L. Tisserand

The 8th International Comparison of Absolute Gravimeters 2009: the first Key Comparison (CCM.G-K1) in the field of absolute gravimetry

The 8th International Comparison of Absolute Gravimeters (ICAG2009) took place at the headquarters of the International Bureau of Weights and Measures (BIPM) from September to October 2009. It was the first ICAG organized as a key comparison in the framework of the CIPM Mutual Recognition Arrangement of the International Committee for Weights and Measures (CIPM MRA) (CIPM 1999). ICAG2009 was composed of a Key Comparison (KC) as defined by the CIPM MRA, organized by the Consultative Committee for Mass and Related Quantities (CCM) and designated as CCM.G-K1. Participating gravimeters and their operators came from national metrology institutes (NMIs) or their designated institutes (DIs) as defined by the CIPM MRA. A Pilot Study (PS) was run in parallel in order to include gravimeters and their operators from other institutes which, while not signatories of the CIPM MRA, nevertheless play important roles in international gravimetry measurements. The aim of the CIPM MRA is to have international acceptance of the measurement capabilities of the participating institutes in various fields of metrology. The results of CCM.G-K1 thus constitute an accurate and consistent gravity reference traceable to the SI (International System of Units), which can be used as the global basis for geodetic, geophysical and metrological observations of gravity. The measurements performed afterwards by the KC participants can be referred to the international metrological reference, i.e. they are SI-traceable.The ICAG2009 was complemented by a number of associated measurements: the Relative Gravity Campaign (RGC2009), high-precision levelling and an accurate gravity survey in support of the BIPM watt balance project. The major measurements took place at the BIPM between July and October 2009. Altogether 24 institutes with 22 absolute gravimeters (one of the 22 AGs was ultimately withdrawn) and nine relative gravimeters participated in the ICAG/RGC campaign.This paper is focused on the absolute gravity campaign. We review the history of the ICAGs and present the organization, data processing and the final results of the ICAG2009.After almost thirty years of hosting eight successive ICAGs, the CIPM decided to transfer the responsibility for piloting the future ICAGs to NMIs, although maintaining a supervisory role through its Consultative Committee for Mass and Related Quantities.

A new prediction algorithm for the generation of International Atomic Time

In this paper we present a new prediction algorithm for the generation of International Atomic Time (TAI). The new prediction algorithm takes into account the frequency drift which affects most of the participating atomic clocks. In particular, we focus on the effect of the application of the new model on the prediction term for the frequency drift affecting the free atomic time scale (EAL). We also present its effect on TAI performance and on atomic clock weights.

Relative Gravity Measurement Campaign during the 7th International Comparison of Absolute Gravimeters (2005)

Since the 1st International Comparison of Absolute Gravimeters (ICAG) and accompanying Relative Gravity Campaign (RGC) held at the BIPM in 1981, repeated ICAG-RGCs have been organized every four years. A total of 19 absolute gravimeters (AG) and 15 relative gravimeters (RG) participated in the 7th ICAG-RGC, which took place in 2005. Co-located absolute and relative gravity measurements as well as precision levelling measurements were carried out. The final version of the absolute g values of the 7th ICAG has been officially released recently. This paper is the final report of the 7th RGC and replaces the preliminary results published earlier. It covers the organization of the RGC and the data processing, analyses RG behaviour, computes g, δg and OAG (offset of AG) and discusses their uncertainties. In preparation for the BIPM key comparison ICAG-2009, a standard data-processing procedure has been developed and installed in the BIPM ICAG-RGC software package, GraviSoft. This was used for the final data processing.

UTCr: a rapid realization of UTC

Considering the evolving needs of time metrology and the convenience of allowing the contributing laboratories access to a realization of UTC more frequently than through the monthly Circular T, the BIPM Time Department started in 2012 to implement the computation of UTCr, a rapid realization of UTC published every week and based on daily data. After 18 months of pilot experiment, this new product has been declared operational and is now an official publication of the BIPM. This paper presents the main characteristics and properties of UTCr.

A new weighting procedure for UTC

In this paper we present a new weighting algorithm for the generation of Coordinated Universal Time (UTC). The new weighting procedure is based on the idea that the best clocks are the most predictable. The effect of the new algorithm on the weight distribution and on the stability of UTC is presented and discussed. An improvement in the frequency stability of UTC at both short and long terms is observed, as well as a better clock weight distribution in the ensemble.

