ESA GNC Conference Papers Repository

BepiColombo is an ESA cornerstone mission to Mercury in collaboration with the Japan Aerospace Exploration Agency (JAXA), with Airbus Defence & Space as prime industrial contractor for the ESA contribution. The two scientific orbiters -ESA's Mercury Planetary Orbiter (MPO) and JAXA's Mercury Magnetospheric Orbiter (MMO)- are launched together in 2018 as a single composite spacecraft, including a module with electric propulsion supporting the 7 year cruise phase. Notwithstanding some design similarity to previous European interplanetary missions like Rosetta, a number of novel GNC challenges had to be addressed. Apart from S/C modularity and the complex propulsion systems under GNC control, a main driver for GNC FDIR are the strong constraints on S/C attitude and solar array pointing in the harsh thermal environment at close sun distances. Key challenges include the following: (1) Even short S/C mispointings can't be tolerated. As there wouldn't be time to perform an attitude acquisition from 'lost in space' conditions at safe mode entry, the GNC is instead relying on attitude context information continuously maintained. Moreover, depending on the failure, a restart of the on-board computer at safe mode entry could lead to an intolerably long outage of attitude control. Therefore a second on-board computer -the Failure Control Electronics- had to be implemented, running a simplified GNC software, ready to take over attitude control temporarily in case of transient outage of the prime on-board computer. (2) The Mercury orbit insertion phase includes a series of highly constrained chemical propulsion manoeuvres, with the aim of delivering firstly the MMO and secondly the MPO into their operational orbits. In case of a premature abort of such a manoeuvre, the spacecraft may end in an orbit different to the planned one. Usage of previously uploaded guidance may hence lead to a violation of thermal constraints, as correct pointing with respect to Mercury is crucial. For these cases a special on-board GNC function is used, which is able to adjust the ground-provided guidance in case of an aborted manoeuvre, depending on the estimated deltaV realised prior to the abort. (3) The S/C solar arrays can't be pointed straight to the sun due to technological limitations. In Mercury orbit, solar array pointing has to be adjusted continuously to achieve sufficiently high power generation without violating thermal constraints. This requires a complex split of activities between spacecraft and ground, with solar array control by GNC according to a ground-provided profile in the form of Chebyshev polynomials, and ground producing the array pointing profile based on a thermal model. Dedicated on-board FDIR is implemented using hardwired alarms on solar array thermistor readings. (4) Attitude and solar array guidance for safe mode are ground-provided, in line with the estimated orbit evolution around Mercury. As the orbit is not controlled, frequent safe mode guidance updates in safeguard memory are required to adhere to the strict pointing constraints with respect to Mercury. This turns the maintenance of on-board guidance into a complex operations activity, for which any mistakes may have serious consequences at the next safe mode entry. To be launched in less than two years, the BepiColombo spacecraft is currently undergoing final integration and testing at ESA/ESTEC, with the S/C design and operations concepts fully finalised, and operations preparations at ESA/ESOC at an advanced stage. The paper will present the concepts, challenges and particularities for BepiColombo GNC FDIR, covering conceptual design aspects as well as the concrete operations approach.