The clock comparisons that provide the data for the calculation of TAI are carried out using GNSS (GPS: Global Positioning System; and GLONASS) and by TWSTFT (Two-Way Satellite Time and Frequency Transfer).
The application of GPS all-in-view and TWSTFT to clock comparisons, as well as the development of more stable commercial caesium standards, has improved the accuracy of timing data by nearly two orders of magnitude over the last two decades. Another improvement is the increasing number of multichannel dual-frequency GPS receivers available in time laboratories. Some GNSS links in TAI are corrected using ionospheric maps and precise operational satellite ephemerides produced by the International GNSS Service (IGS).
The BIPM is involved in the development and application of other methods of time and frequency transfer, such as the use of optical fibres.
The BIPM also carries out studies of many subtle effects that have become important as a result of the improved data. Among these are the effects of changes in ionospheric parameters, the calibration of GPS, GLONASS and TWSTFT equipment, use of post-processed GPS and GLONASS precise ephemerides, improved knowledge of the coordinates of national laboratories, and correlations between clocks.
The BIPM Time Department, in agreement with the Regional Metrology Organizations (RMOs) and the national institutes, has established a procedure for the calibration of time transfer equipment located in time laboratories participating in the calculation of International Atomic Time (TAI). These procedures are available in the BIPM Guidelines for GPS equipment calibration.
Differential time corrections for GPS time equipment are published in Rapports BIPM; click here for an up-to-date list.
The results of comparisons of time links computed from data submitted by TAI contributing laboratories are given on our FTP server.