International Association of Geodesy

Report of the BIPM Time Section to the IAG Section V on the activities during the period 1999-2000 
Elisa Felicitas Arias
Head of the BIPM Time Section 

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 Circular T. Definitive results for 1999 and 2000 have been available, in the form of computer-readable files in the BIPM home-page and on printed volumes of the respective Annual Reports of the BIPM Time Section. 
Work is done to automate the calculation of TAI and UTC, this allowing a shorter delay in the publication of Circular T.


Algorithms for time scale
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. Studies are undertaken to evaluate the feasibility of providing a prediction of UTC in quasi-real time.
Some 80 % of the clocks are now either commercial caesium clocks of the type HP5071A or active, auto-tuned active hydrogen masers, and together they contribute 86 % of the total weight with consequent improvement in the stability of EAL. Since most HP5071A clocks have at present the maximum relative weight, the weighting procedure of clocks in TAI is under revision.
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. 
Nine primary frequency standards 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 the last year, from +2 ´ 1015 to +6 ´ 1015, with an uncertainty of 4 ´ 1015. 
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 were 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
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 one-channel receivers. The commercial availability of newly developed receivers has stimulated interest in extending the classical common-view technique for use of multichannel dual-code dual-system (GPS and GLONASS) observations, with the aim of improving the accuracy of time transfer. The two-way time and frequency transfer via geostationary satellites (TWSTFT) has a performance comparable to that of GPS. Since July 1999 GPS multichannel links and TWSFTF links are being progressively introduced in TAI. Even if the calculation of TAI relies mostly on single channel GPS links , GPS multichannel and TWSTFT links are also included. 
Ionospheric parameters and precise ephemerides provided by the IGS (International GPS Service) 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 are developed in the frame of the IGS/BIPM pilot project to study accurate time and frequency comparison using GPS phase and code measurements.

Space-time references
The BIPM/IAU Joint Committee on general relativity for space-time reference systems and metrology (JCR), created in 1997, continued its work. Two studies have been conducted at the BIPM in collaboration with other members of the JCR. One concerns 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. The second study concerns the realisation of geocentric coordinate times.
Following a Call for Participation of the IERS, the BIPM, jointly with the USNO, will provide its Conventions Product Centre since January 2001.