Use of weak GNSS signals in a mission to the moon

According to the European Space Agency (ESA) Lunar Exploration program, the use of GNSS weak-signal navigation in future lunar exploration missions has the potential to increase the robustness of the navigation during all mission phases and improve considerably its autonomy. The major objectives of the ESA Moon-GNSS project have been to determine the feasibility of using GNSS (GPS/Galileo) weak-signal technology in future lunar missions to improve the navigation performance in terms of accuracy, cost reduction and autonomy. The Moon mission scenario is very challenging for the GNSS signals processing: less visibility compared to an Earth-based receiver, low signal strength, poor satellite geometry, Earth and Moon signal occultation, and spacecraft dynamics. The identification of the Moon-GNSS navigation receiver requirements for the upcoming lunar exploration missions has been performed. The impact of the receiver requirements on the Moon-GNSS receiver module architecture and algorithms has been analyzed (weak signal processing, filtering and navigation), including an overview of the state of the art space-borne GNSS receivers. Besides, the synergies between GNSS signal/navigation processing and other navigation sensors (i.e. accelerometers, optical camera, laser altimeter) have been analyzed, using the state of the art of sensors integration for space missions. A demonstrator of the weak-signal Moon-GNSS navigation has been designed and implemented, showing the main functional and performance capabilities of the Moon-GNSS receiver. A test campaign representative of a real Moon-GNSS mission has been carried out, covering all the mission phases of the real mission conditions in terms of dynamics and signal disturbances, for different configurations: standard sensors, standard sensors plus GNSS and stand-alone GNSS navigation.