Straylight physical model and simulator for a rapid and flexible evaluation of impacts on the products derived from the EPS-SG/3MI instrument

The 3MI instrument is a multi-directional spectro-polarimeter to fly on-board the Metop-SG platform to be launched in 2022 as part of EUMETSAT's EPS-SG system space segment. As for most of radiometers devoted to Earth Observing from space, the radiometric budget is a system budget which includes many contributors, from the raw detector signal-to-noise to the artefact introduced by the ground correction in level-1 processing. The straylight happening in the optics is one of these contributors which could sometimes become dominant in the system budget. In this context, it is required to constrain the instrumental design as well as the ground characterization and its correction by processing. This is supported by maintaining accurate understanding of the straylight and its behaviour from early on in the project in order to check its radiometric level and quantify the potential impact on the products. This paper will present the analysis done to understand and model the physical behaviour of 3MI's straylight (in-field and out-of-field of view contributions). Based on initial inputs from the instrument builders and assuming some simplification, a physical straylight model was derived. This model allows an easy implementation in a simulator able to add this straylight contribution to any type of images, particularly on very realistic scenes. Our physical model shows advantageous complementarity to the builder's simulations in the sense that it allows a better quantification on a wide variety of realistic images while specifications usually assume theoretical targets such as a knife-edge transition. Such a capability is needed for 3MI system activities, in particular to cope with any deviation to the performance requirements. This paper presents examples of 3MI straylight-impacted images simulated at EUMETSAT using as input 1/ a simulation of the straylight provided by the instrument builder (ESA and Leonardo), and 2/ a realistic test data set based on PARASOL and MODIS acquisitions from the A-train observatory coupled with radiative transfer simulations (generated by ICARE/LOA). This simulator will be used during the development of the 3MI instrument and its ground characterization in order to monitor the impact on products.