Precise estimation of the GPS To Galileo Time Offset: an experimental setup and its associated processing method

Although the satellite navigation systems are all in their definition based on the international atomic time (TAI), each system has its own realization. There is therefore a difference, variable in time, between different realizations of the same time systems. Thus, the time of the GPS (GPST) and Galileo (Galileo System Time, GST) systems differ in their realization by some nanoseconds. This difference is called GGTO for GPS to Galileo time offset. An independent, accurate and high resolution knowledge of the GGTO can be useful for precise positioning and orbit determination in a multi-constellation context. However, the GGTO, determined by the Galileo ground segment, is not distributed publicly in a precise way. We present here an experimental setup to estimate the GGTO: the same GNSS antenna is ``split'' on two receivers, one using the GPST, the other the GST as reference time. Both receivers are connected to the same rubidium oscillator, in order to have the same frequency reference. A preliminary analysis has been carried out using collected pseudo-distance observations. The results obtained show a good agreement with the broadcast GGTO, but a constant bias of few nanoseconds remains, requiring further investigations.