Drag-Based Formation Control of Millennium Space Systems Satellites

Aside from gravity, aerodynamic drag is typically the strongest environmental force acting on a spacecraft in low Earth orbit. While often treated as a disturbance whose perturbative effects on the orbit must be mitigated through periodic thruster burns, drag can potentially be harnessed for on-orbit maneuvering.This paper discusses a practical algorithm for achieving and maintaining a desired separation between two satellites using differential aerodynamic drag. By increasing the drag and thereby reducing the mean semi-major axis of “lead” vehicle, the separation between the satellites is increased. Conversely, by increasing the relative drag on the “chase” vehicle, inter-satellite separation is decreased. With this control concept, the derived algorithms compute how long each satellite should stay in either a minimum or maximum drag configuration and when the drag configurations should be changed.Flight results are presented for two propellant-free vehicles operated by Millennium Space Systems where this drag control strategy was successfully applied to achieve a desired inter-satellite separation. Using open loop commands, ephemeris knowledge from freely available TLEs, and the ability to orient the primary solar arrays differently while in eclipse, mission operators were able to achieve a vehicle separation within the required tolerance with minimal drift over time.