Constellation Phasing with Differential Drag on Planet Labs Satellites

A methodology is presented for the differential drag control of a large fleet of propulsionless satellites deployed in the same orbit. The controller places satellites into a constellation with specified angular offsets and zero relative speed. Time-optimal phasing is achieved by first determining an appropriate relative placement because the satellites are interchangeable. A second optimization problem is then solved as a large coupled system to find the drag command profile required for each satellite. The control authority is the available ratio of low-drag to high-drag ballistic coefficients of the satellites given realistic operational constraints. The controller is able to successfully phase constellations with up to 100 satellites in simulations. On-orbit performance of the controller is demonstrated by phasing Planet Labs's Flock 2p constellation of 12 CubeSats, launched in June 2016, into a 510 km sun-synchronous orbit.