Weak GNSS signal navigation in lunar missions

Weak GNSS signals could be exploited in future lunar missions to increase navigation robustness, flexibility and autonomy. In these applications GNSS reception suffers from very low signal levels, partial visibility of the GNSS sources and unfavourable geometry, making use of either secondary lobes or the signals’ spill over around the Earth mask. Objective of a recent ESA study was to evaluate the challenges of such a navigation technique using GPS and future Galileo reception with carrier to signal levels as low as 10 to 15 dBHz. Investigated mission phases included transfer orbit, low lunar orbits, lunar ascent and descent as well as surface operation and navigation at the Lagrangian points. The paper presents the approach pursued during the study and shares its main findings, showing first that GNSS can be actually considered as an available and extremely useful navigation resource in all phases, even if part of the missions, and specifically low lunar orbits and descent, will require a mandatory aiding form other sensors. Moreover, external aiding in terms of the content of the data message has been envisaged in order to adopt a snapshot architecture for the receiver and to overcome limits in tracking loops with extremely low carrier to noise ratio. A proof of concept for the proposed receiver has been built and tested in different simulated scenarios. As a final result, a multi-constellation GNSS receiver software can be considered a suitable option to enable autonomous navigation in lunar missions, allowing for large savings in the expensive – and poorly available – ground-based tracking network.