Weak GNSS Signal Navigation to the Moon

Reception of weak GNSS signals in challenging environments using techniques such as Assisted GNSS has become a reality and today such techniques are considered important enablers of GNSS receivers in mobile devices [1]. Furthermore, the use of GPS and Galileo signals for indoor navigation is also receiving increased attention in recent studies which push the limits of minimum signal to noise ratios [2]. Space users have also seen remarkable achievements, where GPS signal reception has been confirmed and reported in GEO orbits [3] and seen as enablers for increased spacecraft autonomy in GEO orbits [4]. The study of weak GNSS signal reception techniques for lunar missions appears as a logical sequential step which is further supported by the growing international efforts for lunar exploration, which typically involves radiometric range and Doppler measurements from the Earth to perform orbit determination. In the frame of an ongoing ESA study, the Lunar GNSS project is studying the main challenges of a GNSS receiver in different phases of a mission to the Moon including Moon Transfer Orbit, Low Lunar Orbit and Descent. The study analyses expected signal strengths, DOP and number of visible satellites for selected trajectories and space volumes. Factors such as GNSS antenna radiation patterns (transmitter and receiver), frequency, constellations, spacecraft attitude, signal integration times and data assistance are discussed. Requirements for the Lunar Lander mission based on conventional radiometric measurements are presented. An overview of high sensitivity techniques which take benefit of Galileo signals and modernized GPS to cope with the challenges is provided and a description of an orbital filter complementing GNSS measurements in closely-coupled fashion is presented. The paper also describes a dedicated test platform which allows demonstrating the main functional and performance capabilities for weak signal navigation. A first batch of simulation results for selected trajectories of the Lunar Lander mission are presented, highlighting the preliminary achievable navigation performance throughout the trajectories.