Andreas Bauch

Time transfer through optical fibres over a distance of 73 km with an uncertainty below 100 ps

We demonstrate the capability of accurate time transfer using optical fibres over long distances utilizing a dark fibre and hardware which is usually employed in two-way satellite time and frequency transfer (TWSTFT). Our time transfer through optical fibre (TTTOF) system is a variant of the standard TWSTFT by employing an optical fibre in the transmission path instead of free-space transmission of signals between two ground stations through geostationary satellites. As we use a dark fibre there are practically no limitations to the bandwidth of the transmitted signals so that we can use the highest chip rate of the binary phase-shift modulation available from the commercial equipment. This leads to an enhanced precision compared with satellite time transfer where the occupied bandwidth is limited for cost reasons. The TTTOF system has been characterized and calibrated in a common-clock experiment at PTB, and the combined calibration uncertainty is estimated as 74 ps. In a second step the remote part of the system was operated at Leibniz Universitat Hannover, Institut fur Quantenoptik (IQ) separated by 73 km from PTB in Braunschweig. In parallel, a GPS time transfer link between Braunschweig and Hannover was established, and both links connected a passive hydrogen maser at IQ with the reference time scale UTC(PTB) maintained in PTB. The results obtained with both links agree within the 1-σ uncertainty of the GPS link results, which is estimated as 0.72 ns. The fibre link exhibits a nearly ten-fold improved stability compared with the GPS link, and assessment of its performance has been limited by the properties of the passive maser.

Generation of UTC(PTB) as a fountain-clock based time scale

The Physikalisch-Technische Bundesanstalt (PTB) has substantially improved the quality of its local time scale UTC(PTB), which is the national realization of the international time reference Coordinated Universal Time (UTC). It serves as the basis for PTBs time services, for local clock comparisons and for international time comparisons. Since February 2010 UTC(PTB) has been realized using an active hydrogen maser (AHM) steered in frequency via a phase micro-stepper according to an algorithm which combines the frequency comparison data between the AHM and primary and commercial caesium clocks of PTB. Thereby the long-term stability and accuracy of PTBs primary clocks, in particular its fountain clock CSF1, were combined with the short-term frequency stability of the AHM. CSF1 data were used to calculate the steering on all but six days during 15 months. During the time between July 2010 and July 2011, the time difference between UTC(PTB) and UTC was always less than 6?ns and the monthly mean rate differences never exceeded 0.16?ns per day.

First comparison of remote cesium fountains

The frequencies of the cesium fountain primary frequency standards at the National Institute of Standards and Technology and the Physikalisch-Technische Bundesanstalt have been compared. Two-way satellite time and frequency transfer and GPS carrier-phase were the principal frequency-transfer techniques used to make the comparison. For the 15-day interval in which both fountains were in operation the frequencies were compared with an additional uncertainty due to the comparison process of only 6.2 /spl times/ 10/sup -16/. The two standards agree within their stated one-sigma uncertainties of /spl sim/ 1.7 /spl times/ 10/sup -15/.

Caesium atomic clocks: function, performance and applications

For more than four decades, caesium atomic clocks have been the backbone in a variety of demanding applications in science and technology. Neither satellite based navigation systems, like the US Global Positioning System, nor the syntonization of telecommunication networks at the presently prescribed levels, would function without them. Recent years have brought major breakthroughs in the development, operation and mutual comparison of frequency standards based on the same hyperfine transition in caesium as used previously, but now incorporating the technique of laser cooling. Several cold-atom fountains have been developed. Mutual agreement to within about one part in 1015 has been demonstrated for two of them operated side by side, and also for two operated simultaneously, in the US and Germany. This paper gives an overview of currently available commercial caesium clocks and primary standards developed in national metrology institutes.

Carrier-phase two-way satellite frequency transfer over a very long baseline

In this paper we report that carrier-phase two-way satellite time and frequency transfer (TWSTFT) was successfully demonstrated over a very long baseline of 9000 km, established between the National Institute of Information and Communications Technology (NICT) and the Physikalisch-Technische Bundesanstalt (PTB). We verified that the carrier-phase TWSTFT (TWCP) result agreed with those obtained by conventional TWSTFT and GPS carrier-phase (GPSCP) techniques. Moreover, a much improved short-term instability for frequency transfer of 2 × 10−13 at 1 s was achieved, which is at the same level as previously confirmed over a shorter baseline within Japan. The precision achieved was so high that the effects of ionospheric delay became significant; they are ignored in conventional TWSTFT even over a long link. We compensated for these effects using ionospheric delays computed from regional vertical total electron content maps. The agreement between the TWCP and GPSCP results was improved because of this compensation.

