I have two satellites flying in formation geverned by the equations of motion including J2 and drag. Now, one of the satellites has more drag than the other so they become appart. I designed a controller (proportional integral) which meassures the along track position between them and gives Thrust values so the error, which is the difference in the along ttrack position between different orbits, will be zero (that means they will still fly in formation) I have to give a stability proof for the controller, someone can help me? (the equations governing the system are nonlinear, of course). thanks!