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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This paper presents a dilution of precision (DOP) analysis of a clock-aided global navigation satellite system (GNSS). Firstly, the analysis shows that a best linear unbiased estimator should be used in the clock-aided GNSS to get the best improvements in accuracy. Secondly, the influence of the clock on vertical DOP (VDOP) and horizontal DOP (HDOP) is analysed by using the clock characteristics as a weighting matrix. The analysis highlights quantitative improvements in the VDOP and HDOP and explains why improvements in the VDOP are superior to those of the HDOP in the clock-aided GNSS. Finally, the paper analyses how incorrectly known a priori clock variances degrade the level of accuracy and offers guidelines for determining a weighting matrix in the cases where the clock characteristics are not clearly known.

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