Generation of UTC(PTB) as a fountain-clock based time scale

The Physikalisch-Technische Bundesanstalt (PTB) has substantially improved the quality of its local time scale UTC(PTB), which is the national realization of the international time reference Coordinated Universal Time (UTC). It serves as the basis for PTB's time services, for local clock comparisons and for international time comparisons. Since February 2010 UTC(PTB) has been realized using an active hydrogen maser (AHM) steered in frequency via a phase micro-stepper according to an algorithm which combines the frequency comparison data between the AHM and primary and commercial caesium clocks of PTB. Thereby the long-term stability and accuracy of PTB's primary clocks, in particular its fountain clock CSF1, were combined with the short-term frequency stability of the AHM. CSF1 data were used to calculate the steering on all but six days during 15 months. During the time between July 2010 and July 2011, the time difference between UTC(PTB) and UTC was always less than 6 ns and the monthly mean rate differences never exceeded 0.16 ns per day.