Session A4: PNT Solutions for Space Applications

Phased Orbital Design of Lunar Navigation Satellite Constellations for PNT Services in Lunar Exploration
Minjae Kang, Interdisciplinary Program in Space Systems, Seoul National University; Hojoon Jeong, Jaeuk Park, Changdon Kee, Department of Aerospace Engineering and SNU-IAMD, Seoul National University
Location: Beacon A

As interest in lunar exploration continues to grow, the demand for precise positioning on the Moon has become increasingly critical. Legacy methods of relying on Earth-centered GNSS signals to provide navigation for lunar users present significant challenges, including weak signal strength, limited visibility, and signal delays caused by the considerable distance between the two celestial bodies. To address these limitations and ensure reliable Positioning, Navigation, and Timing (PNT) services for users on the lunar surface, research into dedicated lunar-based navigation satellite systems has advanced rapidly. Major space agencies, including NASA with its Lunar Communications Relay and Navigation System (LCRNS), ESA with the Moonlight initiative, and JAXA with the Lunar Navigation Satellite System (LNSS), are actively developing orbital constellations aimed at providing seamless PNT coverage on the Moon. These initiatives will integrate into an interoperable architecture, the Lunar Augmented Navigation Service (LANS), targeting regional PNT services at the lunar South Pole by the late 2020s, with full lunar surface coverage expected by the 2030s.
Designing lunar navigation satellite orbits requires careful consideration of long-term stability and navigation performance. Given the Moon's distance of approximately 384,000 km from Earth, satellite deployment for lunar missions is inherently costly. Since a satellite's operational lifetime is closely linked to fuel consumption, frequent corrective maneuvers caused by orbital instability accelerate fuel depletion and lead to costly replacements. To extend satellite lifetimes and minimize operational costs, orbits must maintain long-term stability while reducing the need for frequent adjustments. At altitudes above 1,700 km, lunar orbits are significantly influenced by Earth's gravity, requiring careful orbit design to counter these perturbations. The geometric configuration of satellites is equally important in determining navigation accuracy, as it directly affects the Dilution of Precision (DOP) metric. Even with stable orbits, the arrangement of the constellation plays a key role in performance, with optimal configurations reducing DOP and improving overall precision. In designing lunar navigation satellite orbits, achieving both orbital stability and reliable navigation performance is crucial to support various exploration missions and ensure the accuracy required for mission success.
This study presents a phased approach to designing orbital constellations for a lunar navigation satellite system, aligned with the coverage expansion plan proposed by LANS. We analyzed orbital constellation conditions in each phase to meet DOP-based navigation requirements over extended periods, while minimizing corrective maneuvers. Phase 1 focuses on developing a constellation to provide zonal coverage at the lunar South Pole, with the primary goal of achieving reliable Horizontal DOP (HDOP) to deliver the essential positioning information needed for surface exploration. Phase 2 focused on extending coverage to the entire lunar surface by building upon the orbits established in Phase 1. With the full constellation deployed, Position DOP (PDOP) was adopted as the primary metric for assessing navigation performance, aligned with GPS performance standards which require PDOP values below 6, enabling precise positioning for more complex and wide-ranging exploration missions. Orbital data were generated using the lunar orbit propagation program developed at Seoul National University's GNSS Lab, based on NASA's DE421 ephemeris data, and subsequently analyzed to evaluate DOP performance for lunar surface users with a mask angle of 5 degrees applied.
In Phase 1, we utilized Elliptical Lunar Frozen Orbits (ELFOs) to achieve stable zonal coverage at the lunar South Pole. By analyzing the variations in orbital elements derived from Lagrange's planetary equations, we identified a wide range of stable orbital conditions that cancel out the perturbative effects of Earth's gravity, with the inclination determining the specific orbital configuration and constraining the other orbital elements accordingly. To prevent lunar surface collisions near the perilune, a minimum altitude of 300 km was incorporated into the design. While many stable constellations exist, selecting those that offer superior DOP performance requires careful assessment of the satellite geometry, which varies with the phasing between orbital planes. We conducted a numerical analysis of HDOP at a single grid point at the South Pole, adjusting the inclination of the ELFOs and the satellite phasing to determine configurations that deliver reliable navigation performance.
In Phase 2, we expanded coverage to the entire lunar surface by building on the orbits established in Phase 1 and adding a minimal number of satellites. Unlike conventional Walker constellations, which distribute satellites evenly across circular orbits for global coverage, our approach leveraged the Phase 1 orbits, initially designed for the South Pole. Although these orbits are not inherently efficient for full coverage, the challenge was to achieve GPS-equivalent PDOP performance while minimizing the total satellite count. We explored various configurations, combining circular orbits with ELFOs, and conducted a comparative analysis to identify the most effective constellation for global coverage. This design minimized the satellite volume while ensuring comprehensive PNT services. PDOP performance was evaluated at equidistant grid points across the lunar surface to confirm that the constellation met the required GPS standards for reliable navigation in future exploration missions.
The results of our study demonstrate that in Phase 1, the designed ELFO constellation can maintain an RMS value of HDOP below 1.3 for over five years at the South Pole without corrective maneuvers. In Phase 2, the full constellation achieved over 98% PDOP availability, meaning PDOP values remained below the GPS standard of 6 for the vast majority of the lunar surface. This theoretically confirms the feasibility of providing reliable PNT services across the entire lunar surface. These findings are expected to provide valuable insights into the development of lunar navigation satellite systems and offer a foundational approach for expanding coverage from the South Pole to the entire Moon.