Time & Frequency Standards Laboratory
The second, symbol s, is the SI unit of time. It is defined by taking the
fixed numerical value of the caesium frequency ΔνCs, 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 NSAI’s National Metrology Laboratory maintains a Microsemi 5071A-C001 High
Performance Primary Caesium Frequency Standard. Accuracy and stability data is assessed
by the BIPM on an ongoing basis and this clock is part of the global ensemble used by the
BIPM to calculate International Atomic Time (TAI) and Coordinated Universal Time (UTC).
NSAI NML’s atomic clock (UTC (NSAI)) is steered when required so that it is always within
500 ns of UTC. The accuracy of the 5071A-C001 is ±5.0 x 10-13.
Microsemi 5071A Atomic Clock
Mesit GTR51 Receiver
Also maintained by the NSAI NML are a number of Free-Running Rubidium Frequency
Standards and Global Positioning System Disciplined Oscillators (GPSDOs) which facilitate
traceable calibration of frequency measuring and generating equipment.
Each of these frequency standards provide a highly stable time base for the calibration of
instruments such as Process Timers, Universal Counters, Frequency Generators,
Tachometers, Stroboscopes, Stopwatches and similar.