Remote atomic clock synchronization via satellites and optical fibers

Abstract. In the global network of institutions engaged with the realization of International Atomic Time (TAI), atomic clocks and time scales are compared by means of the Global Positioning System (GPS) and by employing telecommunication satellites for two-way satellite time and frequency transfer (TWSTFT). The frequencies of the state-of-the-art primary caesium fountain clocks can be compared at the level of 10−15 (relative, 1 day averaging) and time scales can be synchronized with an uncertainty of one nanosecond. Future improvements of worldwide clock comparisons will require also an improvement of the local signal distribution systems. For example, the future ACES (atomic clock ensemble in space) mission shall demonstrate remote time scale comparisons at the uncertainty level of 100 ps. To ensure that the ACES ground instrument will be synchronized to the local time scale at the Physikalisch-Technische Bundesanstalt (PTB) without a significant uncertainty contribution, we have developed a means for calibrated clock comparisons through optical fibers. An uncertainty below 40 ps over a distance of 2 km has been demonstrated on the campus of PTB. This technology is thus in general a promising candidate for synchronization of enhanced time transfer equipment with the local realizations of Coordinated Universal Time UTC. Based on these experiments we estimate the uncertainty level for calibrated time transfer through optical fibers over longer distances. These findings are compared with the current status and developments of satellite based time transfer systems, with a focus on the calibration techniques for operational systems.

The PTB primary clocks CS1 and CS2

The primary clocks CS1 and CS2 have been developed and operated by the Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. By their contributions to the definition of the scale unit of International Atomic Time they have provided access to the realization of the SI second over decades with exceptional accuracy. They have stood out against other primary clocks by the novelty of their design concept, their robustness of construction and last but not the least by their almost continuous operation for many years. Their properties have changed with time, but during the last 7 years their uncertainty, uB, has been estimated as 8 × 10−15 (CS1) and 12 × 10−15 (CS2). Comparisons with PTBs cold-atom frequency standard CSF1 (uB = 1 × 10−15) over 3.5 years revealed that CS2 and CSF1 agreed well within the uncertainty uB(CS2), whereas CS1 frequency deviates slightly more from CSF1 than uB(CS1).

Improved GPS-based time link calibration involving ROA and PTB

The calibration of time transfer links is mandatory in the context of international collaboration for the realization of International Atomic Time. In this paper, we present the results of the calibration of the GPS time transfer link between the Real Instituto y Observatorio de la Armada (ROA) and the Physikalisch-Technische Bundesanstalt (PTB) by means of a traveling geodetic-type GPS receiver and an evaluation of the achieved type A and B uncertainty. The time transfer results were achieved by using CA, P3, and also carrier phase PPP comparison techniques. We finally use these results to re-calibrate the two-way satellite time and frequency transfer (TWSTFT) link between ROA and PTB, using one month of data. We show that a TWSTFT link can be calibrated by means of GPS time comparisons with an uncertainty below 2 ns, and that potentially even sub-nanosecond uncertainty can be achieved. This is a novel and cost-effective approach compared with the more common calibration using a traveling TWSTFT station.

Measurement of the Frequency-Shift Due to Distributed Cavity Phase Difference in an Atomic Clock

The Ramsey cavity of a Cs beam frequency standard was moved vertically and horizontally perpendicular to the beam axis. In the vertical direction the measured frequency of the clock transition depends linearly upon the place of irradiation due to a change in the cavity phase difference. Simplified assumptions are made in the computation of the distributed phase. Experimental and theoretical results agree satisfactorily.

Two-way satellite time transfer between USNO and PTB

Two completely independent two-way time and frequency transfer (TWSTFT) links have been established between the institutions of USNO and PTB, with transponder frequencies in the Ku-band and X-band, respectively. The Ku-band link has some strategic importance, since currently it connects almost one half of the atomic clocks in the BIPM network that are employed for the realization of TAI. The X-band data are provided as a backup. To reach the full potential of TWSTFT, especially for time scale comparisons, repetitive calibrations of the links are necessary. Since 2002, USNO has scheduled semiannual calibration exercises. We report on the three calibration campaigns in 2004 and early 2005. New calibration values were determined in 2004 and 2005. For the first time, combined uncertainties below 1.0 ns for both links were achieved. A change of the TWSTFT transmission frequencies or satellite changes in general cause discontinuities in the series of time transfer data and render the previous calibration useless. We describe how we coped with two such events by bridging with X-band and GPS carrier phase data. The previous calibration could be preserved with sufficient accuracy of about a few tenths of a nanosecond