Absolute frequency measurement of the Sr-87 optical lattice clock at NTSC using international atomic time

We report the absolute frequency measurement of the 5s(21)S(0) -> 5s5p(3)P(0) transition in 87Sr optical lattice clock (Sr1) at National Time Service Center. Its systematic frequency shifts are evaluated carefully with a total relative uncertainty of 5.1 x 10(-17). The measured absolute frequency is 429 228 004 229 872.91(18) Hz with a relative uncertainty of 4.1 x 10(-16), with reference to the ensemble of primary and secondary frequency standards published in the Circular T bulletin by BIPM through a global navigation satellite system link.