ESA GNC Conference Papers Repository

CHEOPS (CHaracterizing ExoPlanet Satellite) is the first mission dedicated to search for transits of exoplanets using ultrahigh precision photometry on bright stars already known to host planets. It is the first ESA S-class (small) mission, and it has been implemented by means of a partnership with Switzerland, which among other tasks was in charge of scientific lead and instrument development. Airbus Defence and Space Spain has been the prime designer and manufacturer of the platform, also conducting the satellite design, integration, validation and tests. Finally, Airbus has been responsible to lead and execute the LEOP and IOC activities and has also acted as Mission Control System provider. CHEOPS was successfully launched from Kourou on December 18th, 2019 as a co-passenger embarked on a Soyuz launcher. At the end of the in-orbit commissioning, the spacecraft started its nominal three years mission. The platform is based on AS250 product line from Airbus, which has been adapted to a scientific mission (a goal different from Earth Observation, which is the aim of this product line). The AOCS features a gyroless solution and it implements a payload in the loop design. The pointing performances highly improve the required APE of 4 arcseconds at 68%, reaching values below 1 arcsecond. Considering the mission needs, the design of the platform does not embark a GPS receiver, and orbit determination is based on Doppler measurements, which allows estimating the orbit on ground. The state vector is periodically and automatically uplinked from ground and subsequently propagated onboard. Besides review of achieved performances, this paper will detail in orbit experience for payload in the loop behaviour for non-standard phases like occultations, atmospheric Line of Sight passes and the update of the navigation function to work without GPS measurements. Based on the high scientific return of the mission and the excellent performance (including AOCS behaviour), CHEOPS lifetime has been extended, potentially up to 2028.