Orbit Design of a Lunar Navigation Satellite System: Enhanced Hybrid Frozen Constellation for Global Lunar PNT Service

With the growing momentum in lunar exploration, establishing a reliable positioning, navigation, and timing infrastructure has become critical. While the planned initial operational capability relies on Elliptical Lunar Frozen Orbits (ELFO) for South Pole coverage, extending this to a global full operational capability remains a challenge due to regional coverage gaps. To address this, this study proposes an enhanced hybrid frozen constellation combining ELFOs and Circular Lunar Frozen Orbits (CLFO). Building upon our previous work, we enhance the constellation design by enlarging the semi-major axis of the CLFOs, which yields two key advantages. First, this adjustment significantly improves overall user visibility, especially in mid-latitude regions. Second, it synchronizes the right ascension of the ascending node drift rates of the ELFO and CLFO segments, ensuring that the relative geometry of the entire constellation is maintained over the long term. A long-term performance analysis based on position dilution of precision validates that this optimized architecture satisfies global navigation requirements over 20 years without orbit correction maneuvers, demonstrating superior and more spatially uniform performance compared to previous baseline designs.