Two-way satellite time transfer between USNO and PTB

Two completely independent two-way time and frequency transfer (TWSTFT) links have been established between the institutions of USNO and PTB, with transponder frequencies in the Ku-band and X-band, respectively. The Ku-band link has some strategic importance, since currently it connects almost one half of the atomic clocks in the BIPM network that are employed for the realization of TAI. The X-band data are provided as a backup. To reach the full potential of TWSTFT, especially for time scale comparisons, repetitive calibrations of the links are necessary. Since 2002, USNO has scheduled semiannual calibration exercises. We report on the three calibration campaigns in 2004 and early 2005. New calibration values were determined in 2004 and 2005. For the first time, combined uncertainties below 1.0 ns for both links were achieved. A change of the TWSTFT transmission frequencies or satellite changes in general cause discontinuities in the series of time transfer data and render the previous calibration useless. We describe how we coped with two such events by bridging with X-band and GPS carrier phase data. The previous calibration could be preserved with sufficient accuracy of about a few tenths of a nanosecond