The 3MI instrument is a multi-directional spectro-polarimeter to fly on-board the Metop-SG platform with a launch scheduled end 2025, as part of EUMETSAT’s EPS-SG system space segment. The performance of the Level-1 products is an important pre-requisite for the quality of the Level-2 products devoted to aerosol and cloud characterisation. Like for all polarimeters, this performance is the result of a system implying many contributors, radiometric as well as geometric. The instrument sensitivity to the polarisation is obviously a crucial part in this budget while it is used in the Level-1 processing to convert the measured radiometry through different polarisers into the physical Stokes vector characterising the observed polarisation. If direct measurements of the polarisation sensitivity, called the Mueller Matrix, can be made through a dedicated on-ground characterisation, a physical understanding of this behaviour is a clear asset. It is indeed important to distinguish in the Mueller Matrix, all the physical contributors which are supposed to play a significant role according the theoretical ray-tracing calculation. These terms are the polariser itself obviously, but also the contribution of the lenses and their polarisation signatures in term of small linear polarisation and retardance. In this paper, the 3MI Proto-Flight Model behaviour measured during the ground campaign, is analysed and a modelling is proposed trying to provide a complete interpretation of the instrumental behaviour. Moreover, this model will be very useful for interpretation of the in-flight vicarious calibration results which will check and monitor this instrumental behaviour.
The Multi-View, Multi-Channel, Multi-Polarisation Imaging (3MI) instrument is a passive scanning radiometer dedicated to aerosol characterisation, air quality and numerical weather prediction, as well as climate monitoring and more generally characterisation of the microphysical properties of the atmosphere, including clouds. The 3MI mission has heritage from the POLarization and Directionality of the Earth's Reflectances (POLDER) on-board the ADEOS and PARASOL satellites. Compared to POLDER, 3MI has improved spatial coverage, higher spatial resolution, and an expanded spectral range with more spectral bands in the reflective part of the spectrum, all bands being polarised (except absorption bands). It is scheduled for launch on the EPS-SG platform in 2024.
3MI’s mission is to provide images of the Earth Top-Of-Atmosphere outgoing radiance for 12 different spectral bands (from 410nm to 2130nm), with 3 different polarisers (-60°, 0° and +60°), and 14 angles. The design consists of two optical heads (SWIR and VNIR) composed by a detector along with a filter and polariser rotating wheel and a wide field-of-view optics. The multi-view is achieved by several successive overlapping acquisitions of the same Earth-Atmosphere target under different angles thanks to the instrument large field of view.
Using the experience acquired for the POLDER missions, CNES is cooperating with EUMETSAT and is providing the necessary analysis and expertise for the in-flight calibration and/or validation of several key parameters, both for geometric and radiometric aspects. Here we present the different methodologies that will be used to achieve that goal.
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