Long-Term Instability of GPS-Based Time Transfer and Proposals for Improvements

GNSS-based time transfer is the prevalent technique used in the generation of Coordinated Universal Time (UTC); in September 2011, 56 links out of the total of 67 were GNSS-only. These links are realized by combining the data from a single GNSS receiver in each of the time laboratories involved. Some modern receivers have shown only sub-nanosecond variations, but it has been observed that the internal calibration reference of other GPS receivers occasionally jumps by several nanoseconds, and GPS receivers can suffer long-term variations of up to 2 ns per year. In certain situations, without periodic calibrations, these variations can accumulate to form much larger offsets. This would be particularly serious at the PTB, the unique pivot laboratory in the worldwide UTC network, because it would in turn bias the associated subsets of participating laboratories, as well as UTC itself. In this paper, we show the long-term delay instability in the GPS receivers revealed by common-clock and near-zero-baseline observations at METAS, NICT, USNO and PTB. These show phase jumps, seasonal variations, and various forms of linear variations. Based on these findings, we consider how the Type B uncertainty of a link calibration as estimated at a certain epoch can be extrapolated over an extended period so as to estimate the combined uncertainty of [UTC UTC (k)]. Combining the results of an ensemble of independent GNSS receivers can reduce the influence of individual devices. As a first step, we investigate double-receiver time transfer, and find that the use of a second time-transfer system increases the time stability with regard to measurement noise and receiver phase jumps. Comparison with a third receiver, or use of a completely independent technique such as Two-Way Satellite Time and Frequency Transfer (TWSTFT), helps to identify which receiver has caused an anomaly in the measurements. Presented at the Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting 2011(43rd), held in Long Beach, California on 14-17 November 2011. The original document contains color images.