CONTROL AND FLIGHT PERFORMANCE OF TETHERED SATELLITE SMALL EXPENDABLEDEPLOYMENT SYSTEM-II

The second mission of the small expendable deployment system (SEDS-II) followed the successful mission of SEDS-I, which deployed freely a sm all instrumented probe on a 20-km tether. Unlike SEDS-I, the deploymen t of SEDS-II was controlled to provide a small libration amplitude and tether velocity at the end of deployment, The preflight goal for SEDS -II was a maximum libration of less than 10 deg and a final velocity o f less than 1 m/s. The control problem was made difficult by the limit ed capabilities of the SEDS sensors and onboard computer and the large uncertainties inherent in the response of the actuator (brake) and th e plant (deployer). The nonlinear, nonautonomous control problem is di vided in two parts by using a numerically formulated feedback lineariz ation, i.e., by devising 1) a nonlinear control (reference) trajectory and 2) a linear control about the reference trajectory. An ad hoc fee dback law that forces the perturbed system to follow the reference tra jectory is derived by using a linearized variational model. The contro ller is then tested, through computer simulations, for large deviation s of the model parameters on the nonlinear model. The relevant flight data are also presented and compared to the reference values to demons trate the validity and robustness of the control law, which provided a maximum libration amplitude of less than 4 deg and a final tether vel ocity of less than 0.02 m/s.