Dynamic lateral entry guidance logic

The lateral motion of an entry vehicle is controlled by the sign of its bank angle, determined by the entry guidance system. The conventional technique to change the bank sign is based on pre-specified threshold values in the heading error of the vehicle with respect to the landing site. This paper develops a new automated lateral guidance logic based on the crossrange which is found to be a more suitable parameter than the heading error for lateral guidance. The present guidance logic determines the bank reversals by constantly evaluating information from the reference crossrange profile, current crossrange, and estimated actual lift-to-drag ratio. Near the end of the trajectory, a non-iterative numerical predictor decides whether a final bank reversal is needed to null the heading error. This algorithm enables the entry guidance system to fly a wide range of missions and vastly different bank angle profiles, and provides reliable and good performance in the presence of significant aerodynamic modeling uncertainty. All these tasks can be accomplished without requiring manual tuning of guidance parameters or using an excessive number of bank reversals. Extensive high fidelity simulations with different mission scenarios and significant dispersions are presented to demonstrate the performance of the proposed method.