The provision of communication and navigation services to lunar orbiting and surface users, for different mission phases (i.e., lunar transfer orbit, descent and landing, proximity operations), is of particular importance considering the growing number of lunar missions planned by private companies and national agencies in the next 20 years.

During the deployment of a lunar navigation satellite system, both its monitoring and its validation will require an innovative approach different from common GNSS terrestrial application, considering the absence of a ground station on the lunar surface. In this paper, a trade-off will be performed between three proposed strategies for the monitoring of navigation signals broadcasted by lunar-orbiting satellites:

• Monitoring from Earth: the direct visibility from a Ground Segment on the Earth will be analyzed in the presentation and a preliminary link budget is computed in order to derive power levels and discuss the feasibility of this strategy.
• Cross-monitoring: the analyzed constellation geometry permits extensive visibility time windows between its satellites. Since FSL (Free-Space Loss) of these links could be more representative of an orbit and/or surface user of the navigation service, link budget is computed but transmission and reception in the same frequency band shall be dealt.
• Monitoring from lunar orbit: both the predominant FSL of a Moon-to-Earth link and the issues related to reception of the navigation signal by the lunar satellites could be overcome by lunar-orbiting assets either equipping a receiver (in-orbit monitoring but without interference) or acting as data relay towards Earth (dedicated communication link with Earth-pointed antennas instead of side lobes exploitation). Link budget and visibility will be analyzed considering different orbits.

Advantages and disadvantages will be presented and discussed in order to propose the best strategy to verify structure and/or content of the navigation signal.