Absolute Frequency Of Sr-87 At 1.8 X 10(-16) Uncertainty By Reference To Remote Primary Frequency Standards

The optical lattice clock NICT-Sr1 regularly reports calibration measurements of the international timescale TAI. By comparing measurement results to the reports of eight primary frequency standards, we find the absolute frequency of the Sr-87 clock transition to be f(Sr) = 429 228 004 229 873.082(76) Hz, with a fractional uncertainty of less than 1.8 x 10(-16) approaching the systematic limits of the best realization of the SI second. Our result is consistent with other recent measurements and further supported by the loop closure over the absolute frequencies of Sr-87, Yb-171 and direct optical measurements of their ratio.