Coherent Tracking of Globalstar Signals Using Partially-Known Spreading Codes

It is nowadays widely accepted that the future of satellite-based positioning will comprise the combination of conventional Global Navigation Satellite Systems (GNSS) with new Low-Earth Orbit (LEO) satellites specifically designed for positioning, navigation and timing (PNT). In the meantime, opportunistic positioning using existing LEO constellations has become a practical way to showcase the benefits of incorporating LEO satellites in the PNT domain. Among these constellations, Globalstar is particularly interesting because it shares the same spread-spectrum modulation principle as most GNSS. Therefore, Globalstar signals could easily be processed by GNSS receivers with some minor modifications. Unfortunately, the spreading codes for Globalstar are not publicly available, thus preventing the optimal receiver implementation. While some attempts have been done to estimate such codes, or to get rid of them, the present paper shows that tracking of Globalstar signals is possible using a set of, presumably, very similar spreading code sequences to the ones actually used by Globalstar satellites. Experimental results with live recorded signals are provided to confirm this statement.