Lunar Navigation Satellite System for Entire Moon Surface: HDOP and SISE Evaluation

Needs of lunar PNT systems are expanding for upcoming lunar missions such as the Artemis programs. NASA, ESA, and JAXA have been working on respective lunar PNT systems whose initial services will start from the lunar South Pole region where the Artemis base camp, lander landing, and rover exploration are expected. NASA and ESA have been also developing the LNIS (LunaNet Interoperability Specification) that is a standardization document for the lunar Comm&Nav (CPNT). By complying the LNIS and its applicable documents, the respective lunar PNT systems will become interoperable and compatible, and together form the moon GNSS called the LANS (Lunar Augmented Navigation Service). We expect that the initial lunar PNT and LANS services will start around the end of the 2020s and each space agency is now aiming to achieve this challenging timeline. At the same time, we also expect that the service expansion from the initial lunar South Pole region to the entire moon surface will be occurred around the middle of the 2030s, which is the goal of the LANS as stated in the LNIS. Therefore, the satellite constellation design to cover the entire moon surface is an imminent research topic for the future lunar PNT and LANS. In this paper, we propose two candidate satellite constellations to provide the PNT service at the entire moon surface. The first candidate constellation uses additional ELFOs (elliptical lunar frozen orbits) to augment our lunar PNT system called the LNSS (Lunar Navigation Satellite System) that was originally designed to provide the PNT service at the lunar South Pole region. The second candidate constellation is based on new circular orbits. In this paper, the long-term stability analyses for these ELFO and circular orbit are conducted and the HDOP values for these two constellations are calculated. The expected SISE values for the satellites flying in these constellation orbits based on the GNSS weak signal navigation technique are also evaluated. Then, based on these results, the expected positioning accuracy at moon surface points is analyzed. After a series of evaluation, we confirmed that our two candidate satellite constellations both provide 10 to 20 meter-level positioning accuracy at the entire moon surface, which supports the use of these constellations to expand the PNT service areas of the initial lunar PNT and LANS that are currently targeting the lunar South Pole region.