Optimal lunar landing and retargeting using a hybrid control strategy

A novel non-linear spacecraft guidance scheme utilizing a hybrid controller for pinpoint lunar landing and retargeting 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 analyzing the ability of the system to perform retargeting maneuvers during the descent to the lunar surface. 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 system approach utilizes the fact that the logic and behavior of switching guidance laws is inherent in the definition of the system. The presented case of a hybrid system 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 and an 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. Results demonstrate the capability of the hybrid guidance law to reach the desired target point on the lunar surface with low residual guidance errors. Further, the Hybrid Guidance Law has been applied to the problem of retargeting in order to examine the performance of the algorithm under such conditions.