Robust Entry Guidance with Atmospheric Adaptation

Abstract

Robust atmospheric entry guidance for blunt-body entry vehicles with bank angle modulation is achieved by combining online atmospheric density estimation with an updated version of the Convex Predictor-corrector Entry Guidance (CPEG) algorithm. During atmospheric entry, a square-root Extended Kalman Filter is used to estimate a ratio between the density of the experienced atmosphere with that of an approximate model, which is spline-fit based on MarsGRAM perturbed data. The information from this filter is used to modify the approximate model used by the guidance algorithm. The proposed update to CPEG includes time as a decision variable, dramatically improving the robustness of the algorithm. CPEG predicts the trajectory at each control call with a nonlinear simulation followed by a single convex trajectory optimization problem that updates the commanded bank angle derivative. The robustness and performance of this estimator and controller guidance architecture are demonstrated on a wide range of realistic Martian atmospheres and is able to achieve state-of-the-art accuracy with respect to altitude-triggered parachute deployment.