT sequence and completion of all tests with delivery in time that pave the way for next generation of Sentinel-2.