Reentry Terminal Guidance Through Sliding Mode Control

Abstract

A guidance scheme has been developed for the terminal guidance of an unpowered lifting reentry vehicle during the approach and landing phase. The proposed approach is quite useful for offline trajectory design and allows for trajectories to be generated online through the use of a closed-loop control law. In scenarios in which the reentry vehicle is significantly deviated from its nominal trajectory upon entry into the landing phase, the usefulness of such an online method can be clearly realized. These types of scenarios are of interest for any reentry vehicle, including the space shuttle, since existing guidance approaches during the approach and landing phase involve tracking trajectories created offline. To solve the approach and landing guidance problem, a novel concept called the sliding mode terminal guidance is used in this work. This approach takes advantage of the finite-time-reaching phase of the sliding mode technique to ensure that any desired state constraints can be fulfilled in a finite time. Further, by using a new approach to second-order sliding mode control, analytic solutions are obtained for both the altitude and flight-path angle during the reentry process. The end result of this approach is a closed-loop guidance (control) law, which can be used to generate trajectories that depend only on the initial and final conditions of the approach and landing phase. Simulations shown indicate that the method provides some robustness to variations in the initial downrange and velocity.