The main objective of the Chandrayaan-3 lunar landing mission was to achieve a safe and soft landing near the lunar south pole. The powered descent trajectory consisted of a braking phase with inertial navigation, an attitude hold, a braking phase with absolute navigation, and a terminal descent phase. This paper presents the flight-implemented closed-loop guidance design for the braking phase with absolute navigation. Our main contribution is the formulation of a robust and fuel-efficient analytical polynomial guidance algorithm under state and control constraints with real-time retargeting capabilities. This is achieved through offline data-driven analysis, which enables a robust online decision-making algorithm to assess the feasibility of pinpoint landing in real time at the start of the phase. If the pinpoint landing is deemed infeasible, the algorithm computes a suitable alternative retargeting site in real time without the need for onboard optimization. Consequently, the proposed algorithm is analytical and extends the operational region of the classical polynomial guidance algorithm.
Kumar et al. (Mon,) studied this question.