Mars atmospheric entry guidance for reference trajectory tracking

To improve the accuracy of Mars landing, a new nonlinear entry guidance law for low lift landers has been developed. A novel Terminal Sliding Mode Control (TSMC) for tracking a reference trajectory is proposed. The guidance method is based on TSMC theory adapted to the problem of entry guidance. The TSMC has been adapted to account for the specific sliding mode exhibited by the longitudinal motion of the entry vehicle, with bank angle control following a reference trajectory. The finite-time stability nature of the TSMC law is proven by using a Lyapunov-based approach. Extended state observer (ESO) is applied to estimate Mars atmospheric density uncertainty and lift-to-drag ratio disturbance existing in the spacecraft system, by which terminal sliding mode controllers are designed combining the two approaches respectively to force the state variables of the closed loop system to converge to the reference trajectory. Also, simulation results are presented to illustrate the effectiveness of the control strategy.