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Two-way satellite time and frequency transfer (TWSTFT) has become an important technical component in the process of the realization of International Atomic Time. To employ the full potential of the technique, especially for true time transfer, a dedicated calibration is necessary. This consists of the calibration either of the operational link at large, including every component involved, or of the involved ground stations' internal delays only. Both modes were successfully employed by circulating and operating a portable reference station between the sites involved. In this paper, we summarize the theoretical background for the different calibration modes applied and report examples of results from the 13 calibration campaigns performed up to now in Europe and between Europe and the United States. In all of these exercises, estimated uncertainties around 1 ns were achieved. Consecutive campaigns showed a very …