Long-term cluster flight of multiple satellites using differential drag

The idea to use differential drag for satellite formation keeping emerged in the mid-1980s, when the feasibility of differential-drag-based control was proven, assuming linearized relative dynamics for two satellites. Unlike previous work in differential-drag-based formation keeping, the present work develops a nonlinear method suitable for missions in excess of a year. It is shown that the differential mean eccentricity is uncontrollable for near-circular orbits, and hence a nonlinear differential-drag-based controller for matching the drag-related secular component of the semimajor axis is developed. An asymptotic stability proof for the controller is provided. Moreover, two new methods for differential-drag-based cluster keeping of multiple modules are developed, thus expanding existing literature, which usually deals with two satellites only. The results are validated using simulations based on the forthcoming Space Autonomous Mission for Swarming and Geolocation with Nanosatellites, showing that differential-drag-based cluster keeping can be effective for altitudes reaching about 600 km.