  Recently, there has been a growing interest in the use of a SmallSat platform
for the future Lunar Navigation Satellite System (LNSS) to allow for
cost-effectiveness and rapid deployment. However, many design choices are yet
to be finalized for the SmallSat-based LNSS, including the onboard clock and
the orbit type. As compared to the legacy Earth-GPS, designing an LNSS poses
unique challenges: (a) restricted Size, Weight, and Power (SWaP) of the onboard
clock, which limits the timing stability; (b) limited lunar ground monitoring
stations, which engenders a greater preference toward stable LNSS satellite
orbits.
  In this current work, we analyze the trade-off between different design
considerations related to the onboard clock and the lunar orbit type for
designing an LNSS with time-transfer from Earth-GPS. Our proposed time-transfer
architecture combines the intermittently available Earth-GPS signals in a
timing filter to alleviate the cost and SWaP requirements of the onboard
clocks. Specifically, we conduct multiple case studies with different grades of
low-SWaP clocks and various previously studied lunar orbit types. We estimate
the lunar User Equivalent Range Error (UERE) metric to characterize the ranging
accuracy of signals transmitted from an LNSS satellite. Using the Systems Tool
Kit (STK)-based simulation setup from Analytical Graphics, Inc. (AGI), we
evaluate the lunar UERE across various case studies of the LNSS design to
demonstrate comparable performance as that of the legacy Earth-GPS, even while
using a low-SWaP onboard clock. We further perform sensitivity analysis to
investigate the variation in the lunar UERE metric across different case
studies as the Earth-GPS measurement update rates are varied.
