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A multiple-spacecraft close-proximity control algorithm was implemented and tested with the Synchronized Position Hold Engage and Reorient Experimental Satellites (SPHERES) facility onboard the International Space Station. During flight testing, a chaser satellite successfully approached a virtual target satellite while avoiding collision with a virtual obstacle satellite. This research contributes to the control of multiple spacecraft for emerging missions, which may require simultaneous gathering, rendezvous, and docking. The unique control algorithm was developed at the U.S. Naval Postgraduate School and integrated onto the Massachusetts Institute of Technology's SPHERES facility. The control algorithm implemented combines the efficiency of the linear quadratic regulator (used for attraction toward goal positions) and the robust collision-avoidance capability of the artificial potential field method (used for repulsion from moving obstacles). The amalgamation of these two control methods into a multiple-spacecraft close-proximity control algorithm yielded promising results, as demonstrated by simulations. Comprehensive simulation evaluation enabled implementation and ground testing of the spacecraft control algorithm on the SPHERES facility. Successful ground testing led to the execution of flight experiments onboard the International Space Station, which demonstrated the proposed algorithm in a microgravity environment. NomenclatureA, B, C = state-space matrices a = acceleration due to linear-quadratic-regulator-and artificial-potential-field-determined control effort a APF = acceleration due to artificial-potential-fielddetermined control effort a LQR = acceleration due to linear-quadratic-regulatordetermined control effort a m = maximum acceleration a obs = acceleration of chaser spacecraft toward an obstacle a x;y;z = acceleration due to the control effort

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A multiple spacecraft close-proximity control algorithm was implemented and tested with the Synchronized Position Hold Engage and Reorient Experimental Satellites (SPHERES) facility onboard the International Space Station (ISS). During flight testing, a chaser satellite successfully approached a virtual target satellite, while avoiding collision with a virtual obstacle satellite. This research contributes to the control of multiple spacecraft for emerging missions, which may require simultaneous gathering, rendezvous, and docking. The unique control algorithm was developed at NPS and integrated onto the MIT SPHERES facility. The control algorithm implemented combines the efficiency of the Linear Quadratic Regulator (LQR), and the robust collision avoidance capability of the Artificial Potential Function method (APF). The LQR control effort serves as the attractive force toward goal positions, while the APF-based repulsive functions provide collision avoidance for both fixed and moving obstacles. The amalgamation of these two control methods into a multiple spacecraft close-proximity control algorithm yielded promising results as demonstrated by simulations performed at NPS. Comprehensive simulation evaluation enabled implementation and testing of the spacecraft control algorithm on the SPHERES facility at MIT. Finally, successful ground testing enabled execution of flight testing onboard the ISS. The NPS's Spacecraft Robotics Laboratory (SRL) and MIT's Space Systems Laboratory (SSL) simulations, the MIT's SSL SPHERES ground testing, and the SPHERES flight testing results are all presented in this paper.

Abstract-An autonomous distributed control algorithm for multiple spacecraft performing simultaneous close proximity maneuvers has been developed. Examples of these maneuvers include automated on-orbit inspection, assembly, or servicing. The proposed control algorithm combines the control effort efficiency of the Linear Quadratic Regulator (LQR) and the robust collision avoidance capability of the Artificial Potential Function (APF) method. The LQR control effort serves as the attractive force toward goal positions, while APF-based repulsive functions provide collision avoidance for both fixed and moving obstacles. Comprehensive validation and performance evaluation of the control algorithm is conducted by numerical simulations. The simulation results show the developed LQR/APF algorithm to be both robust and efficient for controlling multiple spacecraft during simultaneous docking maneuvers.Note to Practitioners-Use of multiple spacecraft for close proximity operations is expected to increase in future space missions. A challenging problem is how to automate motion planning and control of multiple spacecraft in close proximity. This paper presents a distributed control algorithm for simultaneous docking maneuvers of multiple spacecraft.