# Exploiting Lunar Navigation Constellation for GNC Enhancement in Landing Missions
## 1 Introduction  
The International Space Exploration Coordination Group (ISECG) recognises lunar exploration as a pivotal milestone for advancing crucial technologies necessary for sustained human presence in space and deep-space exploration. As a result, lunar missions have regained much interest globally, attracting involvement from both space agencies and private companies. While these missions pursue different objectives, they all demand precise knowledge of spacecraft positioning and trajectory, along with a robust communication infrastructure for efficient data transmission back to Earth. Up to now, lunar missions have heavily relied on Direct-to-Earth (DTE) communications and ranging radiometric measurements from the ground. However, as the number of missions grows, several space agencies have proposed dedicated lunar communication and navigation infrastructures to ease the Earth ground-segment load and establish more efficient and reliable communication links [1; 2; 3]. In this context, ESA initiated the Moonlight project, aimed at offering an affordable and high-performance Lunar Communication and Navigation Service to support upcoming institutional and commercial lunar missions. Such infrastructure will maintain continuous Earth contact, even during DTE link downtime (e.g., on the Moon's far side), and provide high-precision navigation data. Furthermore, a dedicated Global Navigation Satellite System (GNSS) constellation could enable onboard autonomous lunar navigation systems for rovers, landers, and spacecraft, meeting the stringent ISECG requirements for safe Lunar surface operations, such as landing within a 90-metre 3\(\sigma\) uncertainty from the intended location. Looking instead specifically at the LNS constellation design, the research community is currently quite active, focusing on different procedures for the definition of the orbitalconfiguration. When looking for a global coverage, the well-known principle of Walker constellation is a possible alternative [4; 5]. Such a solution, however, may not be optimal in terms of cost effectiveness, in particular if an incremental approach is forsaken for the constellation deployment. A more advanced design proposed in two consecutive publications [6; 7] considered multi-objective optimisation to provide high navigation performance availability to the lunar global surface, while still also considering other system-level objectives such as costs, station-keeping expenditure, and robustness to failure. Other studies considered instead constellations deployed in Lagrange Point Orbits (LPO), which provide many geometrical and dynamical features that classical two-body orbits do not possess [8; 9]. A hybrid approach to the design of the architecture was instead followed in [10], where both Keplerian and LPOs are considered in the optimisation, having performance and coverage as maximisation indexes. In any case, looking at an incremental constellation approach, during the initial phases a primary issue will be the limited availability of navigation signals due to the substantially fewer LNS servicing satellites compared to consolidated Earth GNSS systems. The smaller constellation size implies the impossibility of having a continuous view of at least four satellites from any location on the lunar surface at the same time. For this reason, the initial LNS architecture will be designed to prioritise enhancing the navigation performance of surface and Low Lunar Orbital (LLO) users around the South Pole region (which is a subject of high scientific interest) at the cost of a reduced LNS availability in the northern hemisphere. Nonetheless, even for these South Pole users, custom navigation algorithms will need to be developed to meet the mission navigation requirements and cope with the limited LNS-visibility windows. To counteract such problems, recent years have seen numerous studies investigating achievable performance levels for various lunar missions utilising potential LNS constellations. Notably, for the state estimation, ref.