The fine guidance system of the PLATO mission

Abstract

PLATO - PLAnetary Transits and Oscillation of stars - is a medium-class mission in the European Space Agency (ESA) Cosmic Vision programme, whose launch is foreseen by 2026. The objective is the detection and characterization of terrestrial exoplanets up to the habitable zone of solar-type stars by means of their transit signature in front of a very large sample of bright stars. The seismic oscillations of the parent stars orbited by these planets is measured in order to understand the properties of the exoplanatory systems. The PLATO payload consists of an instrument with 26 cameras for star observation; 24 normal cameras grouped in four subsets with six cameras each and two fast cameras. Besides providing scientific data for very bright stars, the fast cameras also serve as two redundant Fine Guidance System (FGS) and will be an integral part of the Attitude and Orbit Control System (AOCS). This ensures a very high pointing precision which is needed to achieve a high photometric precision. Working as a star-tracker, the attitude calculation is based on guide star positions on the focal plane and their reference directions given by a star catalogue. Compared to predecessor missions like CoRoT, Kepler, or TESS, the precision of the fine guidance algorithm needs to be increased significantly. This is especially challenging as the optical design is identical for all cameras and optimized to meet the science objectives rather than to serve as a star-tracker. Therefore, a novel approach based on a Gaussian fit is proposed. The shown algorithm provides a noise optimal estimation of the guide star positions which propagates to an optimal attitude estimation. Although, computational more expensive than conventional methods, its suitability for a real-time on-board application is proven with an implementation on the target hardware. Furthermore, its robustness and precision is assessed theoretically and with simulated star images sequences.