In this paper, an adaptive, disturbance-based sliding-mode controller for hypersonic-entry vehicles is proposed. The scheme is based on high-order sliding-mode theory, and is coupled to an extended sliding-mode observer, able to reconstruct online the disturbances. The result is a numerically stable control scheme, able to adapt online to reduce the error in the presence of multiple uncertainties. The transformation of a high-order sliding-mode technique into an adaptive law by using the extended sliding-mode observer is, together with the multi-input/multi-output formulation for hypersonic-entry vehicles, the main contribution of this paper. The robustness is verified with respect to perturbations in terms of initial conditions, atmospheric density variations, as well as mass and aerodynamic uncertainties. Results show that the approach is valid, leading to an accurate disturbance reconstruction, to a better transient, and to good tracking performance, improved of about 50% in terms of altitude and range errors with respect to the corresponding standard sliding-mode-control approach.@en

A method is developed to determine station-keeping maneuvers for a fleet of satellites collocated in a geostationary slot. The method is enabled by a linear time-varying formulation of the satellite orbit dynamics in terms of nonsingular orbital elements. A leader–follower control hierarchy is used, such that the motion of the follower satellites is controlled relative to the leader. Key objectives of the station-keeping method are to minimize propellant consumption and to limit the number of maneuvers while guaranteeing safe separation between the satellites. The method is applied in a realistic simulation scenario, including orbit determination, as well as actuation and modeling errors. The method is demonstrated to work for a fleet of four satellites with differences in mass, surface area, and propulsion system for a maneuver cycle of one week. It is then demonstrated that, by reducing the maneuver cycle duration to one day, the method allows collocation of 16 satellites in a single slot, without penalties on propellant consumption.@en