Run Time Assurance for Spacecraft Attitude Control Under Nondeterministic Assumptions

This article presents a comprehensive development and testing of a run time assurance (RTA) filter for a torque-controlled spacecraft in free rotational motion with torque actuation limits for which the objective is to enforce a line-of-sight constraint. A nondeterministic dynamical model is considered for the spacecraft that accounts for disturbance torques, and a guaranteed safe RTA filter is constructed using recent results from mixed monotone systems theory for reachable set overapproximations and optimization-based computation of invariant sets. The RTA filter ensures that the system is always within reach of an a priori safe terminal set by computing reachable sets of the dynamics online at run time. The approach is demonstrated on the Autonomous Spacecraft Testing of Robotic Operations in Space (ASTROS) platform at the Georgia Institute of Technology, Atlanta, GA, USA. In the experiment, potentially unsafe inputs are provided by a human, and the RTA filter overrides the human-commanded inputs when necessary to guarantee safety. The controller update rate for the ASTROS platform is about 10 Hz, while the RTA filter requires about 1 ms of computation time per controller update.