Assisted GNSS navigation in lunar missions

The trend towards autonomous operations will likely extend to include very high orbits and lunar missions. A basic asset for autonomy is the availability of a correct navigation solution. The possibilities offered by the use of GNSS, even at altitudes far higher than expected, are investigated in this paper, showing moderate availability of weak signals with frequent outages. Software receivers would be able to acquire the signals in this scenario, but decoding the navigation message – typically required in order to compute a fix - remains a very difficult task. An approach to obtain a solution, via the iunfrequent uplink of a limited part of the information included in the data message, is proposed and aalyzed, and the relevant requirements for the clock and orbit propagator on-board are discussed. This technique, based on a very moderate aiding, has the potential for saving operational costs related to ground support and tracking which are quite significant for these missions.