A System Study for Cislunar Radio Navigation Leveraging the Use of Realistic Galileo and GPS Signals (2019)

“We are not going back to the Moon, we are going forward to the Moon” said Jan Woerner, Director General of the European Space Agency (ESA) in 2018, as reported by Space News [1]. Indeed, in the context of Deep Space Exploration as planned by the International Space Exploration Coordination Group (ISECG), the Moon represents a significant step to feed forward what could be done on Mars. The main interests for the lunar exploration are related to In-Situ Resource Utilization (ISRU), tourism, science and mining. The most famous lunar projects, such as the “Moon Village” [2] and the Chinese lunar exploration mission, Chang’e 4 [3], to only cite a few, demonstrate how the interest in uncovering our Moon mysteries continues to gain momentum. More specifically, the South Pole region is the target of the vast majority of the upcoming lunar missions, given the potential presence of water under its surface. To ease this process, National Aeronautics and Space Administration (NASA), in collaboration with other international space agencies, is developing a platform called the Deep Space Gateway (DSG) or Lunar Orbital Platform-Gateway (LOP-G) [4]. This gateway, orbiting the Moon in the Near Rectilinear Halo Orbit (NRHO), seeks to facilitate the transfer of space modules to the lunar surface [5]. In this context, communication and navigation will be critical services for making this abundance of future missions possible. In May 2018, the Concurrent Design Facility (CDF) of ESA investigated the most optimal way to provide the South Pole region of the Moon with communication services within the Lunar Comms internal study [6]. The design outcome of their analysis consists in a system of three communication satellites orbiting the Moon in an orbit called Elliptical Lunar Frozen Orbit (ELFO). Figure 1 depicts both the DSG (left) and the Lunar Comms satellites (right) together with the Moon-Earth and the Moon-North Pole vectors. This is a first step towards the development of key-enabling technologies for lunar exploration. However, providing the Moon with navigation services faces a few more challenges. Indeed, simply reproducing what was already implemented on earth, i.e. the wellknown terrestrial Global Navigation Satellite Systems (GNSS), is currently not a viable option for the Moon due to both cost and human maintenance reasons. With this in mind, this paper addresses potential solutions for a Moon Navigation Satellite System (MNSS) to meet the requirements of various scenarios: spacecraft in Earth-Moon Transfer Orbit (MTO), Moon orbiter, lander, launcher and Moon surface user. With respect to other solutions available in the literature such as [7], [8], this contribution assesses the feasibility of a MNSS using the resources already or soon available: i.e. the use of Earth GNSS signals up to Moon altitudes and satellites already planned to orbit the Moon in the upcoming years, including the DSG and those considered in the Lunar Comms constellation. Figure 2 depicts the four Earth GNSS, i.e. Galileo, the Global Positioning System (GPS), BeiDou and the GLObal NAvigation Satellite System (GLONASS) constellations. The paper consists of two distinct parts: • The analysis of the potential use of Earth GNSS signals at Moon altitudes, • The assessment of the performances achievable for a MNSS constellation that includes the DSG in its NRHO and the Lunar Comms satellites in their ELFO.