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Christoph Ehrhart

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Proceedings Article | 11 June 2021 Open Access

Sentinel-4: a geostationary imaging UVN spectrometer for air quality monitoring: integration and alignment of the instrument proto flight model

Pierangelo Marenaci, Luis Estares Otal, Ellart Meijer, Jean-Francois Pittet, Stefan Riedl, Sai Shilpa Suram, Christoph Ehrhart, Grégory Bazalgette Courrèges-Lacoste, Gerard Bulsa, Arnaud Pasquet, Giorgio Bagnasco

Proceedings Volume 11852, 1185267 (2021) https://doi.org/10.1117/12.2600228

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Sentinel-4 is an imaging UVN (UV-VIS-NIR) spectrometer, developed by Airbus Defence and Space GmbH 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 a hyperspectral imager. It is composed of two imaging spectrometers covering the spectral ranges of 305 – 500 nm and 750 – 775 nm and a telescope which projects an image of the Earth onto the slits of the spectrometers. 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 one common Telescope, two Spectrographs and two Focal Plane Assemblies integrated and aligned into a three-dimensional supporting structure. The TSA is later integrated into the main instrument structure which already includes the Scan Mirror Unit, Calibration Assembly and Front Baffle; thus completing the full optical chain. This paper gives an overview of the TSA integration and alignment activities for the Proto Flight Model (PFM) performed at the Airbus premises in Ottobrunn, Germany. It describes the alignment philosophy developed to meet the challenging optical requirements, including for example, the spatial co-registration between the two spectrometers and the spatial sampling distance on ground, after the transition from ambient laboratory conditions to the instrument operating conditions in geostationary orbit. The results of the optical tests in operating conditions produced by the Optical Ground Support Equipment are also included and discussed in this paper.

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