During 2003 the IEN-PTB Two-Way Satellite Time and Frequency Transfer (TWSTFT) link was calibrated using a portable reference station. The calibration activity was conducted in the framework of the Galileo System Test Bed Version 1 (GSTB V1), under a contract with Joanneum Research G.m.b.H. (Austria). The calibration constant was determined with an uncertainty lower than 1 ns. Only few months after the calibration, the satellite provider moved the TWSTFT service to a different satellite; this caused changes non-reciprocal delays of the link (Sagnac and earth stations delays) and the calibration constant had to be re-evaluated. A recalculation of the Sagnac delays is presented together with a re-evaluation of earth stations delays with different measurements techniques.

In 2008, seven European institutes participated in a TWSTFT calibration campaign to determine the internal signal delays of their ground stations relative to the portable reference station of Joanneum Research, Graz. In a second step, the calibration values and their uncertainties for time comparisons of 21 links involving the participating institutes were determined. The following results were obtained. The common clock differences between the two TUG stations measured in agreed within 0.5 ns, proving the suitability of the traveling station as a reliable and stable traveling reference. After substantial changes in the hardware, the links connecting to NPL and VSL were calibrated with uncertainties slightly exceeding 1 ns and 2 ns, respectively. The so-called TAI links OP-PTB, INRIM-PTB, and METAS-PTB were recalibrated. Despite hardware changes and a change of the satellite in use in early 2008, the calibration values measured this time agreed with the values determined in 2005 and 2006 well within the calibration uncertainty of slightly above 1 ns.

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Quartz and rubidium oscillators disciplined by the signals of the Global Positioning System satellites (GPSDOs) are increasingly used as traceable reference standards in calibration centres.These devices were characterised in the past at the Time and Frequency Laboratory of IEN concerning their long-term behaviour.Recently, evidence was found of Short-term frequency instabilities, not previously detected, that must be taken into account in the frequency calibration field. The short-term instability results obtained at IEN on some GPSDOs are reported. An assessment of these features by a field-trial on frequency, was carried on among some Italian calibration centres equipped either with GPSDOs or other frequency references.