SI unit: Time and frequency

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This article is part of a series detailing the SI units and our calibration services. This article centers around time and frequency.

Unit of time: second (s)

​The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ∆νCs, 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 SI unit of frequency is given as the hertz (Hz), implying the unit cycles per second. Hertz is a so-called coherent derived unit in the SI with a special name and symbol.

MIKES is responsible for the traceability of time, time interval, and frequency in Finland​

Coordinated Universal Time (UTC) synchronises the world and provides the unit of time, the SI-second. This global atomic time scale is calculated by the International Bureau of Weights and Measures (BIPM) using data from national time laboratories. MIKES is one of the seventy four national laboratories contributing to the world-wide timekeeping and provides the Finnish realisation of the Coordinated Universal Time, the UTC(MIKE).

Keeping of the national timescale UTC(MIKE) is based on a clock ensemble composed of two complementary types of commercial atomic clocks, caesium beam standards and hydrogen masers, the latter being the primary source of
UTC(MIKE). An international network of time links that continuously compares the differences between national time scales, is organised by the BIPM. MIKES participates through navigation satellite observations, continuously sent to BIPM. The results of this key comparison are published monthly through BIPM Circular T  that is also used to disseminate the UTC for synchronisation of the national time laboratories.

Over the years MIKES researchers have accumulated considerable expertise in clock prediction, yielding a hydrogen maser steering algorithm that makes UTC(MIKE) an optimal estimate of UTC. Although our only link to primary frequency standards is through Circular T, accurate maser steering combined with the exceptionally well-controlled MIKES building environment for the clocks has ​allowed us to keep UTC(MIKE) within 10 ns of UTC for the past few years.

Calibration of time, time interval, and frequency

We calibrate e.g. GPS receivers (frequency), oscillators, time interval counters, stopwatches, stroboscopes, and optical tachometers. The frequency range is 1 mHz to 5 GHz. We make time interval measurements according to customer’s need, with a lower limit of approximately one nanosecond. Furthermore, MIKES has a transmitter for time code and precise 25 MHz frequency for those near the Helsinki metropolitan area who need precise time and frequency.

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