Abstract
A status report is given on current practice and trends in time and frequency metrology. Emphasis is laid on such fields of activity that are of interest in the context of relativistic geodesy. In consequence, several topics of a priori general relevance will not be dealt with. Clocks and the means of comparing their reading are equally important in practically all applications and thus dealt with in this contribution. The performance of commercial atomic clocks did not change significantly during the last 20 years. Progress is noted in the direction of miniaturization, leading to the wide-spread use of chip-scale atomic clocks. On the other hand, research institutes invested considerably into the perfection of their instrumentation. Cold-atom caesium fountain clocks realize the SI-second with a relative uncertainty of close to \(1\times 10^{-16}\), and with a relative frequency instability of the same magnitude after averaging over a few days only. Optical frequency standards are getting closer to being useful in practice: outstanding accuracy combined with improved technological readiness can be noted. So one necessary ingredient for relativistic geodesy has become available. Satellite-based time and frequency comparison is here still somewhat behind: Time transfer with ns-accuracy and frequency transfer with \(1\times 10^{-15}\) per day relative instability have become routine. Better performance requires new signal structures and processing schemes, some appear on the horizon.
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Notes
- 1.
Note: Although this is a review article, my intention was not to provide an exhaustive list of references, but rather to limit myself to text books and previous review articles of other authors with few exceptions.
- 2.
In order to understand the word “leap”: Only very few fountain clocks achieve an instability as shown, see discussion in Sect. 3.2.
- 3.
For improved hold-over capability, some models include a Rb frequency standard (see Sect. 3.1) that is steered with a few hours time constant.
- 4.
As an aside, I remember well that my start as PhD student at PTB in 1983 coincided with the installation of the first receiver of this kind in the PTB time-unit laboratory, a single-channel GPS receiver for time transfer provided by the then National Bureau of Standards (now NIST). This receiver is now at display in the Deutsche Uhrenmuseum (German clock museum), Furtwangen, Black Forest.
- 5.
Source of data: BIPM ftp server at ftp://ftp2.bipm.org/pub/tai/timelinks/lkc/, data in monthly files, Read Me file provided.
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Acknowledgements
This review paper reports mostly on achievements of colleagues from all over the world. The fruitful collaboration belonged to the pleasures of the author’s business life. Special thanks go to Ekkehard Peik, Dirk Piester and Stefan Weyers of PTB for critical reading of the manuscript.
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Bauch, A. (2019). Time and Frequency Metrology in the Context of Relativistic Geodesy. In: Puetzfeld, D., Lämmerzahl, C. (eds) Relativistic Geodesy. Fundamental Theories of Physics, vol 196. Springer, Cham. https://doi.org/10.1007/978-3-030-11500-5_1
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