**Name of Standard:** National Time and Frequency Standard

**Code designation:** ECM 440-1/97-002

**Year of publication:** 1997

**Departmnet:** Institute of Photonics and Electronics AS CR, v. v. i.

**Guarantor: **Ing. Alexander Kuna Ph.D.

**Number of CMS lines provided:** 17

The definition of the second SI:

The second, symbol *s*, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, ∆*ν*_{Cs}, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to *s*^{−1}.

The National Time and Frequency Standard (NTFS) in the Czech Republic is maintained by the Laboratory of the National Time and Frequency Standard (NTFS) within the Institute of Photonics and Electronics AS CR.

LNTFS generates the time scale denoted as UTC(TP) as the national approximation (prediction) of the universal coordinated time UTC. The UTC(TP) second and its coherent frequencies are realized in the industrial high-performance cesium-beam atomic clocks 5071A/001.

The 5071A/001 clock is currently the most accurate industrial atomic clock with the best long-term frequency stability. Its natural (not steered) frequency is guaranteed by the manufacturer to be offset by less than 5·10^{−}^{13} with respect to the ideal realization according to the definition of the SI second.

The uncertainty of the national approximation of the SI second is given namely by the frequency stability of the cesium clock, i.e. 6∙10^{−14} for the averaging interval of 1 day.

LNTFS links several timescales generated from the atomic clocks operated in the partner laboratories in the Czech Republic. Such timescales together with the UTC(TP) timescale contribute to the calculation of the universal coordinated time UTC in collaboration with the International Bureau of Weights and Measures (BIPM).

LNTFS provides calibration of the primary and/or secondary frequency standards such as atomic clocks, ultra-precise quartz crystal oscillators with their frequency free running or disciplined to GNSS. Time transfer receivers using signals from the global satellite navigation or augmentation systems (GPS, GLONASS, Galileo, SBAS, BeiDou) are also calibrated in the LNTFS.