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The PTB has assembled an easy-to-use GPS calibration set-up intended for relative time link calibrations, consisting of a GTR50 time and frequency GPS receiver and an SR620 time-interval counter integrated in a small rack together with a monitor and a keyboard. Usually, locally available time interval counters are used to measure the offset t0 between the 1 PPS signal representing the local time scale and the signal that is connected to the traveling receiver in each visited laboratory. In each time-interval counter, a distinct error for time-interval measurement shows up. If only one single counter is used at all participating laboratories, this error cancels out, assuming that the properties of this counter do not change during the travel, as it is assumed for the properties of the traveling receiver as well. The calibration setup was shipped to USNO in mid June 2010 and was used to calibrate the time transfer links between USNO and PTB, in total 16. This large number reflects the multitude of fixed receivers involved and the different kinds of data processing. For all cases, the calibration uncertainty is at the level of 2 ns and below. The results have been used to validate an ad-hoc calibration adjustment made earlier by the BIPM to the Ku-band TWSTFT link. A deviation of 3.7 ns compared to the 2008 result was found