Hazard avoidance guidance for planetary landing using a dynamic safety margin index

This study presents a novel hazard avoidance guidance method using a dynamic safety margin index, to enhance robustness in the highly uncertain environments of planet and small body landing. As future planetary landing and sample return missions will seek to land in areas with high scientific value which may be located in hazardous terrains, onboard hazard avoidance capability is indispensable. Moreover, the dynamics environment of planet or small body landing is very uncertain due to many sources of perturbations, and the accuracy of lander state estimation in real time is limited. To cope with the impact of state uncertainty on hazard avoidance performance, this study introduces a dynamic safety margin index that changes with the lander state uncertainty, and derives the hazard avoidance guidance law based on evaluation of this index. The safety margin index takes into account the state uncertainty and control constraints of the lander, and quantitatively describes the safety state of the lander with respect to the hazards around. The index is then used to derive the guidance law that makes the system globally stable and guides the lander to the desired final landing state without collision with any hazard. The impact of the lander state uncertainty on trajectory safety is considered and quantified in safety margin index evaluation and in the development of the guidance law, so the proposed algorithm is adaptive to the lander state uncertainty exhibited in planetary landing practice. The hazard avoidance performance with limited control ability is also improved as the control constraints are considered. No offline trajectory generation is required, so the method is appropriate for real-time hazard avoidance following online hazard detection. The behavior and performance of the proposed guidance method is investigated via a set of numerical simulations, and the results show that the hazard avoidance performance with state uncertainty and control constraints is improved using the proposed method, advantageous to practical implementation in the uncertain dynamics environment of planetary landing.