The drift of Le Grand K relative to the others could be explained by the fact that it is taken out of its vault and handled less often than the other objects.

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For GPS, this is an enormous issue because it turns out that the clocks on the satellites drift by almost 40, 000 nanoseconds per day relative to the clocks on the ground because they are high above the earth's surface (and therefore in a weaker gravitational field) and are moving fast relative to the ground.

BBC: Meet the world's director of time