# Moon Sensor Station to Improve the Performance of Lunar Satellite Navigation Systems
## 1 Introduction  
Nowadays, there is a renewed interest in lunar exploration and colonization, driven by the desire to establish a human presence beyond Earth and to exploit the resources available on the Moon [1]. However, the success of such missions is contingent on the development of reliable navigation systems that can provide accurate and continuous positioning information on the lunar surface. Current Earth-based tracking stations, such as the NASA Deep Space Network--DSN [2]--have several limitations in supporting a large number of users, in accuracy, and in availability, e.g., the Moon south pole and the far side exhibit limited or no visibility from Earth-based stations, system capacity, etc. [3, 4]. In this regard, the European Space Agency (ESA) is developing the Moonlight initiative, which will use satellite ranging signals to determine the position and velocity of Moon users, similar to the Global Navigation Satellite System (GNSS) [5, 6], NASA is also working on the Cislunar Autonomous Positioning System (CAPS) project, with similar aims [7]. Moreover, the Japan Aerospace Exploration Agency (JAXA) has recently launched a research project to explore potential satellite systems for lunar navigation [8], Roscosmoshas revealed a plan to establish a lunar satellite navigation system in the time frame from 2036 to 2040 [9]. Finally, China has recently disclosed plans through its space agency, the China National Space Administration (CNSA), to develop a satellite constellation orbiting the Moon, aimed at offering communication and navigation services [10]. Extensive research has been conducted on assessing satellite-based navigation systems for the Moon, focusing on exploiting various orbits to achieve optimal performance. Different investigations, such as in [11; 12; 13; 14; 15; 16], have highlighted two types of constellations as the best choices for lunar applications, the Elliptical Lunar Frozen Orbits (ELFOs) and the Halo orbits. Recent advances in satellite coverage analysis, as demonstrated in [17] along with complementary research on elliptical-orbit satellite coverage and agile satellite visibility prediction [18; 19], highlight significant steps toward optimizing lunar navigation systems. These works collectively contribute to a more refined understanding of satellite coverage dynamics, addressing both orbital perturbations and geometric considerations, vital to ensure comprehensive coverage and reliable navigation on the Moon. The most popular solutions [6; 14; 20] adopt the same approach used in terrestrial satellite navigation systems, i.e., the measurement of at least four pseudoranges from synchronized orbiting satellites. Many evaluations have been conducted on this and other working principles in recent years [12; 14; 16]. Last but not least, the initial phase of lunar exploration has planned the deployment of a limited number of satellites in lunar orbit [21]. For this reason, the accuracy and availability of the navigation system may not always meet the mission requirements. In particular, as shown in the previously cited work, in the initial phase and with a reduced number of satellites, the accuracy and availability obtained are limited and do not match the requirements for more demanding scenarios, such as the landing and rover phases of a typical Moon mission. For this reason, this paper proposes the use of local sensors to improve user positioning performance and compares three configurations with different working methods and complexities. A local sensor can be deployed at a fixed location on the lunar surface and can be used to provide different types of additional information.