20 June 2022
19 May 2020
The 3MI instrument will provide a multi-spectral (from 410 to 2130nm), multi-polarisation (−60°, 0°, and +60°), and multi-angular (14 views) image of the Earth outgoing radiance at the top of the atmosphere.
The 3MI mission has heritage from the POLarization and Directionality of the Earth's Reflectances (POLDER) onboard the ADEOS satellites. Compared to POLDER, 3MI will have improved spatial coverage, higher spatial resolution, and an expanded spectral range with more spectral bands having polarisation measurements.
The primary objective of the 3MI mission is to provide high quality imagery of aerosols variables for climate records, etc. through the provision of:
- Aerosol optical depths for accumulation, coarse and total modes at high horizontal resolution.
- Aerosol particle size for accumulation, coarse and total modes.
- Aerosol type through Ǻngström exponent, refractive index, non-sphericity index.
- Aerosol height index.
- Aerosol absorption.
When used as constraints to the models these products will be used to provide:
- Improved Air Quality Index
- PM (Aerosol Load mass for particles smaller than 2.5µm (PM2.5) or 10µm (PM10).
Secondary mission objectives include:
- Surface albedo
- Improved cloud characterisation: cloud phase, cloud microphysics (phase and effective particle size) cloud height, cloud optical depth.
- Ocean colour
The VII provides information on most cloud variables, but cirrus clouds observation requires multi-viewing and multi-polarisation capability of 3MI to characterise:
- extension, optical depth, particle size;
- a-sphericity factor, crystal orientation, phase function (side and backscattering part).
Spectral characteristics of 3MI mission
The 3MI instrument is a passive satellite radiometer capable of measuring polarised radiances reflected by the Earth, under different viewing geometries, in specified spectral bands, from the visible to the shortwave infrared parts of the electromagnetic spectrum.
The table below shows the central wavelength, bandwidth, polarisation and noise parameter for 3MI. There are 12 spectral channels, of which nine gives polarised measurements. The spectral range is extended as well as a number of polarised channels increased in comparison to POLDER. Like POLDER, 3MI does not have an on-board calibration system, therefore, radiometric performance will be dependent on the accuracy of the vicarious calibration techniques).
| Channel | Central Wavelength (in micrometres) |
FWHM (in MICROmetres) | Polarisation (I, Q, U) | SNR |
|---|---|---|---|---|
| 3MI-2b | 0.410µm | 0.02 | Y | 200@ 46.64W m-2 sr-1 µm-1 |
| 3MI-3 | 0.443µm | 0.02 | Y | 200@ 56.61W m-2 sr-1 µm-1 |
| 3MI-4 | 0.490µm | 0.02 | Y | 200@ 57.74W m-2 sr-1 µm-1 |
| 3MI-5 | 0.555µm | 0.02 | Y | 200@ 55.24W m-2 sr-1 µm-1 |
| 3MI-6 | 0.670µm | 0.02 | Y | 200@ 44.57W m-2 sr-1 µm-1 |
| 3MI-7 | 0.763µm | 0.01 | N | 200@ 36.11W m-2 sr-1 µm-1 |
| 3MI-8 | 0.76 µm | 0.04 | N | 200@ 36.11W m-2 sr-1 µm-1 |
| 3MI-9 | 0.865µm | 0.04 | Y | 200@ 28.20W m-2 sr-1 µm-1 |
| 3MI-9a | 0.910µm | 0.02 | N | 200@ 25.18W m-2 sr-1 µm-1 |
| 3MI-10 | 1.370µm | 0.04 | Y | 200@ 10.73W m-2 sr-1 µm-1 |
| 3MI-11 | 1.650µm | 0.04 | Y | 200@ 6.80W m-2 sr-1 µm-1 |
| 3MI-12 | 2.130µm | 0.04 | Y | 200@ 2.50W m-2 sr-1 µm-1 |
3MI measuring principle
The measuring concept of 3MI is similar to that of POLDER. The multi-viewing capability will be achieved by acquiring 2D images of the same target on Earth, using the same spectral channel at 14 different angles at regular points along the along-track acquisition points. At every along-track acquisition point, a full set of spectral and polarisation images are recorded by a rotating filter wheel.
At nadir, the 3MI SSD is better than 4km square (along-track x across-track) over the full EPS-SG orbit. This compares with approximately 6km of POLDER3 on the PARASOL orbit. Furthermore, 3MI features a minimum swath of 2200km over the EPS-SG orbit. This compares with the 1366km of the POLDER3/PARASOL instrument, thus, 3MI on board EPS-SG provides better coverage.
The first three components of the Stokes vector of the reflected light (I, Q, and U) will be measured at nine channels (see the table above) in the spectral range from 410 to 2130nm for up to 14 observation directions. The multi-polarisation (three acquisitions within one second for the polarised channel) and multi-spectral acquisitions, are done within a wheel rotation of less than seven seconds. The observation in one direction takes 21 seconds (three rotations of a wheel with the duration of seven seconds) and it is requires about five minutes to observe the same ground scene from 14 directions.
