Bearing-Only Navigation for Proximity Operations on Cis-Lunar Non-Keplerian Orbits

In the next years many scientific and demonstration missions will target the cislunar environment to consolidate existing technologies and foster the development of new space systems to support a future human exploration of Mars. Among those, the Lunar Orbital Platform Gateway (LOP-G) will operate as a long-term modular infrastructure in deep-space, offering services to both manned and unmanned missions to the Moon. To safely accomplish many of its tasks, rendezvous and docking/undocking operations must be reliably performed on highly non-Keplerian orbits. However, despite rendezvous and relative navigation are well-known for Earth-centred missions, no autonomous proximity operation has ever been performed in the cislunar domain. Within this framework, the paper assesses the applicability of Bearing-Only measurements to perform relative navigation between distant heterogeneous non-Keplerian orbits. Starting from the absolute motion, a useful discrete-time approximation of the non-Keplerian relative dynamics is presented to support the development of a computationally efficient Guidance, Navigation and Control (GNC) architecture based on a Shrinking Horizon - Model Predictive Control (SH-MPC) strategy. The outcomes of two different test scenarios are critically discussed to prove the effectiveness of the proposed method and highlight potential solutions that enhance the filter observability with minimum fuel consumption.