Design Considerations of a Lunar Navigation Satellite System with Time-Transfer from Earth-GPS

Abstract

After more than fifty years since the Apollo program, we are encountering a revived, second Space Race with international space agencies seeking to not only return humans to the Moon, but to further establish a sustainable human presence. Correspondingly, NASA and the ESA have conceptualized the initial framework for a GPS-like constellation for the Moon, which will ensure uninterrupted navigation and communication services for future lunar missions. For the future Lunar Navigation Satellite System (LNSS), NASA expresses interest in using a SmallSat platform, due to its cost-effectiveness and its potential for rapid deployment; however, many design decisions still need to be finalized, including the lunar orbit, constellation, payload, and onboard clock. Furthermore, designing a dedicated LNSS involves additional challenges, which are unique to the lunar environment, including limited payload capacity for the SmallSat platform, i.e. the Size, Weight, and Power (SWaP) of the onboard clock, limited lunar ground monitoring stations and limited financial investment as compared to the legacy Earth-GPS. Given these unique challenges for the lunar environment, we propose the design of a SmallSat-based LNSS with an inexpensive, low-SWaP onboard clock, which harnesses the satellite signals from the well-established Earth-GPS constellation to maintain precise timing. In particular, we develop a timing filter to correct the lower grade onboard clock, while leveraging the intermittently available Earth-GPS signals. Additionally, we devise a lunar User Equivalent Range Error (UERE) metric to characterize the ranging accuracy of an LNSS satellite. We further validate our proposed time-transfer technique with simulations of an LNSS satellite in an Elliptical Lunar Frozen Orbit (ELFO) with an onboard Chip Scale Atomic Clock (CSAC), while analyzing the visibility effects of Earth-GPS, timing stability, and lunar UERE across different configurations of ELFOs.