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Four primary PTB clocks contribute to UTC

The traditionally great contribution of PTB clocks to Coordinated Universal Time (UTC) has again been clearly increased: Since December 2009, the data of the caesium fountain clock CSF2 of PTB have been included in the determination of UTC. PTB and the French time-keeping institute are thus the only institutes worldwide which operate four primary atomic clocks.

Coordinated Universal Time UTC is the basis for the time zones used in daily life. But also in navigation (GPS), astronomy, telecommunications, geodesy and physical fundamental research, accurate timing is of essential importance. For the calculation of UTC, the International Bureau of Weights and Measures (BIPM) in Sèvres near Paris draws upon the weighted mean of approx. 300 atomic clocks worldwide in more than 60 time institutes. Here, particularly accurate, so-called primary caesium atomic clocks have a special control function. Now, four PTB clocks are among the comparably small number of currently 13 primary caesium atomic clocks in seven time institutes: the two caesium beam clocks CS1 and CS2 as well as the caesium fountain clocks CSF1 and, recently, CSF2.

The caesium fountain clocks are clearly more precise than the caesium beam clocks, because considerably slower, laser-cooled atoms are used in them. Thus, more time is available to measure the resonance frequency of the caesium atoms with the aid of microwave radiation. If the frequency of the microwaves agrees with the resonance frequency, exactly one SI second has past after 9 192 631 770 microwave oscillations.

The second fountain clock, CSF2, had been developed to reduce the uncertainties even more by the performance of comparison measurements with the first, already very accurate, fountain clock CSF1. Already in the first evaluation, the new clock succeeded in advancing into the leading group of primary caesium atomic clocks with a relative systematic uncertainty of 0.8 · 10–15. For the first CSF2 measurements, which the BIPM took into account for the calculation of UTC, the clock could be operated almost continuously over many days. CSF2 thus already belongs to the most reliable primary caesium fountain clocks worldwide.


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