A major role of the national standards labs is to keep the world's clocks running on the proper time. That means making a continual comparison of the primary clocks that are used as the standards. But some of the accuracy achieved in a given lab's measurement of time is lost as that information is communicated to another location on Earth's surface. As good as it is, the Global Positioning System (GPS) system (which at its best enables comparison of clocks with accuracies of ), is becoming a weak link in this relay of information.
Barbara Goss Levi; Space Clock to Fly on the International Space Station. Physics Today 1 February 1998; 51 (2): 22. https://doi.org/10.1063/1.2802917
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