Two-stage path planning approach for designing multiple spacecraft reconfiguration maneuvers and application to SPHERES onboard ISS

Abstract

The thesis presents a two-stage approach for designing optimal reconfiguration maneuvers for multiple spacecraft. These maneuvers involve well-coordinated and highly-coupled motions of the entire fleet of spacecraft while satisfying an arbitrary number of constraints. This problem is particularly difficult because of the nonlinearity of the attitude dynamics, the non-convexity of some of the constraints, and the coupling between the positions and attitudes of all spacecraft. As a result, the trajectory design must be solved as a single 6N DOF problem instead of N separate 6 DOF problems. The first stage of the solution approach quickly provides a feasible initial solution by solving a simplified version without differential constraints using a bi-directional Rapidly-exploring Random Tree (RRT) planner. A transition algorithm then augments this guess with feasible dynamics that are propagated from the beginning to the end of the trajectory. The resulting output is a feasible initial guess to the complete optimal control problem that is discretized in the second stage using a Gauss pseudospectral method (GPM) and solved using an off-the-shelf nonlinear solver. This thesis also places emphasis on the importance of the initialization step in pseudospectral methods in order to decrease their computation times. It demonstrates the improvement that an initial guess based on an RRT planner brings to an optimal control problem solved using pseudospectral methods. Finally, this thesis presents the successful results of several reconfiguration maneuver experiments performed using the Synchronized Position Hold Engage and Reorient Experimental Satellites (SPHERES) hardware testbed onboard the International Space Station (ISS). The maneuvers were designed using two different two-stage algorithms presented in this work. It also discusses the lessons learned from these tests, and the recommendations to improve future ISS reconfiguration experiments.