On the gravimetric contribution to watt balance experiments

It has been recommended that the relative standard uncertainty of the numerical value of the Planck constant required for the redefinition of the kilogram should not exceed 2???10?8. To reach this goal using experiments based on a watt balance, the free-fall acceleration (g) traceable to the SI, at a given point and a given time, needs to be known with a sufficiently small uncertainty well below 2???10?8. Reducing the uncertainty in g allows the other uncertainties related to the watt balance to be increased. Instead of a simultaneous operation of an absolute gravimeter with a watt balance, we propose an alternative approach and demonstrate that a standard uncertainty below 5??Gal (relative uncertainty of 5???10?9) is reachable under the conditions at BIPM. Further decreasing the uncertainty could significantly increase commitments in terms of personnel and equipment and would not significantly improve the uncertainty targeted for the BIPM watt balance experiment. A 5??Gal uncertainty might also satisfy the needs of other watt balance experiments underway or planned. In our approach we combine the following information: (1) the Key Comparison Reference Values obtained from the CCM.G-K1, a key comparison carried out in the frame of the International Comparison of Absolute Gravimeters in 2009 (ICAG2009); (2) the accurate gravity network established using the qualified absolute and relative gravimeters; (3) temporal gravity variations based on observed Earth-tide parameters and modelled effects of polar motion and atmospheric mass redistribution; (4) uncertainty estimates that account for non-modelled effects; (5) the option to carry out absolute gravity measurements once every one or two years with two or more gravimeters for monitoring the stability of the gravity field at the BIPM.

UTCr: A rapid realization of UTC

Considering the evolving needs of time metrology and the convenience of allowing the contributing laboratories access to a realization UTC more frequently than through the monthly Circular T, the BIPM Time Department has started to implement the computation of UTCr, a rapid realization of UTC published every week and based on daily clock and time transfer data. Results of the first weeks of a pilot experimentation of this new product are presented.

A new prediction algorithm for EAL

In this paper the new algorithm for the prediction of EAL (Echelle Atomique Libre) is presented. The effect of the application of the new model for the prediction term on the EAL frequency drift is quantified and presented together with its effect on TAI and on the atomic clock weights.

Progress in the link calibration for UTC time transfer

The dominant part of the total uncertainty budget in the [UTC-UTC(k)] is from the type B uncertainty (uB) in time link calibration. In fact, 60% of the type A uncertainty (ua) in UTC time links are <; 1 ns while the 93% of uB ≥ 5ns. In 2011, the BIPM launched a pilot experiment to reduce the uB from 5 ns at present to ≤ 2 ns and has developed a standard calibration scheme, namely METODE, associated with an integrated portable GNSS calibrator together with the reference receivers stationary at BIPM. Similar setups are made in TL. Using these systems, we performed a series experiments at BIPM, OP and TL to study the attainable uncertainty of METODE. The results are compared to that of the traditional differential receiver calibrations.

Accurate Gravimetry at the BIPM Watt Balance Site

Associated with the 8th International Comparison of Absolute Gravimeters (ICAG) at the Bureau International des Poids et Mesures (BIPM) in 2009, accurate gravity measurements were made to support the BIPM watt balance (WB) project, aiming at determining the Planck constant h for the future realization of a new definition of the kilogram based on the Planck constant and the accurate g values. The ICAG 2009 was organized as a metrological Key Comparison (KC) as defined by the Committee International des Poids et Mesures (CIPM) Mutual Recognition Arrangement (MRA). Its results will thus constitute a precise and consistent gravity reference in SI units which then can be used as the global basis for geodetic, geophysical and metrological observations of gravity. Four absolute and six relative gravimeters took part in the WB gravity campaign. Their results can therefore be converted to the international reference of the KC results, i.e., they are SI-traceable. The WB network is a 3-D grid over the site in the WB laboratory. In order to monitor the stability of the sites, repeated precise leveling has being carried out. In addition, a set of accurate Earth tide parameters were determined based on six months of gravimetric tidal observations. They will enable the tidal prediction, which is needed in order to use gravity values to produce the momentary values of the acceleration of free fall that are required by the WB experiment. We present in this paper the organization, the measurements, the data processing and the preliminary results.