Multi-GNSS time transfer based on the CGGTTS

This paper studies the use of multiple GNSS constellations for time transfer. The performances of each constellation are compared to one another. We show that, with its current constellation, Galileo already provides time transfer results with the same or better performances than GPS, for which some time links are affected by a diurnal variation. The paper also investigates different ways of combining the constellations into one global time transfer solution. We show that in a global AV solution, only one constellation should be calibrated, due to the inherent inter-system biases in the multi-GNSS clock products. Our results show that the best combination is to use the Galileo solution as a reference, and to combine the observations applying one bias (with respect to Galileo) per satellite of the other constellations. This allows reducing the impact of diurnal variations present in some GPS clock solutions. Combining Galileo and GPS satellites in this way improves the time transfer link with respect to the GPS-only or Galileo-only solutions. Adding moreover BeiDou-2 and GLONASS satellites gives, depending on the link, a solution similar or worse than the solution obtained from GPS and Galileo.