from this pdf (published 1999) found on this page of the NIST PML time and frequency division's website. NIST is the "national institute of standards and technology", although they generally deal with more sciency stuff, and their PML is the "physical measurement laboratory".

NIST does a lot of very cool stuff, especially NIST PML. you should check out the site! their writings are surprisingly easy to read and they actually have a sense of whimsy. ofc they are a US government organization and sadly some bits of their site has been subject to the wills of the trumpster, so be aware of that.