  We have measured the absolute frequency of the optical lattice clock based on
$^{87}$Sr at PTB with an uncertainty of $3.9\times 10^{-16}$ 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 5s$^2$ $^1$S$_0$-5s5p $^3$P$_0$
transition in $^{87}$Sr 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 $3\times 10^{-17}$, opening
new prospects for frequency ratio measurements between optical clocks for
fundamental research, geodesy, or optical clock evaluation.
