Absolute frequency measurement with uncertainty below $1\times 10^{-15}$   using International Atomic Time
  The absolute frequency of the $^{87}{\rm Sr}$ clock transition measured in 2015 was reevaluated using an improved frequency link to the SI second. The scale interval of International Atomic Time (TAI) that we used as the reference was calibrated for an evaluation interval of five days instead of the conventional interval of one month which is regularly employed in Circular T. The calibration on a five-day basis removed the uncertainty in assimilating the TAI scale of the five-day mean to that of the one-month mean. The reevaluation resulted in the total uncertainty of $10^{-16}$ level for the first time without local cesium fountains. Since there are presumably no correlations among systematic shifts of cesium fountains worldwide, the measurement is not limited by the systematic uncertainty of a specific primary frequency standard. 
Hidekazu Hachisu, G\'erard Petit, and Tetsuya Ido
physics.atom-ph physics.optics