OpenSky: a modular and open-source simulator of sky polarization measurements

International audience Autonomous navigation requires robust strategies, particularly in GPS-denied environments. Of great interest for geolocation or to estimate North is the measurement and processing of polarized skylight patterns. In this study, we fully describe an open source and easily upgradable simulator, named OpenSky, which can simulate the measurement of sky polarization properties, i.e., the light intensity, the angle of polarization and the degree of linear polarization of light seen through a polarimetric camera. OpenSky is an open source simulator available on open repositories (Github and Open Science Framework). It structured around 5 blocks, each of which can easily be improved, replaced or completed by users. These blocks individually simulate sky polarization properties, skylight intensity, optical conjugation, polarizing filters and sensors. The high fidelity of OpenSky was assessed by comparing the simulated camera captures to experimental raw images, from which both the angle and the degree of polarization were extracted by image processing. OpenSky will be useful for developing novel celestialbased sensors and estimating their potential relevance to scientific communities (fundamental or applied sciences). OpenSky could also become a powerful tool to train or validate deep neural networks. One major interest is the ability of the OpenSky simulator to generate many sky conditions at various positions on Earth, which can be difficult and costly to obtain in real world.