GLOBALLY STABLE FEEDBACK LAWS FOR NEAR-MINIMUM-FUEL AND NEAR-MINIMUM-TIME POINTING MANEUVERS FOR A LANDMARK-TRACKING SPACECRAFT

Utilizing unique properties of a recently developed set of attitude pa rameters, the modified Rodrigues parameters, feedforward/feedback type control laws are developed for a spacecraft undergoing large nonlinea r motions using three reaction wheels. The method is suitable for trac king given reference trajectories that spline smoothly into a target s tate; these reference trajectories may be exact or approximate solutio ns of the system equations of motion. An associated asymptotically sta ble nonlinear observer is formulated for state estimation. In particul ar, we illustrate the ideas using both near-minimum-time and near-mini mum fuel rotations about Euler's principal rotation axis, with paramet erization of the sharpness of the control switching for each class of reference maneuvers. Lyapunov stability theory is used to prove rigoro us global asymptotic stability of the closed-loop tracking error dynam ics in the absence of external torques. If external torques are presen t, then the system is Lagrange stable. The methodology is illustrated by designing example control laws for a prototype landmark-tracking sp acecraft; simulations are reported that show this approach to be attra ctive for practical applications. The inputs to the reference trajecto ry are designed with user-controlled sharpness of all control switches to enhance the trackability of the reference maneuvers in the presenc e of structural flexibility.