First comparison of primary frequency standards between Europe and Asia

In December 2006, PTB and NICT simultaneously operated their caesium atomic fountain primary frequency standards, PTB-CSF1 and NICT-CsF1 for 15 days starting from MJD 54079. This was the first direct comparison of primary frequency standards between Europe and Asia. The fountains were compared to local H-masers and the frequency difference between the H-masers was determined directly by TWSTFT with frequency transfer uncertainty below 10
^-15
. PTB-CSF1 and NICT-CsF1 showed good agreement within the stated uncertainties.