GNSS to Lunar navigation systems: Fused approaches and handover based on a lunar beacon

The renewed international interest in lunar exploration has made the establishment of reliable Positioning, Navigation and Timing (PNT) services beyond Earth orbit a strategic priority. As upcoming missions extend further into cislunar space and onto the lunar surface, autonomous navigation becomes essential to ensure operational continuity without constant reliance on ground-based tracking networks. Global Navigation Satellite Systems (GNSS), although originally designed for terrestrial and near-Earth users, have already demonstrated their usefulness for a variety of space applications. The Lunar GNSS Receiver Experiment (LuGRE), developed under an agreement between the Italian Space Agency (ASI) and NASA, has confirmed the feasibility of receiving and processing GPS and Galileo signals in cislunar space and on the lunar surface, demonstrating the potential of GNSS to support navigation well beyond Earth orbit. However, at these distances, signals are extremely weak and all visible satellites appear clustered within a narrow angular region around Earth, leading to unfavorable geometry and degraded positioning accuracy. This thesis provides a quantitative assessment of GNSS performance in the cislunar environment and investigates how it can be enhanced through the inclusion of a lunar surface beacon transmitting GNSS-like signals. A dedicated MATLAB-based simulation framework was developed and extensively improved for this purpose. The simulator was upgraded to support all major global and regional constellations (GPS, Galileo, GLONASS, BeiDou, QZSS and IRNSS), realistic multi-frequency antenna radiation patterns, and detailed link-budget modeling. A configurable lunar beacon model was integrated, allowing the evaluation of different transmitter powers, antenna patterns and three pointing strategies: mutual pointing, dual Earth-pointing and a hybrid configuration combining the two approaches. The simulator predictions of satellite visibility were validated using real GNSS data collected with a high-end Septentrio receiver connected to the reference antenna located on the roof of the DET building at Politecnico di Torino. Simulation results in the cislunar environment show that, while GNSS constellations ensure nearly continuous signal availability along an Earth–Moon transfer orbit, the satellite geometry remains weak, leading to Geometric Dilution of Precision (GDOP) values typically ranging between 50 and 300, with peaks exceeding 600 during the worst geometric conditions. In this regard, the inclusion of a single beacon on the lunar surface markedly improves the overall geometry. In particular, with the hybrid pointing configuration, GDOP values are reduced by more than an order of magnitude, leading to positioning errors in the order of a few tens to a few hundreds of metres, compared to several kilometres in the GNSS-only case. These findings highlight the effectiveness of surface beacons as lightweight and cost-efficient augmentation elements for future lunar navigation systems, contributing to the design of reliable and autonomous PNT architectures for the upcoming era of deep-space exploration.