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Improvements of the systematic uncertainty, frequency instability, andlong-term reliability of the two caesium fountain primary frequency standardsCSF1 and CSF2 at PTB (Physikalisch-Technische Bundesanstalt) are described. Wehave further investigated many of the systematic effects and made a number ofmodifications of the fountains. With an optically stabilized microwaveoscillator, the quantum projection noise limited frequency instabilities areimproved to $7.2 \times 10^{-14} (\tau/1\,\mathrm{s})^{-1/2}$ for CSF1 and $2.5\times 10^{-14} (\tau/1\,\mathrm{s})^{-1/2}$ for CSF2 at high atom density. Thesystematic uncertainties of CSF1 and CSF2 are reduced to $2.74 \times 10^{-16}$and $1.71 \times 10^{-16}$, respectively. Both fountain clocks regularlycalibrate the scale unit of International Atomic Time (TAI) and the localrealization of Coordinated Universal Time, UTC(PTB), and serve as references tomeasure the frequencies of local and remote optical frequency standards.

Advances in the accuracy, stability, and reliability of the PTB primary fountain clocks