The clock comparisons that provide the data for the calculation of TAI are carried out using GPS (Global Positioning System) satellites and by TWSTFT (Two-Way Satellite Time and Frequency Transfer).
The application of GPS common-view C/A-code 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 GPS receivers available in time laboratories. All GPS 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 of other methods of time and frequency transfer, such as the use of GPS carrier phase and GLONASS (Global Navigation Satellite System) observations, and the use of GLONASS precise code.
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 conducts calibrations of time transfer equipment located in time laboratories participating in the calculation of International Atomic Time (TAI).
Differential time corrections for GPS time equipment are published in Rapports BIPM; click here for an up-to-date list.
International GPS tracking schedules are issued about every six months, and information on time-link comparisons is given on our ftp server.