Minimize MultiSpectral Instrument (MSI) Overview

The design of the Multispectral Instrument (MSI) on-board SENTINEL-2 has been driven by the requirement for large swath high geometrical and spectral performance of the measurements.

The MSI measures the Earth's reflected radiance in 13 spectral bands from VNIR to SWIR (Table 1).

Table 1: Spectral bands for the SENTINEL-2 sensors (S2A & S2B)

 

S2A

S2B

 

Band Number

Central wavelength (nm)

Bandwidth (nm)

Central wavelength (nm)

Bandwidth (nm)

Spatial resolution (m)

1

443.9

27

442.3

45

60

2

496.6

98

492.1

98

10

3

560.0

45

559

46

10

4

664.5

38

665

39

10

5

703.9

19

703.8

20

20

6

740.2

18

739.1

18

20

7

782.5

28

779.7

28

20

8

835.1

145

833

133

10

8a

864.8

33

864

32

20

9

945.0

26

943.2

27

60

10

1373.5

75

1376.9

76

60

11

1613.7

143

1610.4

141

20

12

2202.4

242

2185.7

238

20

 

Figure 1 (below) shows the S2A spectral response functions (SRF) and transmission due to vertical column Water Vapour (WV) absorption for subarctic winter atmosphere (light blue, lowest WV content) and tropical atmosphere (darker blue, highest WV content). WV transmission data comes from MODTRAN.

Note: In order to better view the Figure, right click on the image, and select 'View Image'. 

Figure 1: SRF and transmission due to vertical column WV absorption

S2A-spectral response functions (SRF) and transmission due to vertical column Water Vapour (WV) absorption for subarctic winter atmosphere (light blue, lowest WV content) and tropical atmosphere (darker blue, highest WV content). WV transmission data comes from MODTRAN.

MSI Description

The MSI is designed and built by Airbus Defence and Space, France, and is based on a push-broom concept (Figure 1).

MSI instrument features

  • A Three-Mirror Anastigmat (TMA) telescope with a pupil diameter equivalent to 150 mm, isostatically mounted on the platform to minimise thermo-elastic distortions. The optical design has been optimised to achieve state-of-the-art imaging quality across its very wide field of view. Its 290 km swath width is a larger swath than previous multi-spectral optical missions such as SPOT and LANDSAT. The telescope structure and the mirrors are made of silicon carbide, which provides a very high optical stability to mass ratio and minimises thermal deformation.
  • Two focal planes based on:
    • monolithic CMOS detectors for VNIR
    • mercury-cadmium-telluride detectors hybridised on a CMOS read-out circuit for SWIR (the SWIR focal plane is passively thermo-controlled at temperatures below 195 K).
  • A dichroic VNIR/SWIR beam splitter. Spectral separation in the various bands is provided by stripe filters mounted on top of the detectors.
  • Two distinct arrays of 12 detectors mounted on each focal plane covering VNIR and SWIR channels respectively. The 12 detectors on each focal plane are in a staggered configuration to cover the entire field of view.
  • A full-field and full-pupil on-board diffuser for radiometric calibration to guarantee high-quality radiometric performance.
  • A shutter mechanism preventing the instrument from direct illumination by the sun. This mechanism is also used as a calibration device by collecting the sunlight after reflection by a diffuser.

Figure 2: The MultiSpectral Instrument (Airbus Defence and Space)

Figure 3: MSI Internal Configuration

The 12 detectors on each focal plane are mounted in a staggered formation (Figure 3) to cover the whole 20.6o instrument field of view, resulting in a compound swath width of 290 km on the ground track. As illustrated in the figure below, due to the staggered positioning of the detectors on the focal planes, a parallax angle between the two alternating odd and even clusters of detectors is induced in the measurements, resulting in a shift along track of approximately 46 km (maximum) inter-detector. Likewise, the hardware design of both the VNIR and SWIR detectors imposes a relative displacement of each spectral channel sensor within the detector resulting in an inter-band measurement parallax amounting to a maximum along track displacement of approximately 14 km.

Figure 4: Staggered Detector Configuration and Inter-Detector / Inter-Band Parallax Angles (parallax figures derived from MSI instrument documentation)

 

Due to the particular geometrical layout of the focal plane, each spectral band of the MSI observes the ground surface at different times.

Table 2: The temporal offset (in seconds) between selected Bands

Inter-band

Pair

Temporal offset

between bands

(seconds)

B08 / B02

0.264

B03 / B08

0.264

B03 / B02

0.527

B10 / B03

0.324

B10 / B02

0.851

B04 / B10

0.154

B04 / B02

1.005

B05 / B04

0.264

B05 / B02

1.269

B11 / B05

0.199

B11 / B02

1.468

B06 / B11

0.057

B06 / B02

1.525

B07 / B06

0.265

B07 / B02

1.790

B8a / B07

0.265

B8a / B02

2.055

B12 / B8a

0.030

B12 / B02

2.085

B01 / B12

0.229

B01 / B02

2.314

B09 / B01

0.271

B09 / B02

2.586