Architecting Autonomy for Safe Microgravity Free-Flyer Inspection

Small free-flying spacecraft have the potential to provide vital extravehicular activity (EVA) services like inspection and repair for future orbital outposts such as the planned Lunar Gateway. Operating adjacent to delicate space station and other microgravity targets, these spacecraft require formalization to describe the autonomy that a free-flyer inspection mission must provide. This work explores the transformation of general mission requirements for this class of free-flyer into a set of concrete decisions for the planning and control autonomy architectures that will power such missions. Flowing down from operator commands for inspection of important regions and mission time-criticality, a motion planning problem emerges that provides the basis for developing autonomy solutions. Unique constraints are considered such as typical velocity limitations, pointing, and keep-in/keep-out zones, accompanied by a discussion of mission fallback techniques for providing hierarchical safety guarantees under model uncertainties and failure. Planning considerations such as cost function design and path versus trajectory control are discussed. The typical inputs and outputs of the planning and control autonomy stack of such a mission are also provided. Finally, notional system requirements such as solve times and propellant use are documented to inform planning and control design. The entire proposed autonomy framework for free-flyer inspection is realized in the SmallSatSim simulation environment, providing a reference example of free-flyer inspection autonomy. The proposed autonomy architecture serves as a blueprint for future implementations of small satellite autonomous inspection in proximity to mission-critical hardware, going beyond the existing literature in terms of both (1) providing realistic system requirements for an autonomous inspection mission and; (2) translating these requirements into autonomy design decisions for inspection planning and control.