Time comparisons between USNO and PTB: a model for the determination of the time offset between GPS time and the future Galileo system time

The interoperability of the two satellite navigation systems Galileo and GPS shall be assured by, among other measures, broadcasting the actual difference between the two system times, Galileo system time (GST) and GPS time (GPST). The properties of PTB´s time scale UTC(PTB) are in many aspects similar to what is expected for GST. Thus, the difference UTC(PTB)-GPST can be considered to be representative for the GST-GPST difference. UTC(PTB)-GPST was determined with the help of two methods, a TWSTFT link with USNO, and the restitution of GPST from the reception of the GPS signal-in-space (SIS) with GPS time receivers. All links and receivers were calibrated. Therefore, in an ideal world, all links should give the same result. At the current stage of data analysis, we notice a difference of about 7 ns between the UTC(PTB)-GPST difference obtained via the link with USNO and the one obtained from reception of GPS SIS with the dual frequency geodetic receiver.