BIPM comparison of time transfer techniques

Clock comparison for International Atomic Time (TAI) is based on common views of satellites of the GPS constellation and on the technique of two-way satellite time and frequency transfer (TWSTFT). All the TWSTFT links are backed up by the GPS ones. Due to the duplicity of techniques in many baselines, the international network of time links is today highly redundant. The improvement of GPS satellite ephemerides and clock values produced by the International GNSS Service (IGS) might lead to the utilisation of the all-in-view (AV) method for the computation of GPS links instead of the common-view (CV) one currently used for TAI. Studies started at the BIPM to validate the GPS AV method for introduction in the calculation of TAI. To assist related studies, we developed a procedure to compare the links measured with different techniques and calculated with different methods. Results of these comparisons obtained since January 2005 are published on the BIPM ftp site, with monthly updates after the calculation of BIPM circular T.