UVN

The Sentinel-4 mission is part of the Copernicus programme, a joint undertaking of the European Community and the European Space Agency. This mission covers the needs for continuous monitoring of Earth atmospheric composition and air pollution using a high resolution Ultraviolet/Visible/Near - Infrared (UVN) sounder instrument to be deployed on the two geostationary MTG-S satellites. S4-UVN will be embarked on the MTG-S platform closely located to the IR- Sounder (IRS) and will be operated by EUMETSAT.

T

he Sentinel-4/UVN instrument is a high resolution spectrometer system operating with 3 designated bands in the solar reflectance spectrum, covering the UV (305-400nm), VIS (400-500 nm) and NIR (750-775 nm) bands.

The radiometric accuracy of the UVN instrument partly relies on regular in-orbit re-calibrations, over the useful spectral range, in order to determine for example the sensor gain factors and their drifts. The selected approach for in-flight calibration is based on 1°- measurements of the Sun’s irradiance as transmitted by a diffusing device (diffuser), and 2°- measurements of a calibration lamp source. Those functions are provided by the Calibration Assembly (CAA).

UVN is developped by Airbus DS Ottobrunn and OHB Munich is dealing the procurement of the sub-systems for Airbus including the CAA sub-contracted to CSL.

CSL’s task in this project is to design, qualify the CAA for UVN and to deliver one qualification model (QM) for test at system level and two flight models (FM1 and FM2).

The developpment of the CAA started in CSL in June 2013 and progressed up to a preliminary design which was successfully reviewed during a Preliminary Design Review (PDR) in September 2014. The design finalisation was successfully reviewed at the Critical Design Review (CDR) in February 2016. The manufacturing of  the qualification model to be delivered and a Life Test Model (LTM) for additional qualification testing started in the mean time in April 2015, qualification campaign was concluded by a Qualification Review (QR) in May 2017, fully releasing FM activities. FMs acceptance test and calibration campaigns started in September 2017 to end in July 2018.

QM was delivered  in December 2016 to OHB/ADS, Flight Models will be delivered in December 2018.

This project has been challenging due to the various critical elements including:

• A high reliability mechanism for mode selection;
• High performance diffusers for which specific test set-ups have been built to be able to evaluate the impact of spectral features on the performances;
• Bidirectional Transmission Distribution Function measurement realised on our modified optical bench in ISO 5 condition;
• A highly stable White Light Source (WLS) requiring re-qualification of some of its components.

Aknowledgment

UVN project at CSL has been run under contract with OHB Munich.