The international time scales TAI and UTC are computed regularly and the results published monthly in Circular T, which serves as a monthly update of key comparison CCTF-K001.UTC (the re-named CCTF-K2001UTC). The stability of TAI, expressed in terms of an Allan deviation, is estimated to be at or below 0.4 x 10–15 for averaging times of one month. During the period of this report, eleven primary frequency standards contributed to the improvement of the accuracy of TAI; this included seven caesium fountains (IT CSF1, LNE-SYRTE FO1, LNE-SYRTE FO2, LNE-SYRTE FOM, NIST-F1, NMIJ F1 and PTB CSF1). A total correction of –2.4 x 10–15 has been applied throughout the year to [f(EAL) – f(TAI)]. Since July 2005, the scale unit of TAI has been estimated to match the SI second to about 1 x 10–15. The Section has worked closely with the CCTF Working Group on Primary Frequency Standards on improving the accuracy of TAI. Within the activities of this working group, a workshop was held in June with the participation of laboratories involved in the development and operation of primary frequency standards and BIPM staff.
The computation of time links using common-views of GPS satellites has been replaced by the GPS all-in-view method in the regular calculation of the time scales. Clock comparisons based on GPS phase and code observations have been studied for future application in the calculation of TAI. Extensive comparisons of the different techniques and methods for clock comparisons are computed regularly and published on the internet. Calibration programmes of GPS receivers have been organized and run by the Section.
The Section has provided support to the Joint CCL/CCTF Working Group on Frequency Standards (former Joint CCL/CCTF Working Group on Secondary Representations of the Second), in which some members of the staff have responsibilities.
Research work is also dedicated to space-time reference systems. The cooperation with the USNO (United States) for the provision of the Conventions Product Centre of the International Earth Rotation and Reference Systems Service (IERS) continues, and a workshop on the IERS Conventions is being organized for September 2007 at the BIPM.
At the end of 2006, the BIPM piloted for the last time key comparison BIPM.L-K11. Staff of the Section have contributed to the transfer of this key comparison to NMIs. A new protocol has been elaborated, and the key comparison, under the name CCL-K11, will in the future be under the responsibility of BEV (Austria) as pilot laboratory. The Section has also provided calibration and measurement service of lasers for both internal and external users.
An important number of requests for iodine cells have been satisfied in the year covered by this report. Many of the demands concern specially designed cells. Studies on the realisation of fibre-cells have started.
Improvements of the gravimeter FG5-108 have progressed in cooperation with the VNIIM (Russian Federation). Some theoretical investigations have been conducted to improve corrections of the position of the free-falling mass in the gravimeter.
Cooperation between the Section and the special projects at the BIPM continues. Preliminary studies have been made for accurate gravity measurements for the BIPM watt balance. A member of the Section assists in the winding of the coils for the watt balance. In addition, the Section is involved in the construction of the interferometer for length measurements in the calculable capacitor.
Summary
