A renewed scientific interest has been growing in the exploration of small asteroids in addition to larger planetary bodies such as Mars, since their weaker gravitational field makes them more easily accessible. However, such exploratory missions are very challenging from an engineering perspective, particularly when striving for optimal propellant consumption. This is mostly due to the perturbed $ This work is funded by the UK Space Agency through a 2016 NSTP-2 Space Technology Fast Track grant entitled "Robust and Nonlinear Guidance and Control for Landing on Small Bodies". Mr. Simplício is also the recipient of a Doctoral Training Partnership award by the Engineering and Physical Sciences Research Council.
While common Descent and Landing strategies involve extended periods of forced motion, significant fuel savings could be achieved by exploiting the natural dynamics in the vicinity of the target. However, small bodies are characterised by perturbed and poorly known dynamics environments, calling for robust autonomous guidance, navigation and control. Airbus Defence and Space and the University of Bristol have been contracted by the UK Space Agency to investigate the optimisation of landing trajectories, including novel approaches from the dynamical systems theory, and robust nonlinear control techniques, with an application to the case of a landing on the Martian moon Phobos.
Descent & landing (D&L) on small planetary bodies are scientifically rewarding exploration missions but they are technically challenging due to the complex and poorly-known environment around those bodies. The standard guidance synthesis approach considers nominal conditions and applies optimal control theory to obtain guidance law gains, followed by intensive verification and validation. In this article, it is shown that the standard approach may yield gains that are not optimal once dispersions (and/or other optimality metrics) are taken into account and a tuning approach is then proposed based on a priori methodological system assessment. The proposed approach employs systematic high-fidelity simulations to generate trade-off maps. These maps can be generated by on ground operators based on the best estimated conditions and uploaded to the spacecraft as it approaches the target. The proposed systematic guidance tuning and resulting maps also provide a valuable understanding of the system dynamics towards the application of other industry-oriented tools such as structured H ∞ optimisation.It is shown that the proposed tuning enables propellant consumption reductions of around 40% compared to state-of-practice gain selections.
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