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Session B3: Lunar Navigation and Time

Analysis of Navigation Performance with Lunar GNSS Evolution
Filipe Pereira, Cornell University; Daniel Selva, Texas A&M
Location: Beacon B

Peer Reviewed

Peer Reviewed

The gradual expansion of infrastructure on and around the Moon (e.g., Lunar Base Camp and Gateway in the lunar Near Rectilinear Halo Orbit (NRHO)) is key to establishing the long-term exploration of the Moon through e.g., NASA’s Artemis and Commercial Lunar Payload service missions. Crucial components of a future Lunar infrastructure are navigation systems that enable the execution of complex maneuvers —such as landing, orbit rendezvous, and docking — in cislunar space.
Current missions are largely dependent on the support of the Deep Space Network (DSN), for communication and navigation services at lunar distances. The future increase in demand for these services is likely to be beyond the capabilities of the DSN, which is operating close to capacity. Despite recent work showing that Earthbound GNSS signals can be received at lunar distances, these signals are mostly relevant only for time synchronization, since the poor Geometric Dilution Of Precision (GDOP), severely limits their usefulness for precise navigation purposes. Thus, we argue that a dedicated satellite navigation system in lunar orbit capable of delivering GPS-like performance in the entire lunar surface (including the lunar far side not covered by existing infrastructure) would be a key asset. A Lunar GNSS can be designed to prevent interference with protected radio astronomy bands on the shielded zone of the Moon. Finally, such a system could also aid orbit maintenance operations in the Lunar Gateway and assist deep space missions requiring low-thrust transfers involving the Earth-Moon system.
In a previous study, we performed a multi-objective design optimization for a Lunar GNSS space segment considering GDOP, GDOP Availability, space segment cost, station-keeping delta-V, and robustness to single-satellite failure. Results showed that Pareto approximate solutions with a global GDOP availability (GDOP < 6.0) greater than 98% and station-keeping delta-V <= 250 m/s per satellite per year consisted of Walker constellations in near-circular polar orbits at an altitude of ~2 lunar radii. For example, a 24-satellite constellation could achieve a GDOP (98th PCTL) of 3.6 and 99.5% GDOP availability on a global user grid over the lunar surface.
In this work, we assess the level of user navigation performance that can be achieved with a Lunar GNSS over a set of reference trajectories —NRHO, static/dynamic users on the lunar surface, and a satellite in low lunar orbit. Importantly, the contribution of orbit ephemeris and clock errors on navigation errors are analyzed. Finally, we show how the navigation performance could evolve over the constellation deployment period.
Linear covariance analysis is used to compute navigation error budgets around a reference trajectory in a computationally efficient manner. This method has been shown to provide identical results to traditional Monte-Carlo analysis in a fraction of the time. Thus, it provides a valuable tool for sensitivity analysis e.g., to evaluate the effect of sensor specifications and disturbance levels on the navigation errors. Pseudorange and delta range measurements from Lunar GNSS and Earth GNSS are modeled, and the impact of different measurement update rates is assessed. An Epoch-Era analysis is used to analyze navigation performance at different stages of Lunar GNSS space segment evolution. To this end, several Epochs with fixed context and value expectations are defined, and a robust strategy for orbit deployment can be obtained by optimizing Eras (Epoch sequences) across several plausible future scenarios.
This analysis is meant to define the target levels for orbit determination and clock synchronization errors to achieve position accuracy goals such as those in precise landing. The definition of these performance targets can inform the design of autonomous orbit determination, and timekeeping strategies. Insights into robust value delivery throughout Lunar GNSS constellation deployment can be obtained from the Epoch-Era analysis.



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