Safe operations in proximity of space debris: relative motion design and pose estimation

This paper deals with the problem of designing relative trajectories in Earth orbit between an active spacecraft (chaser) and a non-functional, man-made object (target), in the framework of active debris removal space missions. Indeed, these activities involve relative orbital manoeuvres, e.g., monitoring, station keeping, rendezvous and docking, in which the target and the chaser move in close-proximity to each other, i.e., the relative distance ranges from a few tens of meters (depending on the target size) up to contact (in the case of docking). Pose estimation, i.e., the problem of determining the target-chaser relative attitude and position parameters, is a complex technical challenge related to these manoeuvres. Since space debris are non-cooperative targets, this task must be entrusted to either passive or active Electro-optical sensors, thus its performance is affected by the relative observation geometry between the chaser and the target. For these reasons, the relative trajectories must be designed to minimize the risk of collisions or uncontrolled contact, on one side, and to optimize relative navigation performance, on the other side. The importance of this latter aspect is motivated by the fact that the capability to estimate the target-chaser pose with high accuracy allows relaxing the chaser control requirements, thus having a valuable impact on mission and satellite costs. The design approach presented in this paper is formulated in mean orbit parameters and it is based on a relative motion model relevant to two-satellite formations which includes the non-Keplerian perturbations due to secular Earth oblateness, as well as the possibility of considering targets moving along a small-eccentricity orbit. It allows designing innovative trajectories for target monitoring, based on the concept of safety ellipse, which satisfy both safety and pose estimation requirements. This latter aspect is demonstrated within a numerical simulation environment capable of realistically reproducing target-chaser relative dynamics, the operation of a scanning LIDAR selected on board the chaser as relative navigation sensor, and pose estimation algorithms based on the processing of 3D point clouds. A potential target on Geostationary Earth Orbit is selected as test case