The BIPM (Bureau International des Poids et Mesures) officially published the system results of two-way satellite time and frequency transfer (TWSTFT) of the NTSC in the latest Circular T 257; thus, the TWSTFT results of NTSC are officially applied to the reduction of international atomic time (TAI: Temps Atomique International), and the class-A uncertainty of comparison between UTC (NTSC) and UTC of the international standard time is reduced to 0.5ns from 1.5ns.
TWSTFT is a technology using the communication satellite to achieve the two-way transfer of time and frequency signals, and is most precise method in the world currently. Its precision can be 0.3--0.5ns with the accuracy being 1ns. In order to the global time synchronization, the BIPM recommended and organized the implementation of “Global Two-way Satellite Time and Frequency Transfer” Plan in the late 1990s, and since Apr., 1999 it has been publishing regularly the Monthly Bulletin of TWSTFT. Since July, 1999, the TWSTFT data of the German Federal Institute for Physical Technology (PTB) and TUG have been officially applied to the calculation of TAI.
The NTSC started the TWSTFT between the NICT in Oct., 1998. For the purpose of ensuring our country’s keeping in line with the international advanced level in the field of time and frequency transfer and exerting NTSC’s effect in the fields of TAI and global TWSTFT, the NTSC is actively developing the link with European TWSTFT while joining in the comparison chain of TWSTFT in the Asian-Pacific region. After the TWSTFT link experiments with the Paris Observatory (OP: Observatoire de Paris) of France and VSL of the Netherlands, the NTSC established officially a common TWSTFT link with the PTB -- the central station of international TWSTFT -- in 2009. At present, the NTSC-PTB link has been officially used for the TAI reduction after the stable operation for several months. The official application of the NTSC’s TWSTFT data to the TAI reduction is a great thing in the time-keeping of our country which improves significantly the comparison precision between the standard time of our country and that of the world.