Lunar Transfer Missions Navigation using GNSS Software Receivers

The work deals with the possibility to use GNSS as a mean to navigate missions targeted to the Moon. Two critical aspects arise: the availability of the signal (very limited signal power at high altitudes), and the dilution of precision (sources are viewed in a limited portion of the sky) which deeply affect the expected performance. With respect to the availability of the signal, in order to recovery even faint levels, the use of software receiver techniques can help to significantly lower the threshold of the useful power to noise ratio by integrating along extended time intervals. The substantial computation effort required is softened by the availability of long time windows due to very slow dynamics. Taking into account codeless pilot tones (e.g. Galileo), the combination of two signals (pilot and data) provides better performance than the classical extended coherent integration in both acquisition and tracking tasks. Doppler issue is critical: in fact the lower the acquisition threshold, granted by long coherent integration, the higher is the limitation on maximum admissible Doppler. Numerical simulations for a selected lunar transfer strategy suggest confidence on the Doppler rate values, but still point out how the number of valid sources can be extremely limited. As far as it concerns the accuracy,, resulting values for DOPs are quite high and lead to poor performance as the spacecraft approaches the Moon. Even to take into account the upcoming additions to GPS does not improve the DOP issue. The possibility of augmenting beacons to be located on the lunar surface is therefore considered to help in the most critical phases.