Once it is there, the pips’ journey time through the fibre-optic cable can be worked out, by logging the discrepancy between what the pips say and what the caesium clock says.

Even if the clocks pass this later test, usurping the caesium clock to create a more precise definition of the second will not be easy.

It's because this transition frequency is so much higher than the resonant frequency of quartz that a caesium clock is so much more accurate.

And if the fibre connection bringing the pips should be cut, the caesium clock takes over until a connection is restored.

Without the caesium clock, for example, satellite navigation would be impossible.