Felicitas Arias

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 Altogether 24 institutes with 22 absolute gravimeters (one of the 22 AGs was ultimately withdrawn) and nine relative gravimeters participated in the ICAG/RGC campaign.This paper is focused on the absolute gravity campaign.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.

BIPM comparison of time transfer techniques

Clock comparison for International Atomic Time (TAI) is based on common views of satellites of the GPS constellation and on the technique of two-way satellite time and frequency transfer (TWSTFT). All the TWSTFT links are backed up by the GPS ones. Due to the duplicity of techniques in many baselines, the international network of time links is today highly redundant. The improvement of GPS satellite ephemerides and clock values produced by the International GNSS Service (IGS) might lead to the utilisation of the all-in-view (AV) method for the computation of GPS links instead of the common-view (CV) one currently used for TAI. Studies started at the BIPM to validate the GPS AV method for introduction in the calculation of TAI. To assist related studies, we developed a procedure to compare the links measured with different techniques and calculated with different methods. Results of these comparisons obtained since January 2005 are published on the BIPM ftp site, with monthly updates after the calculation of BIPM circular T.

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.

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.

Toward new procedures in TWSTFT and GNSS delay characterization for UTC time transfer

UTC generation includes the computation of UTC-UTC(k) and its uncertainty estimation. A significant part of the uncertainty of the UTC approximations UTC(k) in national contributing laboratories is based on accurate metrological measurement of time transfer equipment delays (so called “equipment calibration”). Organizing and maintaining the calibration of the time transfer facilities contributing to UTC is among the responsibilities of the BIPM. At present, the time transfer techniques used for UTC generation are based on the two-way satellite time and frequency transfer (TWSTFT or TW) and the global navigation satellite systems (GNSS), i.e. GPS and GLN (Glonass). They are used for calculating the differences [UTC(k) - UTC (1)] between any participating laboratory and that chosen as a pivot (at present the PTB). In the 1980s, GPS C/A technique dominated the UTC time transfer. Since 2000, TW and GPS MC, P3 and PPP techniques as well as GLN have been successively introduced in the UTC generation. In consequence, the calibrations of the different time transfer equipment were introduced and are performed separately. Today, there are four parallel types of independent calibrations based on different strategies that can be defined either as site-based or link-based. The BIPM has assigned values of uB of about 1 ns for the link-based TW calibrations, based on the values reported by those performing the calibration. However, for the sited-based GNSS calibrations, a conventional value of 5 ns has generally been assigned. This choice has been motivated by studies which found that values of [UTC(k) - UTC (1)] bigger the respective uB may exist when the link as calculated by different techniques, and because long-term instability of the standard receivers may cause inconstancy in individual calibrations carried out in different periods. On the other side, due to the development in technology, the statistical uncertainty uA has been reduced by a factor of 10 since a dozen of years. The state of the art of uA is 0.5 ns for TW and 0.3 ns for GPS PPP. The uB calibration uncertainty is dominant in the total uncertainty of [UTC-UTC(k)], and several authors have investigated how to improve the calibration of time transfer equipment to decrease its value. Also the BIPM has undertaken studies for improving the current calibration policy. The goal of this studies are: 1) to reduce the inconsistency between different techniques by making a combined use of the respective calibrations; 2) to reduce influence of the long-term instability of the BIPM standards by a special designed schedule; 3) to obtain more realistic uB values than the conventional 5-ns and, in consequence, to reduce the total uncertainty of UTC-UTC(k); 4) to easy the calibration organization and reduce its cost; 5) to simplify the calibration monitoring and the combination of different time transfer techniques. Uncertainty estimation is an important part of the UTC computation and hence carefully discussed. We present hereafter a study, which in no way means the adoption of a new calibration policy at the BIPM.

