Assessing Utility of Different Orbits for a Lunar PNT Constellation

"As the Artemis program establishes a long-term presence on the lunar surface, lunar surface users will need an accurate source of Position, Navigation, and Timing (PNT). Under the LunaNet framework, NASA has proposed the Lunar Navigation System (LNS), a constellation of satellites in lunar orbit which will broadcast LunaNet’s Augmented Forward Signal (AFS) [1] to the lunar surface. LNS will have the objective of providing user position accuracies of 10 m and timing accuracies of 10 microseconds on the lunar surface and up to 200 km above the surface.

Several classes of orbits are possibilities for the LNS constellation, including circular orbits, eccentric orbits, and various families of 3-body orbits. Each of these orbit classes have both advantages and disadvantages in their utility as PNT constellations. In this study we examine a wide variety of potential LNS constellations and evaluate their performance against several metrics, including:
a. Dilution of Precision (DOP) performance over the lunar service volume and the South Pole region, during both the constellation buildup and after the constellation is completed.
b. Number of satellites required to achieve required DOP performance.
c. Relative dynamics of LNS satellites with respect to surface users.
d. Signal in space error (SISE) performance, assuming orbit determination that includes the Global Positioning System (GPS), crosslinks, and monitor sites on the surface of the moon.
e. Constellation insertion, station keeping, and disposal costs.

f. Specific Concept of Operations (CONOPS) and engineering challenges associated with establishing and operating the constellations. (E.g., frequent station keeping, variable signal strength, divergent CONOPS for different planes of the constellation.)
The system’s performance is considered both independently and in combination with other proposed systems, such as NASA’s Lunar Communication Relay and Navigation System (LCRNS) and ESA’s Moonlight constellation.

The study concludes that a 3-plane Walker constellation at 6 lunar radii (slightly higher than GPS’s 4.2 earth radii from a geometry perspective, but significantly closer in an absolute sense) is a good candidate for the LNS constellation, providing the user with good SISE and DOP performance, while not requiring excessive station keeping or complex CONOPS. However, other viable concepts exist and could be chosen if early South Pole coverage is a driving factor in the system design."