Pseudo-Waypoint Guidance for Proximity Spacecraft Maneuvers

A paper describes algorithms for guidance and control (G&C) of a spacecraft maneuvering near a planet, moon, asteroid, comet, or other small astronomical body. The algorithms were developed following a model-predictive-control approach along with a convexification of the governing dynamical equations, control constraints, and trajectory and state constraints

Controlling Attitude of a Solar-Sail Spacecraft Using Vanes

A paper discusses a concept for controlling the attitude and thrust vector of a three-axis stabilized Solar Sail spacecraft using only four single degree-of-freedom articulated spar-tip vanes. The vanes, at the corners of the sail, would be turned to commanded angles about the diagonals of the square sail. Commands would be generated by an adaptive controller that would track a given trajectory while rejecting effects of such disturbance torques as those attributable to offsets between the center of pressure on the sail and the center of mass. The controller would include a standard proportional + derivative part, a feedforward part, and a dynamic component that would act like a generalized integrator. The controller would globally track reference signals, and in the presence of such control-actuator constraints as saturation and delay, the controller would utilize strategies to cancel or reduce their effects. The control scheme would be embodied in a robust, nonlinear algorithm that would allocate torques among the vanes, always finding a stable solution arbitrarily close to the global optimum solution of the control effort allocation problem. The solution would include an acceptably small angle, slow limit-cycle oscillation of the vanes, while providing overall thrust vector pointing stability and performance

Stabilization, observation, tracking and disturbance rejection for uncertain/nonlinear and time-varying systems

In the first part of this study, we consider the problem of designing observers, stabilizing state and observer based output feedback controllers for a general class of nonlinear systems. The nonlinearities for this general class are described in terms of a quadratic inequality. First, analysis results are obtained. Basic Lyapunov stability theory is used to derive analysis results for quadratic stability of these systems. The controller and observer design procedures are established which involve solution of linear matrix inequalities (LMI\u27s). In the second part of this study, we consider uncertain/nonlinear systems subject to unknown constant disturbance inputs. We wish to design state feedback controllers which ensure that the system output asymptotically tracks a specified constant reference signal and all states are bounded. Our main result reduces the original problem to a stabilization problem for an associated augmented system which we call the “derivative augmented system”. This system describes the dynamics of the derivative of an augmented state. Then, we study the same problem by using a different approach. In this case, the dynamics between any two trajectories of an augmented system is considered. The stabilization of this dynamics leads to the desired tracking results. We obtain state and output feedback PI controllers to achieve this objective. The design procedures involve solution of LMI\u27s. In the third part, we apply the same controller structure on tracking problems where disturbance and reference signals have bounded derivatives. It is shown that tracking can be achieved with a bound on the tracking error, which is determined by the bound on the derivatives of disturbance and reference signals. It is also shown that the state vector and control input stay bounded if the disturbance and reference signals are bounded

Observers for Systems with Nonlinearities Satisfying an Incremental Quadratic Inequality

We consider the problem of state estimation for nonlinear time-varying systems whose nonlinearities satisfy an incremental quadratic inequality. These observer results unifies earlier results in the literature; and extend it to some additional classes of nonlinearities. Observers are presented which guarantee that the state estimation error exponentially converges to zero. Observer design involves solving linear matrix inequalities for the observer gain matrices. Results are illustrated by application to a simple model of an underwater

L2 Earth Atmosphere Observatory: Formation Guidance, Metrology and Control Synthesis

The Earth Observatory Formation at L2, a Lagrange libration point, is a unique large aperture (25 m diameter) space telescope concept that will improve the knowledge and understanding of dynamic, chemical and radiative mechanisms that cause changes in the atmosphere, and can lead to the development of models and techniques to predict short and long-term climate changes. The results of this concept definition study show that the telescope concept is feasible, and can have technology readiness in the 2020 time frame. Further advanced development in several subsystems is needed, such as higher efficiency Xenon ion thrusters with throttling, and optical quality large membrane mirror with active shape control. It presents an analysis and solution of guidance, sensing, control, and propulsion problems for a formation of two spacecraft on the Sun-Earth line in the neighborhood of the Sun-Earth L2 point, that observes Earth s atmosphere during continuous solar occultation by the Earth. A system architecture is described for the observatory, and its components that include unique mission specific metrology. The formation must follow a powered trajectory with strictly limited fuel use to observe solar occultation. A configuration of ion thrusters and reaction wheels for translation and attitude control is designed along with algorithms for orbit following and formation control. Simulation results of the orbital and formation dynamics are presented that verify performance of the control systems