Evaluation of different satellite navigation methods for the Moon in the future exploration age

Numerous lunar missions are planned for the next decade and the current deep space tracking system could not be sufficient to support all of them, since it has inherent limits in terms of complexity, availability, costs and user capacity. Consequently, new semi-autonomous or autonomous navigation systems are under study for the Moon, using satellites deployed in lunar orbit. In this work, a general and modular architecture for a satellite based navigation system, compatible with the state-of-the-art recommendations, is defined and three different localization methods, exploiting One-Way and Two-Way ranging, are proposed. The Two-Way ranging is proposed with two different implementation techniques: Earth based and user based round trip delay. These solutions were evaluated for two types of constellations that are, at the moment, the most promising for a future lunar navigation system, the Halo and ELFO constellations, by the use of the Cramer–Rao Lower Bound. Moreover, models for measurement errors affecting the lunar navigation system are proposed. The results show that Two-Way localization has slightly better accuracy and availability w.r.t. the One-Way localization, but it has also some limits in terms of capacity and latency. In the author’s opinion, the two methods might coexist for different user requirements and services in a future lunar navigation system.