A strontium lattice clock with 3 x 10-17 inaccuracy and its frequency

We have measured the absolute frequency of the optical lattice clock based on Sr-87 at PTB with an uncertainty of 3.9 x 10(-16) using two caesium fountain clocks. This is close to the accuracy of today's best realizations of the SI second. The absolute frequency of the 5 s(2) S-1(0) - 5s5p P-3(0) transition in Sr-87 is 429 228 004 229 873.13(17) Hz. Our result is in excellent agreement with recent measurements performed in different laboratories worldwide. We improved the total systematic uncertainty of our Sr frequency standard by a factor of five and reach 3 x 10(-17), opening new prospects for frequency ratio measurements between optical clocks for fundamental research, geodesy or optical clock evaluation.