Restoring a TWSTFT calibration with a GPS bridge - A standard procedure for UTC time transfer

Two-Way Satellite Time and Frequency Transfer (TW for short) is one of the primary techniques for UTC (Coordinated Universal Time) time transfer. One of its advantages is its ability of accurate calibration, reflected by its low achievable Type B uncertainty, u_B(TW). Of the time links included in the calculation of UTC, the best u_B(TW) is about 1 ns, as compared to 5 ns for u_B(GPS). Moreover, the TW calibration is characterized by long-term stability and reproducibility within the stated calibration uncertainty. However TW calibration is rather expensive and time consuming. It also depends on the complete configuration of the triangle Lab(1)-satellite-Lab(2). Any change in any of the segments necessitates a change in the calibration. Such a situation has occurred several times over the last years, due to exchanges of the used satellites. After changes, a calibration can be restored by means of a bridge based on another, continued, time-transfer technique. In this paper, we discuss the restoration of TW calibrations using GPS PPP and other bridges, and estimate their uncertainties. A standard procedure has been developed for the restoration operation. This procedure was use d to restore TW calibrations following the change in satellite from IS-3R to T-11N in mid-2009, and the results were implemented in the TW links as of September 2009.