Architecting Autonomy for Safe Microgravity Free-Flyer Inspection

Small free-flying spacecraft have the potential to pro-vide vital extravehicular activity (EVA) services like inspectionand repair for future orbital outposts such as the planned Lu-nar Gateway. Operating adjacent to delicate space station andother microgravity targets, these spacecraft require formalizationto describe the autonomy desiderata that a free-flyer inspectionmission must provide. This work explores the transformation ofgeneral mission requirements for this class of free-flyer into aset of concrete decisions for the planning and control autonomyarchitectures that will power such missions. Flowing down fromoperator commands for inspection of important regions and missiontime-criticality, a motion planning problem emerges that providesthe basis for developing autonomy solutions. Unique constraintsare considered such as typical velocity limitations, pointing andplume impingement, and keep-in/keep-out zones, accompanied by adiscussion of mission fallback techniques for providing hierarchicalsafety guarantees under model uncertainties and failure. Planningconsiderations such as planning horizons, cost function design,and path vs. trajectory control are discussed. The typical inputsand outputs of the planning and control autonomy stack of sucha mission are also provided. Finally, notional system require-ments such as anticipated SWAP-C and fuel are documented toinform planning and control design. The entire proposed autonomyframework for free-flyer inspection is realized in the SmallSatSimsimulation environment, providing a reference example of free-flyerinspection autonomy. The proposed autonomy architecture serves asa blueprint for future implementations of small satellite autonomousinspection in proximity to mission-critical hardware, going beyondthe existing literature in terms of both (1) providing realistic systemrequirements and desiderata for an autonomous inspection missionand; (2) their translation into algorithmic structuring and autonomydesign decisions for inspection planning and control.

小型自由飞行航天器有望为未来的轨道前哨（如规划中的月球门户（Lunar Gateway））提供关键的舱外活动（EVA）服务，例如检查和维修。由于需在精密空间站及其他微重力目标附近运行，这类航天器需要通过形式化方法来描述自由飞行检查任务必须满足的自主性需求（desiderata）。本研究探讨了如何将这类自由飞行航天器的通用任务需求转化为规划与控制自主架构的一系列具体决策，这些架构将为这类任务提供支撑。从对重要区域进行检查的操作员指令和任务时间关键性推导而来，一个运动规划问题应运而生，为自主解决方案的开发提供了基础。研究考虑了独特的约束条件，如典型的速度限制、指向与羽流冲击（plume impingement）以及准入/禁入区域（keep-in/keep-out zones），同时讨论了在模型不确定性和故障情况下提供分层安全保障的任务后备技术。文中还讨论了规划时域、代价函数设计以及路径与轨迹控制对比等规划相关考量。此外，本文还给出了这类任务的规划与控制自主栈的典型输入和输出。最后，本文记录了预期的SWAP-C及燃料等概念性系统需求，为规划与控制设计提供参考。所提出的自由飞行检查自主框架已在SmallSatSim仿真环境中实现，为自由飞行检查自主性提供了参考实例。该自主架构可作为未来在关键任务硬件附近实施小型卫星自主检查的蓝图，其超越现有文献之处体现在两个方面：（1）为自主检查任务提供了真实的系统需求和desiderata；（2）将这些需求转化为检查规划与控制的算法结构和自主设计决策。