Satellite Formation Control using Differential Drag

Abstract

The ability to operate in formations is a great advantage of small satellites as many small satellites can be built and launched for the cost of one large satellite. Formation operations are valuable because they allow for an observational resolution in time and space not possible with a single large satellite. However, traditional methods of formation control generally rely on thrusters which often consume significant power and are difficult and expensive to include on small satellites such as CubeSats. This paper discusses a thrusterfree means of controlling the in-track separations between satellites in a formation. Below about 700 km, aerodynamic forces can be harnessed to establish and maintain satellite formations. Through orientation changes, the drag forces experienced by satellites can be varied. Because orbit energy is reduced by an amount equal to the magnitude of the work done by the aerodynamic drag force, a satellite with greater drag will experience a decrease in semi-major axis and travel more quickly than a satellite initially in an identical orbit but with less drag. The effects of aerodynamic drag on satellite separation over time were analyzed and equations were derived governing satellite formation dynamics in the presence of drag. Simulations were conducted using AGI’s Systems Toolkit (STK) controlled by a MATLAB script. These simulations modeled the general case of elliptical orbits and included factors such as J2 effects, a non-standard atmosphere, non-standard drag coefficients, solar pressure, and third body gravitational forces that are difficult to model analytically. Simulation results verified the analytical solutions and indicated that formation control using aerodynamic forces was indeed feasible and practical for many missions. At altitudes between 700 km and 500 km, maneuver completion times were reasonable for many scientific missions but still long enough for satellite orientation changes to be commanded entirely from the ground based on observed satellite behavior.