Modeling And Control Of Spacecraft Systems With Coupled Orbital And Attitude Dynamics

Spacecraft formation flying with coupled orbital-attitude dynamics is one of the most intriguing topics in the field of astronautics. Orbital-attitude coupling is induced when a non-symmetrical spacecraft in orbit is disturbed by means of active maneuvering or by external disturbances. Direct contributing factors to the coupled dynamics include the orbital radius, the gravitational parameter and the orbital angular velocity. Disturbance due to coupling is inherently weak in nature (in the order of magnitudes of 10−13 Newtons) for Earth orbit, which majority of spacecraft attitude-orbital control system (AOCS) can easily overcome or can be eliminated by means of system dynamics linearization. For very large spacecraft that have very high moment of inertia, coupled dynamics can impose strong nonlinear disturbance and can affect orbital trajectory. Numerical simulations of the coupled dynamics for a rigid-body single spacecraft system, a dumbbell spacecraft system and a multiple spacecraft formation flying system are conducted for Earth and asteroid 4 Vesta orbits. Simulation results suggest that dumbbell spacecraft systems are the most severely affected by the orbital-attitude coupling due to the connecting tether. Nonlinear coupled orbital-attitude equations of motion are fully developed and are used to formulate a nonlinear controller using feedback linearization. Feedback linearization control method is perfect for this system because the spacecraft’s nonlinear coupled dynamics is preserved and not approximated. The controller is validated by numerical simulations as well as implemented in a hardware-in-the-loop experiment using the Ryerson University’s Satellite Airbed Formation Experiment. For asteroid-related missions, orbital-attitude coupling can be several magnitudes times larger than the coupling experienced for Earth orbit depending on the properties of the asteroid and thus in turn, can severely affect the performance of the spacecraft control system.