The performance of carrier-phase based GPS precision time transfer was investigated by conducting a 7-month campaign of time transfer among four national timing laboratories. The Common-View and the All-in-View techniques, which are two typical techniques for the GPS carrier-phase time transfer, are examined in terms of time transfer and frequency transfer together with other independent time transfer systems such as TWSTFT. Comparison of clock estimates with BIPM Circular-T was performed and the modified Allan deviation of clock estimates was calculated to assess the relative frequency instability as a measure of frequency transfer performance. Also an issue of baseline limitation for the carrier-phase Common-View methods has been investigated by conducting a 1-month GPS time transfer to a set of IGS stations with different baselines. Finally, the potential of the GPS carrier-phase time transfer as a real-time synchronization method was assessed by carrying out the GPS time transfer with the IGS Ultra-rapid orbits.
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