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
The MIT Space Systems Laboratory (SSL) has developed a testbed for the testing of formation flight and autonomous docking algorithms in both 1-g and microgravity environments. The SPHERES testbed consists of multiple microsatellites, or Spheres, which can autonomously control their position and attitude. The testbed can be operated on an air table in a 1-g laboratory environment, in NASA's KC-135 reduced gravity research aircraft and inside the International Space Station (ISS). SPHERES launch to the ISS is currently manifested for May 19 2004 on Progress 14P. Various types of docking maneuvers, ranging from docking with a cooperative target to docking with a tumbling target, have been developed. The ultimate objective of this research is to integrate the different algorithms into one program that can assess the health status of the target vehicle, plan an optimal docking maneuver while accounting for the existing constraints and finally, execute that maneuver even in the presence of simulated failures. In this paper, results obtained to date on the ground based air table using the initial version of the program will be presented, as well as results obtained from microgravity experiments onboard the KC-135.
SPHERES (Synchronized Position Hold Engage Reorient Experimental Satellites) is a formation flight testing facility consisting of three satellites operating inside the International Space Station (ISS).The goal is to use the long term microgravity environment of the ISS to mature formation flight and docking algorithms. The operations processes of SPHERES have also matured over the course of the first seven test sessions. This paper describes the evolution of the SPHERES program operations processes from conception to implementation to refinement through flight experience. Modifications to the operations processes were based on experience and feedback from Marshall Space Flight Center Payload Operations Center, USAF Space Test Program office at Johnson Space Center, and the crew of Expedition 13 (first to operate SPHERES on station). Important lessons learned were on aspects such as test session frequency, determination of session success, and contingency operations. The paper describes the tests sessions; then it details the lessons learned, the change in processes, and the impact on the outcome of later test sessions. SPHERES had very successful initial test sessions which allowed for modification and tailoring of the operations processes to streamline the code delivery and to tailor responses based on flight experiences.
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.
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