The PTB primary clocks CS1 and CS2

The primary clocks CS1 and CS2 have been developed and operated by the Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. By their contributions to the definition of the scale unit of International Atomic Time they have provided access to the realization of the SI second over decades with exceptional accuracy. They have stood out against other primary clocks by the novelty of their design concept, their robustness of construction and last but not the least by their almost continuous operation for many years. Their properties have changed with time, but during the last 7 years their uncertainty, uB, has been estimated as 8 × 10−15 (CS1) and 12 × 10−15 (CS2). Comparisons with PTB’s cold-atom frequency standard CSF1 (uB = 1 × 10−15) over 3.5 years revealed that CS2 and CSF1 agreed well within the uncertainty uB(CS2), whereas CS1 frequency deviates slightly more from CSF1 than uB(CS1).