The second is realised with time and frequency standards („Atomic Clocks“) at the BEV.
By way of international time comparisons via GPS-Satellites the BEV contributes to the International Atomic Time TAI and thus to the Coordinated Universal Time UTC. The TAI arises from the definition of the second while the UTC is derived from the TAI allowing for the variation in earth rotation by appoximation.
The SI unit second can be realised with the lowest uncertainty of measurement. Therefore, other base units like the unit of length and various electrical units can easily be derived from this accurate time source.