First evaluation of the Primary Frequency Standard NTSC-CsF2

Abstract Caesium fountain primary frequency standards have been used to realize the definition of the second in the International System of units for more than two decades. This paper reports the structure and uncertainty evaluation of the NTSC-CsF2, developed by National Time Service Center (NTSC), Chinese Academy of Sciences. The short-term stability at high atomic density is 2.1 × 10 −13 at 1 s of averaging time. The fractional combined type B uncertainty is evaluated at 4.3 × 10 −16 . A frequency comparison has been conducted between NTSC-CsF2 and the SI second through a Global Navigation Satellite System link for eight months whose results show a fractional frequency difference of 4.2(5.2) × 10 −16 . With a reduced chi-square of 0.8, NTSC-CsF2 is found to be consistent with that of the other fountains within the uncertainty.