Progress in Carrier Phase Time Transfer

The progress of the joint Pilot Project for time transfer, formed by the International GPS Service (IGS) and the Bureal International des Poids et Mesures (BIPM), was recently reviewed. Three notable milestones were set. (1) The IGS will implement, at least in a test mode, an internally realized time scale based on an integration of combined frequency standards within the IGS network. This will eventually become the reference time scale for all IGS clock products (instead of the current GPS broadcast time). (2) A new procedure for combined receiver and satellite clock products will be implemented officially in November 2000. Receiver clocks are an entirely new product of the IGS. (3) The BIPM will coordinate an effort to calibrate all Ashtech Z12-T (and possibly other) receivers suitable for time transfer applications, either differentially or absolutely. Progress reports will be presented publicly in the spring 2001.

Towards accurate optical fibre time transfer in UTC

Accurate time transfers are now fully based on space techniques. The recently developed optical fibre technique is expected to be used in the UTC generation in the future. Some UTC labs operate optical fibre links using different approaches. In particular, the here called Two-Way Optical Fibre Time (and Frequency) Transfer (TWOFT/TWOTFT) is of special interest. The symmetry of the propagation conditions in both directions allows to substantially reducing the instability due to fluctuations of the propagation, and its capability of self-calibration. The first operational TWOTT between the UTC laboratories has been established in Poland between AOS and GUM since January 2012. The result has being submitted to the BIPM since March 2013. The BIPM started the monthly data processing and compares to the GNSS links. The TWOTT can reach an accuracy of hundred ps in a few minutes and therefore provide an effective new tool for the assessment of GNSS and TWSTT CP time link calibration and bring substantial improvement to the [UTC-UTC(k)]. However, time transfer by optical fibres today does not match the structure of the UTC time transfer network with a unique pivot. This requires the new network configuration, algorithm and will provoke fundamental changes in the UTC time link computation.

BIPM calibration scheme for UTC time links — BIPM pilot expermiment to strenghen Asia-Europe very long baselines

The calibration uncertainty (uB) is the dominant part in the total uncertainty budget of the UTC generation [1,3]. There are two calibration strategies: (a) Receiver calibration and (b) Time link calibration. The state-of-the-art [1,4,5] of the uB is of order 5 ns for GNSS time transfer based on the strategy (a) and is 1 ns for TWSTFT based on the strategy (b) in which the absolute offset is canceled. In the BIPM Circular T, the uncertainty in UTC-UTC(k) is obtained from the uncertainties in the links [3]. To reduce the uncertainty in UTC-UTC(k), it is optimal to use the calibration strategy (b). Earlier investigations gave theoretical and practical proofs [6–12]. Based on these studies, we propose the BIPM calibration scheme using a calibration system composed of three GNSS geodesic receivers: one is stationary at BIPM and the other two travel between the laboratories. We describe the set-up of the system, the schedule, the uncertainty uB and aging of the uB. The uB is expected to be better than 2 ns for even very long inter-continental UTC baselines.

Experimental research on BeiDou time transfer using the NIM made GNSS time and frequency receivers at the BIPM in Euro-Asia link

A new time and frequency transfer system compatible with BDS has been developed at NIM, and two of them are in operation at the BIPM since Jan 2017. Experiments with this equipment have been implemented to characterize the time and frequency transfer by BDS. This includes analysis of the satellites signal coverage statistics in the BIPM area, the error correction, the measurement noise of the NIM time and frequency transfer receivers, the short and long-term instabilities in the European and Asian baselines, the time transfer link calibration and the accuracy of the time transfer and and AV and CV modes.

Atomic Time Scales and Their Applications in Astronomy

The unit of time is defined as a multiple of the period of the hyperfine transition of the atom of cesium 133 and realized at the level of a few parts in 1016 by about a dozen cesium fountains maintained in national metrology institutes. International Atomic Time (TAI) takes its accuracy from the primary frequency standards, but preserves its high stability over intervals of 1 month making use of the largest industrial clock ensemble in the world. Seventy-four institutes worldwide spread or disseminate atomic time for different applications; the International Bureau of Weights and Measures (BIPM) collects their data and integrates them in an algorithm which produces TAI.