International Association of Geodesy

International Association of Geodesy

9. BIPM Time Section
Felicitas Arias, Head

International time scales
Reference time scales International Atomic Time (TAI) and Universal Coordinated Time (UTC) have been computed regularly and have been published in the monthly BIPM Circular T. Definitive results for 1999, 2000, 2001 and 2002 have been available, in the form of computer-readable files from the BIPM website (http://www.bipm.org), and on printed volumes of the respective Annual Reports of the BIPM Time Section. Work has been done to automate the calculation of the time scales TAI and UTC published monthly in the BIPM Circular T. Several modifications have been introduced in Circular T, starting in May 2003: results are given to 0.1 ns; the list of time links used in the calculation of the current circular is provided; and a new format has been adopted for the layout of the circular.

Algorithms for time scales
Research concerning time scale algorithms includes studies to improve the long-term stability of the free atomic time scale EAL and the accuracy of TAI. The weighting procedure of clocks participating in TAI has been revised and modified. Until 31 December 2000, the maximum relative weight of clocks participating into TAI was fixed to 0.700%. With the improvement of the commercial atomic clocks, this method became inefficient to discriminate between the best clocks. Since January 2001 the maximum relative clock weight has been established each month, depending on the number of participating clocks. This modification improves the stability of the international time scales.The medium-term stability of EAL, expressed in terms of the Allan deviation, is estimated to be 0.6 ´ 10-15 for averaging times of 20 to 40 days over the period.
Primary frequency standards developed and operated by the National Institute of Standards and Technology (NIST, USA), the Communications Research Laboratory (CRL, Japan), the Paris Observatory (OP, France) and the Physikalish-Technische Bundesanstalt (PTB, Germany) reported their measures to the BIPM. The global treatment of these individual measurements led to a relative departure of the duration of the TAI scale unit from the SI second on the geoid ranging, in 2002, from +6 ´ 10-15 to +11´ 10-15, with an uncertainty smaller than 3 ´ 10-15. The frequency offset between TAI and EAL is changed when necessary; this operation is referred to as the “steering of TAI”. Following the recommendations of the Consultative Committee on Time and Frequency, changes were implemented to render the data used in TAI, as well as the results, more accessible to the users and to make the procedures of calculation even more transparent and traceable. Since April 2000 two modifications had been implemented: a new model to characterise the instability of the free atomic scale EAL, and a more complete representation of the uncertainty of the deviation of the TAI scale interval relative to that of the Terrestrial Time TT.

Time links
Two techniques of time transfer are used at present to compare clocks in TAI: GPS common-views based on C/A measurements and two-way satellite time and frequency transfer (TWSTFT). In the last decade the time links computed at the BIPM used the classical GPS common-view technique based on C/A-code measurements obtained from single-channel receivers. The commercial availability of newly developed receivers has stimulated interest in extending the classical common-view technique for use of multi-channel dual-code dual-system (GPS and GLONASS) observations, with the aim of improving the accuracy of time transfer. Since July 1999 GPS multi-channel links and TWSFTF links have been progressively introduced in TAI. Ionospheric maps and precise operational satellite ephemerides provided by the International GPS Service (IGS) are routinely used to correct all links in regular TAI calculations since May 2000. In addition, the BIPM Time section carries on research on new techniques of time transfer, such as the utilisation of geodetic type receivers. These activities had been developed in the period 1999-2002 in the frame of the IGS/BIPM pilot project to study accurate time and frequency comparison using GPS phase and code measurements, and have been incorporated to the IGS as a current activity since 2003. A pilot experiment (TAIP3) was proposed in April 2002 to laboratories participating to TAI. The goal was to study time links computed with GPS P3 data obtained from geodetic-type dual-frequency receivers. Comparisons of such P3 time links with other techniques presently used for TAI have been conducted in the aim of evaluating the long term stability of P3 time links and, as a matter of fact, to assess the long-term stability of the other techniques. It has been concluded that the reliability and long-term stability of P3 links are adequate for their use in TAI computation.

Space-time references
The BIPM/IAU Joint Committee on general relativity for space-time reference systems and metrology (JCR), concluded his work in January 2001. Its activities have been undertaken by the IAU working group on Relativity, Celestial Mechanics, Astrometry and Metrology (RCMAM). Studies have been conducted at the BIPM in collaboration with other members of the JCR/RCMAM, they concern the extension of the relativistic framework to allow a correct treatment for time transformations and the realisation of barycentric coordinate time at the full post Newtonian level and the realisation of geocentric coordinate times. Since January 2001 the BIPM, jointly with the USNO, is the Conventions Product Centre of the International Earth Rotation Service (IERS).