Sentinel 4: a geostationary imaging UVN spectrometer for air quality monitoring: optical alignment of the Instrument Flight Model 2

Abstract

Sentinel-4 is an imaging UVN (UV-VIS-NIR) spectrometer, developed by Airbus under ESA contract in the frame of the joint EU/ESA COPERNICUS program. The mission objective is the operational monitoring of trace gas concentrations for atmospheric chemistry and climate over Europe. Sentinel-4 will provide accurate measurements of key atmospheric constituents such as ozone, nitrogen dioxide, sulfur dioxide, methane, and aerosol properties.The instrument is composed of two dispersive spectrometers covering the spectral ranges of 305-500nm and 750-775nm and a common telescope which projects an image of the Earth onto the two slits. The instrument will be placed in a geostationary orbit, and a 3D spectral-spatial data-cube will be acquired thanks to the mirror scanning in East-West direction.The Telescope Spectrograph Assembly (TSA) represents the core of the optical system inside the Optical Instrument Module (OIM). It is composed of the telescope, the two spectrographs and the two Focal Plane Assemblies (FPAs) which need to be integrated and aligned into the supporting structure. After verification of the optical performance parameters, in operating conditions, the TSA is finally integrated in the OIM.The Instrument Proto Flight Model (PFM) was aligned in 2020 and after undergoing the Calibration & Characterization campaign in 2022 it was finally integrated onto the Meteosat Third Generation (MTG-S) platform next to the Infrared Sounder (IRS) instrument in 2023. This paper gives an overview of the Flight Model 2 (FM2) TSA integration and alignment activities performed at the Airbus premises in Ottobrunn, Germany, during the first half of 2023. It describes the alignment philosophy developed to meet the challenging optical requirements, including spectral resolution, ground spatial sampling distance and spatial coregistration between the two channels. The results of the optical tests, performed before and after transition from ambient laboratory conditions to the operating conditions reproduced in a thermal vacuum chamber, will be presented and compared to those of the PFM instrument.