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Distributed Satellite Systems Algorithm Maturation with SPHERES Aboard the ISS

Since beginning operations aboard the International Space Station (ISS) in May 2006, the Synchronized Position Hold Engage Re-orient Experimental Satellites (SPHERES) facility has completed a total of twelve test session with over 150 tests. The operational lessons learned from the first year of tests were presented at the 2007 IAC conference; this new paper will present a summary of the most relevant results of SPHERES research aboard the ISS. As a testing environment for distributed satellites systems, SPHERES research concentrates on the development of estimation, control, and autonomy algorithms for missions that include docking, formation flight, close proximity operations, and in-space assembly. By operating in the risk-tolerant environment created by the ISS, the SPHERES scientists can push the limits of the algorithms and attempt tests which would never be conducted as part of normal missions. In this manner SPHERES complements missions such as Orbital Express, while at the same time paving the way to future missions such as TPF, Darwin, and in-space assembly of large space structures, such as an inter-planetary stack. The first four test sessions (through August 2006) helped to validate the operation of the hardware and allowed the team to become efficient during each test session. Since then eight more test sessions have taken place. During these test sessions the SPHERES team has achieved multiple space-firsts, such as: docking to a tumbling target, demonstrated plume impingement effects in spacecraft of similar size, on-line path planning for a docking maneuver, coordinated formation flight of three satellites in microgravity, and formation reconfigurations. The team has also began tests of reconfiguration algorithms which allow the satellites to control each other after docking. Further, research has started on tests that simulate robotic inspection of other satellites. This research has been supplemented by demonstrations of obstacle avoidance algorithms operating in real-time. This paper will present results of the most advanced research of SPHERES by the time of publication in each of the principal areas of research: docking, in-space assembly, formation flight, and robotic inspection. The presentation of the results will be accompanied by high level descriptions of the algorithms used in each test, with references to other publications with detailed explanations of the algorithms. The applicability of the algorithms to future missions will be presented. The paper will conclude with a description of future test sessions to complete the goals of this phase of the SPHERES program.

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