Differential Drag as a Means of Spacecraft Formation Control

This paper investigates the feasibility of using differential drag as a means of nano-satellite formation control. Differential drag is caused when the ballistic coefficients of the spacecraft in a formation are not equal. The magnitude of differential drag depends on the difference in ballistic coefficients and also the altitude of the spacecraft formation. AGI's Satellite Tool Kit is used initially to assess the magnitude of drifts caused due to differential drag for different altitudes. This information is then used to show that it is feasible to use differential drag for spacecraft formation control. A simple PID controller is then implemented that adjusts the cross sectional areas of the satellites such that the energies of the orbits remain equal. Results are presented that show that the control law can maintain the formation separation with reasonable accuracy.