1.
THE MSI MAIN FEATURES
The design of the MSI instrument is mainly driven by the Spatial Sampling Distance (SSD) of 10 m; the swath of 290 km which requires a large field of view of 20.6°and the 13 spectral bands within a large spectral domain from 0.4 to 2.4 µm.
Figure-1 :
Sentinel-2 Mission spectral and SSD requirements
The MSI instrument is based on a push-broom concept. It features a unique mirror silicon carbide off-axis telescope with a 150 mm pupil feeding two focal planes spectrally separated by a dichroic filter. CMOS and hybrid HgCdTe detectors are selected to cover the Visible and Near Infra Red (VNIR) and Short Wavelength Infra Red (SWIR) channels. The MSI instrument includes a sun calibration and shutter mechanism. The 1.4 Tbits image video stream, once acquired and digitized is compressed inside the instrument.
The instrument carries one external sensor assembly that provides the attitude and pointing reference to ensure a 20 m pointing accuracy on the ground before image correction.
Figure-2:
MSI internal configuration
The main instrument design drivers are recalled here after:
Figure-3;
The overall mass of the MSI instrument is 230Kg; its power consumption is 200W in Imaging mode and 60 W in stand-by mode
2.
OPTOMECHANICAL ARRANGEMENT
The optical configuration is based on a Three-Mirror Anastigmat (TMA) telecentric telescope, which can achieve the requested performance and geometric constraints with a minimum of optical elements. The telescope comprises three aspheric mirrors: M2 mirror is a simple conic surface, whereas the other mirrors need more aspherisation terms.
Figure-4:
Dimensions of the MSI mirrors
The entrance pupil is rectangular. It is equivalent to a 150 mm diameter full pupil. It is located on the M2 mirror, which gives the best balance between M1 and M3 dimensions, and is suitable for image telecentricity.
Since the VNIR and SWIR detectors are different, the complete imaging of the required spectral bands is done using a dichroic separation inside the splitter unit.
Figure-5:
The spectral filtering onto the different VNIR and SWIR spectral bands is ensured by slit filters mounted on top of the detectors. These filters provides the required spectral isolation
3.
MECHANICAL AND THERMAL ARCHITECTURE
This design aims at maintaining separated functions to allow parallel development of the main assemblies and simple alignment at instrument level. It also minimises number of structural items to cope with stability, manufacturing, and assembly constraints. The separated assemblies (TMA telescope, SWIR and VNIR focal planes, Calibration and Shutter Mechanism, Primary and Secondary Structure can be developed, integrated and tested separately prior final integration of the instrument.
Figure-5:
Main assemblies of the MSI
The telescope mirrors and structural baseplate are made of Silicon Carbide material in order to minimise thermo-elastic distortions. Isostatic mounts decouple the instrument from potential deformations of the platform upper plate.
Figure-6:
Telescope mechanical configuration
The thermal design ensures an homogenous environment for the telescope and a high temperature stability of both focal planes.
Figure :
SWIR detectors temperature evolution during imaging
4.
VNIR AND SWIR FOCAL PLANE ASSEMBLIES
Both focal planes accommodate 12 elementary detectors in two staggered rows to get the required swath The SWIR focal plane operates at -80°C whereas the VNIR focal plane operates at 20°C. Both focal planes are passively cooled down. A monolithic SiC structure provides support to the detectors, the filters and their adjustment devices and offers a direct thermal link to the radiator
Figure-6:
Focal plane configuration
5.
KEY COMPONENTS: FILTERS AND DETECTORS
Key components have been identified as key performance drivers for the mission Dedicated Strip filters mounted on top of each VNIR or SWIR detector provides the required spectral templates for each spectral bands.
Figure-7:
VNIR and SWIR spectral filters (Courtesy of Iéna-Optronik)
The VNIR detector is made of a CMOS die, using the 0.35 µm CMOS technology, integrated in a ceramic package. The detector architecture enables Correlated Double
Sampling for the 10 VNR spectral bands and Time Delay Integration (TDI) mode for the 10m bands. Black coating on the die eliminates scattering.
Figure-8:
CMOS detector with back coating
The SWIR detector is made of an HgCdTe photosensitive material hybridized to a silicon readout circuit (ROIC) and integrated into a dedicated hermetic package. The SWIR detector has three spectral bands for which the spectral efficiency is optimized. B11 and B12 bands are operated in (TDI) mode.
Figure-9:
SWIR detector EM model at hybridization stage (Courtesy of Sofradir)
6.
DETECTION CHAIN AND ELECTRONIC ARCHITECTURE
The main driver of the detection chain architecture is the implementation of 48 analogue-to-digital low noise video chains.
The Front End Electronics Modules (FEEM) extract, condition and transmit the video signals towards the Video and Compression Unit (VCU) The VCU controls the FEEMs and receives analogue video data from the FEEMs; it digitizes and pre-processes the data received from the FEEMs; it performs pixel equalisation, video data compression and formatting, and transmits the data packets to the Spacecraft; it performs the thermal control and housekeeping tasks; Finally it distributes the power supply to the FEEMs and the detectors.
Figure-10:
Schematic of the detection chain including the FEE modules distribution, the
nominal and redundant VCU functions.
7.
MAIN PERFORMANCE OF THE INSTRUMENT
The Signal to Noise performance is above 100 for all spectral bands.
Figure –11 :
radiometric performance of the MSI instrument
The Modulation Transfer Function is above or close to 0.15 for all 10m bands. It is above 0.20 for the other bands.
Figure –12 :
Along track and across track Modulation Transfer Function of the MSI instrument
8.
CONCLUSION
The design phase of the MSI instrument is currently under finalisation. The completion of the industrial team is underway in order to deliver the flight model mid 2011.
The MultiSpectral Instrument is the next generation of the European land imagers. Its performance will set new standards for the future multispectral / hyperspectral space cameras.
