The two-way satellite time and frequency transfer (TWSTFT) is a kind of time transfer and comparison technology of the highest precision at present, and its basic working principal is that the two labs in the comparison send simultaneously time signals respectively of their local clocks to the other, and receive and measure the clock time signals from the other side at the same time; after the data of measurement and comparison are exchanged, the bias between the clocks at the two locations can be calculated. Thank to the absolutely same path of two-way satellite time transfer signals, the affect of changing path delay of time can be eliminated to the largest extent, therefore a higher comparison precision can be achieved. The NTSC has set up TWSTFT links in succession with labs of five countries including the NICT.

The PTB is the host node of TAI system’s global TWSTFT, so only the center compare with the time of PTB and follow the two-way satellite data of the Time Lab of Data Exchange Standard under the BIPM (Bureau International des Poids et Mesures) can it participate officially in the reduction of TAI. In an attempt to realize the TWSTFT link with PTB, the NTSC established experimental links successively with VSL of the Netherlands and OP of France in Europe, both of which achieved success. The center carried out successfully the uplink test required by IS Satellite Company in mid June, 2008, and performed link adjustment with PTB. At present, NTSC-PTB have carried out common TWSTFT observation and data accumulation, and the comparison precision is 0.3ns, equivalent to those of other international two-way links. The official application of NTSC-PTB two-way satellite data will further improve the comparison precision between the standard time UTC (NTSC) of our country and the international standard time. The NTSC has become an important part in the Europe-Asia section of the global time comparison link, for it has simultaneously the two-way satellite comparison links with Japan and Europe at present.