  The absolute frequency of the 1S0-3P0 clock transition of 87Sr has been
measured to be 429 228 004 229 873.65 (37) Hz using lattice-confined atoms,
where the fractional uncertainty of 8.6x10-16 represents one of the most
accurate measurements of an atomic transition frequency to date. After a
detailed study of systematic effects, which reduced the total systematic
uncertainty of the Sr lattice clock to 1.5x10-16, the clock frequency is
measured against a hydrogen maser which is simultaneously calibrated to the US
primary frequency standard, the NIST Cs fountain clock, NIST-F1. The comparison
is made possible using a femtosecond laser based optical frequency comb to
phase coherently connect the optical and microwave spectral regions and by a
3.5 km fiber transfer scheme to compare the remotely located clock signals.
