Control and flight performance of tethered satellite small expendable deployment system-II

Abstract. The second mission of the small expendable deployment system (SEDS-II) followed the successful mission of SEDS-I, which deployed freely a small instrumented probe on a 20-km tether. Unlike SEDS-I, the deployment 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 of less than 1 m/s. The control problem was made difficult by the limited capabilities of the SEDS sensors and onboard computer and the large uncertainties inherent in the response of the actuator (brake) and the plant (deployer). The nonlinear, nonautonomous control problem is divided in two parts by using a numerically formulated feedback linearization, i.e., by devising 1) a nonlinear control (reference) trajectory and 2) a linear control about the reference trajectory. An ad hoc feedback law that forces the perturbed system to follow the reference trajectory is derived by using a linearized variational model. The controller is then tested, through computer simulations, for large deviations of the model parameters on the nonlinear model. The relevant flight data are also presented and compared to the reference values to demonstrate the validity and robustness of the control law, which provided a maximum libration amplitude of less than 4 deg and a final tether velocity of less than 0.02 m/s.