Calibration of six European TWSTFT earth stations using a portable station

Two-way satellite time and frequency transfer (TWSTFT) has become an important component in the international network for comparing time scales. To employ the full potential of the technique a calibration of the internal delay of each ground station is necessary. Only a few calibration campaigns have previously been carried out in the European network of TWSTFT links. We report on the first recalibration of TWSTFT links during a campaign involving six European time institutes. The campaign was performed using a portable ground station assembled and operated by TUG/Joanneum Research, Graz, that visited the sites of INRIM, NPL, OP, PTB, SP, and VSL, travelling a total distance over 7000 km during a three-week period in October/November 2005. Differential delays of earth stations were determined in the common clock mode relative to the portable station. Combined uncertainties ranging from 0.9 ns to 1.3 ns for all calibrated links were achieved in this campaign.