An Approach to the Uncertainty Estimation of [UTC-UTC(k)]

Z. Jiang, W. Lewandowski

Abstract:	UTC is represented by the differences [UTC-UTC(k)] between UTC and the UTC realization of a laboratory k, to which uncertainties are associated. The dominant part of the uncertainty comes from the time transfer. Because PTB is the only pivot of the present UTC time transfer network, the estimation of the uncertainty in [UTC-UTC(k)] becomes therefore that of the time links of Lab(k)-PTB. The uncertainty consists of two parts: the Type A component (uA) is estimated by statistical means and the Type B component (uB) is estimated by other methods, mainly the calibration. The value of uA depends on the time transfer techniques and has been carefully investigated in an early study [4]. The results have been applied in the BIPM Circular T since Nov. 2011. The value of uB depends on the calibration technique. In the present Circular T, the uB are about 1 ns for the TWSTT (Ywo-Way Satellite Time Transfer) links and 5-7 ns for the GNSS links. These conventional uB are the values corresponding to the moments when the calibrations were performed. Are they still valuable? Or are not they overestimated or underestimated? We try to answer the question in this paper. In this paper, we first review the last global uA evaluations made in 2011, of which the result is still valuable; and then we estimate the accuracy of [UTC-UTC(k)] using the newly developed accurate time link calibration techniques of which the uB is about 0.6 ~ 1.5 ns. After proving the dependence of the uncertainty in [UTC-UTC(k)] on that of the related time transfer link Lab(k)-PTB, we analyze the uncertainties of the TWSTT and GNSS time link calibrations performed in the recent years by several independent organizations, such as the TimeTech, PTB, USNO, ROA and BIPM etc. Some seventeen leading UTC laboratories in Europe, Asia and North America are involved which contribute 84% clock weight in the UTC generation. We evaluate the real uncertainty in [UTC-UTC(k)] which is of 2.5 ns in general for both of the TWSTT and GNSS links. It is bigger than the uB(TW)=1 ns and much smaller than the uB(GPS)=5 ns. The evolution of the uB(TW) may explain the first and the overestimation the latter. By this study, we do not find convincible examples proving that the firmware variation in the TWSTT or GNSS receivers may cause the calibrations change bigger than 2 ns. It is however easy to find evidences that most of the outliers > 2 ns in the calibration corrections are probably caused by the equipment setups changes and/or sub-delay measurement errors/biases in the UTC laboratories as well as the events such as the satellite or frequency changes, etc. KEY WORDS: UTC, Time transfer, [UTC-UTC(k)], Calibration, Time link calibration, METODE, Type A uncertainty (uA), Type B uncertainty (uB), combined uncertainty (u)
Published in:	Proceedings of the 46th Annual Precise Time and Time Interval Systems and Applications Meeting December 1 - 4, 2014 Seaport Boston Hotel Boston, Massachusetts
Pages:	79 - 85
Cite this article:	Jiang, Z., Lewandowski,  W., "An Approach to the Uncertainty Estimation of [UTC-UTC(k)]," Proceedings of the 46th Annual Precise Time and Time Interval Systems and Applications Meeting, Boston, Massachusetts, December 2014, pp. 79-85.
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