Peter Whibberley

Evaluation of the primary frequency standard NPL-CsF1

A new caesium fountain frequency standard (NPL-CsF1) at the National Physical Laboratory is described. Procedures for evaluation of the systematic frequency shifts are presented. The NPL-CsF1 has a short-term stability σy(τ) = 1.4 × 10−13τ−1/2, predominantly due to the local oscillator phase noise. The accuracy of 1 part in 1015 is limited by the uncertainty of the frequency shift due to collisions between cold atoms.

Local representations of UTC in national laboratories

Local representations of the international time scale, Coordinated Universal Time (UTC), are maintained by approximately 69 national measurement institutes and other time laboratories. These laboratories contribute their clock and time transfer measurements for use in the computation of UTC. Although local representations of UTC, commonly called UTC(k) time scales, vary considerably, for example in the numbers of atomic clocks available, they also share many characteristics. In this paper, we examine the rationale and requirements for maintaining a local representation of UTC. Its applications might range from underpinning the reference time scale of a Global Navigation Satellite System to providing traceability for frequency and time dissemination services. We address the practical aspects of setting up and operating a UTC(k) time scale, including the equipment and algorithms that generate the time scale, optimize its performance and measure its offset from the similar time scales maintained by other laboratories. We conclude by considering briefly some future developments that may have an impact on the laboratories operating local representations of UTC.

A Kalman filter UTC(k) prediction and steering algorithm

A clock prediction and steering algorithm has been developed to produce a UTC(k) timescale that may be maintained close to UTC. The algorithm is based on predicting individual (UTC — CI) clock offsets which are then combined to form a physical UTC(k) timescale. The algorithm is designed to use data from all the available laboratory clocks without the use of a clock ensemble algorithm. The performance of the underlying Kalman filter predictor is examined in depth, along with initial results using three years of historical NPL clock and UTC data combined with a simulated steering process.

Results of the 2008 TWSTFT calibration of seven european stations

In 2008, seven European institutes participated in a TWSTFT calibration campaign to determine the internal signal delays of their ground stations relative to the portable reference station of Joanneum Research, Graz. In a second step, the calibration values and their uncertainties for time comparisons of 21 links involving the participating institutes were determined. The following results were obtained. The common clock differences between the two TUG stations measured in October 2007, August 2008 and October 2008, respectively, agreed within 0.5 ns, proving the suitability of the traveling station as a reliable and stable traveling reference. After substantial changes in the hardware, the links connecting to NPL and VSL were calibrated with uncertainties slightly exceeding 1 ns and 2 ns, respectively. The so-called TAI links OP-PTB, INRIM-PTB, and METAS-PTB were recalibrated. Despite hardware changes and a change of the satellite in use in early 2008, the calibration values measured this time agreed with the values determined in 2005 and 2006 well within the calibration uncertainty of slightly above 1 ns.

International timescales with optical clocks (ITOC)

A new collaborative European project “International timescales with optical clocks” (ITOC) aims to tackle the key challenges that must be addressed prior to a redefinition of the SI second. A coordinated programme of comparisons will be carried out between European optical clocks developed in five different laboratories, enabling their performance levels to be validated at an unprecedented level of accuracy. Supporting work will be carried out to evaluate relativistic effects that influence the comparisons, including the gravitational redshift of the clock transition frequencies. A proof-of-principle experiment will also be performed to demonstrate that optical clocks could be used to make direct measurements of the Earths gravity potential with high temporal resolution.

Development of a caesium fountain primary frequency standard at the NPL

An experimental caesium fountain frequency standard has been constructed and evaluated. Some results obtained on this device are reported here. Experience gained in its operation is contributing to the design of a second caesium fountain which will ultimately operate as a primary frequency standard at the NPL.

GPS time link calibrations in the frame of EURAMET Project 1156

Since 2010 ROA has supported the coordination of the EURAMET Technical Committee for Time and Frequency (TC-TF) Project 1156, a response from EURAMET TC-TF to Recommendation 2 of CCTF 2009: to study the characterization of GNSS equipment in use for establishing the time links between institutes contributing with their clocks to TAI. Starting that year, a GPS calibration campaign was organized between three contributing laboratories: ROA (Spain), PTB (Germany) and INRIM (Italy). The time transfer results were achieved by using the P3 method, and also carrier phase PPP comparison techniques. These results were also used to re-calibrate the TWSTFT (Two-Way Satellite Time and Frequency Transfer, TW for short) links between labs, with an uncertainty slightly higher than that of the GPS links. During 2011 and 2012, the campaign was repeated, and in 2012 two other laboratories were included in the calibration trip: NPL (United Kingdom) and OP (France). In this paper we report the calibration results, with a focus on the long term stabilities of the GPS and TW links between the visited labs.

Preliminary results from NPL'S clock ensemble algorithm using hydrogen masers and caesium clocks

In recent years the National Physical Laboratory (NPL) has been developing a Kalman filter-based atomic clock ensemble algorithm. The algorithm is being used to form a composite time scale which will ultimately be employed to improve the stability of the UKs national time scale UTC(NPL) and its robustness to hardware failures. The algorithm has been optimised for both short-term and long-term stability. Flicker Frequency Modulation (FFM) clock noise is modelled using a linear combination of integrated Markov noise processes. We present here recent results obtained for the composite time scale using data from an ensemble of three active hydrogen maser clocks and two caesium clocks. An initial assessment of the ensemble algorithm is carried out using a 150-day period of data and two subsets of the data. The results show that the output of the composite is consistent between different periods of clock data and also different lengths of clock data. Initial indications are that the composite is performing well.

Reply to the comment on 'Evaluation of the primary frequency standard NPL-CsF1'

In response to the comment by Jefferts and Levi on page L11 of this issue, we review the uncertainty budget quoted in our earlier paper on the evaluation of the caesium primary frequency standard NPL-CsF1. As a result of this re-evaluation, performed in the light of new analyses and experimental data on the microwave leakage shift, we expand the fractional frequency uncertainty for this standard to 2.0 × 10−15 (1σ).

CLONETS - clock network services: Strategy and innovation for clock services over optical-fibre networks

Methods for long-distance time and frequency transfer over optical fibers have demonstrated excellent performances and are evolving rapidly. CLONETS is a European Union-funded coordination and support action intended to facilitate the vision of a sustainable, pan-European optical fiber network for precise time and frequency reference dissemination.