A GPS calibration trip experience between ROA and PTB

For a GPS calibration campaign between ROA and PTB a GTR50 time and frequency transfer receiver from ROA was chosen. The operation performance of GTR50 type receivers has been validated in long term operation during the last years. The travelling GTR50 was also tested in a sequence of shutdowns and restarts to detect time jumps and malfunctions just after the restart and during the following days of operation. In September 2008, the receiver was shipped to PTB and then was operated there for eleven days. One week after it had returned back at ROA, the common-view data showed only a 100 ps difference with respect to the same configuration carried out one week before the trip. In this paper, we will present the calibration results of this GPS link, and an evaluation of its type A and B uncertainty. The time transfer results were achieved by using C/A, P3 and also carrier phase PPP comparison techniques. We finally extrapolate these results in order to calibrate the two-way satellite time and frequency transfer (TWSTFT) link between ROA and PTB, using one month of data, and involving one GTR50 from ROA and one Ashtech Z-12T from PTB. We show that a TWSTFT link can be calibrated by means of GPS time comparisons with an uncertainty below 2 ns, and that potentially even sub-nanosecond uncertainty can be achieved.