Switching system model for pinpoint lunar landing guidance using a hybrid control strategy

A novel non-linear spacecraft guidance scheme utilizing a hybrid controller for pinpoint lunar landing is presented. The development of this algorithm is motivated by a) the desire to satisfy more stringent landing accuracies required by future lunar mission architectures, and b) the interest in the ability of a system with multiple controllers to provide robustness and performance that cannot be obtained with a single controller. Based on Hybrid System theory, the proposed Hybrid Guidance algorithm utilizes both a global and local controller to bring the lander safely to the desired target on the lunar surface with zero velocity in a finite time. The hybrid approach is used generally to provide flexibility; many stable controllers can be used for the global and local controllers in the hybrid framework, creating options that allow the algorithm to be tailored to meet mission requirements. The presented case utilizes a global controller that implements an optimal guidance law augmented with a sliding mode to bring the lander from an initial state to a predetermined reference trajectory, at which point the guidance law will switch to that of a LQR-based local controller to bring the lander to the desired point on the lunar surface. The individual controllers are shown to be stable in their respective regions. The behavior and performance of the Hybrid Guidance Law (HGL) is examined in a set of Monte Carlo simulations under realistic conditions. The simulations demonstrate that the HGL is very accurate and results in low residual guidance errors.