Instrument polarisation model for the EPS-SG/3MI polarimeter: theoretical assumption and adjustment using on-ground measurements

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.