Path constrained explicit guidance algorithm for lunar pinpoint landing mission

A lunar soft landing mission has the requirement that the space exploration vehicle must land on the surface of the Moon with zero relative velocity. When a specific landing point is preferred, it becomes pinpoint landing. For precise and autonomous landing, other limits are added to the acceleration demanded, shape of the trajectory and state of the vehicle, thus forming a path constrained pinpoint landing mission. The controllability of the mission depends on rocket steering and the throttling commands given. The paper formulates and analyses various aspects of explicit guidance algorithms including that used in the Apollo mission, for soft landing and pinpoint landing on the Moon. The inadequacies in using fixed throttling and having unconstrained acceleration demands are examined. Also, an attempt at achieving a path constrained trajectory using the Apollo lunar descent guidance algorithm is executed.