Rendezvous and Docking Sensor and Effector System Design Considerations for the Lunar Lander Altair

One key function performed by the Guidance, Navigation, and Control (GN&C) system of the lunar lander Altair is to rendezvous and dock with the Crew Exploration Vehicle named Orion, in both low Earth orbit and low lunar orbit. The rendezvous and docking process consists of a series of orbital maneuvers and spacecraft attitude controls that successively bring the active vehicle into the vicinity of, and eventually into contact with, the passive vehicle. This paper provides an overview of the rendezvous and docking performance of the Altair GN&C system. To this end, key rendezvous and docking requirements will first be described. These requirements drive the selections of the Altair GN&C sensors, attitude control effectors, and the rendezvous trajectory design. Measurements needed for the rendezvous and docking task will come from the star tracker, inertial measurement unit, docking camera, LIDAR, two-way S-band radiometric ranging data, as well as crew inputs. Measurements from these sensors will be used by the onboard control systems to control well-selected and well-placed Reaction Control System (RCS) thrusters to achieve a safe docking. The final contact must be made with the docking axes of the mating vehicles well aligned, both translationally and angularly. Before a high-fidelity simulation test bed is made available, error budgeting is a cost-effective way to assess the docking capability of the Altair and Orion GN&C systems. In docking error budgeting, we first identified all key docking error sources (e.g., attitude control deadbands of mating vehicles, docking sensor measurement error, misalignment between docking sensor and docking mechanism, and others). Results of an error budget study provides the system engineers with a good estimate of whether the GN&C system can achieve the constraints imposed by the docking system. If the docking requirements cannot be met, the relevant error budgets will reveal the source(s) of the problem. Before a high-fidelity simulation test bed is built to evaluate the docking capability, error budgeting is one of the most costeffective ways to assess the vehicle’s ability to successfully dock. Nomenclature