Unpowered landing guidance with large initial condition errors

An auto-landing guidance law on the basis of sliding mode control (SMC) is presented in this paper for landing an unpowered reusable launch vehicle (URLV) at the predetermined landing site. The guidance scheme, which takes the full nonlinear point-mass dynamics into account, is developed in three-dimensional space. For the sake of dealing with large initial condition errors, the guidance process is divided into two phases, i.e., the error correction phase and the approach and landing phase. For each phase, a specific sliding mode function is designed according to the terminal constraints. The desired goal can be achieved by employing a backstepping approach which is capable of driving the sliding mode function together with its derivative to zero at the fixed downrange. Combining with the Lyapunov theory, the closed-loop guidance commands can be obtained for online implementation. This guidance approach which is characterized by its simple format and less information demand possesses very high precision at touchdown. Furthermore, strong robustness of the guidance method is guaranteed by the virtue of SMC. Simulation results are given to illustrate the superior performance of the proposed method.