Attitude Regulation of a Leo Rigid Satellite
In this chapter, we consider the attitude control problem for a spacecraft operating in a low-Earth orbit (LEO). When operating in low terrestrial orbits, the spacecraft attitude control problem is significantly more difficult than in the case of high-altitude orbits. As a matter of fact, for orbits of radius of the order of 103 km or less, the influence of the orbital environment on the spacecraft dynamics in no longer negligible. The Earth atmosphere and the gravitational field produce disturbances which may significantly alter the satellite rotational dynamics, and must be actively rejected by the control system. What renders the problem challenging is the large uncertainty which characterizes the disturbance model, and its significant variability with the orbital radius and inclination, and with the specific operating conditions of the spacecraft. Fixed controllers based on proportional and derivative feedback fail to achieve good performance in the presence of persistent disturbances, or require unacceptably high controller gains. Adaptive control techniques, while suitable for dealing with parametric uncertainties, require modifications to cope with external disturbances, which lead to complex and computationally extensive controllers. The solution we present, based on an internal model approach, is able to cope effectively with uncertainties on both the vehicle dynamics and the orbital environment. The control system generates autonomously the control input that offsets asymptotically the influence of the external disturbances to the vehicle dynamics, and provides global convergence of the spacecraft attitude to the desired configuration. The overall controller has the structure of a linear dynamical system with a nonlinear output map, and constitutes a simpler and viable alternative to more complex nonlinear control architectures.
KeywordsInternal Model Gravity Gradient Aerodynamic Drag Attitude Regulation Angular Velocity Vector
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