SPHERES Demonstrations of Satellite Formations aboard the ISS
Saenz-Otero A, et al. (2009). "SPHERES Demonstrations of Satellite Formations aboard the ISS." 32nd Annual American Asronautical Society Guidance and Navigation Conference
Starting in 2007 the SPHERES team expanded its research operations aboard the ISS to include algorithms for formation flight systems. Based on the experiences learned by developing complex docking algorithms, the team began research to mature algorithms to perform imaging maneuvers and add autonomy to allow a formation system initialization and safing in case of failures. The imaging maneuvers research started with basic circular motion of independent satellites; these tests were highly successful. Next spiral maneuvers were performed. These proved more challenging as two things were changed in the tests: the dynamics of the system and the controller. Initial fixes to the controller improved performance, but not sufficiently. A distributed control algorithm, cyclic pursuit, was implemented, yielding high quality results. The autonomy algorithms included formation initialization from random locations and in random orders. The tests were partially successful, allowing the team to propose a potential algorithm and providing insight into potential failures modes of such algorithms (which were not exhibited during simulation). The scatter of a formation was demonstrated and subsequently used in the recovery part of a simulated thruster failure. A communications failure algorithm was implemented, but the tests aboard the ISS were not successful. A collision avoidance algorithm with low overhead was demonstrated. The data obtained from this test enables the team to validate the methods used to determine the size of safety areas between the satellites in a formation flight system, especially during reconfiguration of the constellation.