Sentinel-2 MultiSpectral Instrument Performance Results
Martimort, Philippe; Fernandez, V.; Kirschner, V.
GMES and Sentinel-2 Introduction
The GMES programme is a European initiative for the implementation of information services dealing with environment and security, based on observation data received from Earth Observation (EO) satellites and ground based information [http://www.copernicus.eu]. Within this context, ESA is responsible in particular, for the implementation of the GMES Sentinel missions, feeding the GMES services with operational EO data [http://www.esa.int/gmes]. The GMES Sentinel-2 optical imaging mission will be devoted to the operational monitoring of land and coastal areas. This system will deliver a new generation of optical data products designed to directly feed downstream services related to land monitoring, emergency management and security.
The Sentinel-2 MultiSpectral Instrument
The Multi Spectral Instrument embarked on Sentinel-2 satellite is an optical payload based on the push broom image acquisition concept. It features a Three Mirror Anastigmatic (TMA) telescope with a pupil diameter equivalent to 150 mm, which is iso-statically mounted on the platform to minimize thermo-elastic distortions. The optical design has been optimized to achieve state-of-the-art imaging quality all across its very wide Field of View: 290 km swath width which is significantly enlarged with respect to the 120 Km swath of SPOT-5, or to the 185 Km of the Operational Land Imager (OLI) embarked on the LANDSAT Data Continuity Mission (LDCM). The telescope structure and the mirrors are made of Silicon Carbide (SiC) which provides a very high optical stability to mass ratio and allows minimizing thermo-elastic deformations.
The Visible and Near Infra Red (VNIR) Focal plane is based on monolithic CMOS detectors while the Short Wave Infra Red (SWIR) Focal plane is based on Mercury Cadmium Telluride detectors (MCT) hybridized on a Complementary Metal Oxide Semi-conductor (CMOS) read-out circuit. Twelve VNIR and SWIR detectors are employed in a staggered configuration to cover the Field of View. The SWIR focal plane is passively thermo-controlled at a temperature below 195 K. A dichroic beam-splitter provides the spectral separation of VNIR and SWIR channels. Spectral separation in the various bands is provided by stripe filters mounted on top of the detectors.
A full field and full pupil on-board diffuser will be employed for radiometric calibration to guarantee a high quality radiometric performance. The observation data are digitized onto 12 bits. State-of-the-art lossy compression based on wavelet transform is applied to reduce the data volume. The compression ratio between 2 and 3 has been fine tuned for each spectral band to ensure that there is no significant impact on image quality. A shutter mechanism is implemented to prevent the instrument from direct viewing the sun in orbit and from contamination during launch. The same mechanism is also functioning as a calibration device by collecting the sunlight after reflection by a diffuser.
The Sentinel-2 MultiSpectral Instrument Performance Results
During the assembly, integration and test phase of the MSI Proto-Flight Model (PFM) that will be embarked on Sentinel-2A satellite, a comprehensive on-ground performance characterization and verification campaign will be performed encompassing testing at unit level (e.g. detectors, optical filters, electronics), at sub-system level (VNIR FPA, SWIR FPA, telescope) and finally at integrated MSI level. Radiometric and geometric performance will be characterized on-ground and detailed performance budgets will be established both for worst case condi-tions and also for typical conditions. In particular key image quality parameters will be measured such as Modulation Transfer Function (MTF), Signal to Noise Ratio (SNR), linearity, straylight and crosstalk. At the time of writing of this abstract, performance tests at unit and sub-system level have already been performed and tests at instrument level are about to start. A synthesis of all available performance results and related budgets will be presented at the Living Planet Symposium.