Use of GNSS for lunar missions and plans for lunar in-orbit development

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. Navigation is a key-technology to enable the cislunar and lunar volume discovery. The use of Earth Global Navigation Satellite System (GNSS) signals at Moon altitude has been extensively studied in the past, within and outside the European Space Agency (ESA). Namely, this interest has been reflected in "The Interoperable Global Navigation Satellite Systems Space Service Volume" booklet issued by the International Committee on GNSS (ICG) in 2018 (United Nations - Office for Outer Space Affairs, 2018). However, the main limitation on accurately performing this kind of analysis has always been the limited knowledge of the actual GNSS transmit antenna patterns outside the main lobe, i.e. side-lobes. ESA published the first patterns of GPS satellites using data collected in GIOVE-A (Unwin et al., 2013) and, more recently, the National Aeronautics and Space Administration (NASA) shared very detailed 3D patterns of all GPS Blocks (Donaldson et al., 2018). This allows both ESA and NASA to perform detailed analysis of the expected visibility of GNSS signals at moon altitude. In particular, NASA published visibility results using a GPS-only receiver in a Moon Transfer Orbit (MTO) demonstrating the potential of GPS-based navigation up to the Moon (considering signals above 22 dB-Hz with a 14dBi antenna) (Ashman, 2018). The present paper presents the results of numerical simulations for a Single-Frequency (SF) receiver in the Deep Space Gateway (DSG) orbit, which is an Earth-Moon L2 Halo orbit, using both Galileo and GPS, for which detailed 3D antenna patterns were used. It demonstrates the importance of considering the azimuthal asymmetry of the GNSS antenna patterns and the necessity of using an interoperable Galileo-GPS receiver at such altitudes. Additionally, a comparative analysis between the frequency bands E1/L1 and E5a/L5 is performed to select the one providing the best results. In conclusion, it shows that a high number of satellites is visible at Moon altitude using a receiver with a 14 dBi antenna and a 15 dB-Hz Carrier-to-Noise density ratio acquisition and tracking threshold. Finally, the second part of the paper provides ESA plans for In-Orbit Demonstration (IOD) of the use of GNSS at Moon altitude, covering both CubeSat missions and the DSG. (c) 2020 COSPAR. Published by Elsevier Ltd. All rights reserved.