The 50th Anniversary of the Atomic Second.

This paper gives a brief account of the history of time standards and timekeeping beginning with John Harrison's seagoing clocks for navigation up to today's optical frequency standards and prospects for the future definition of the second.

Analysis of correlations, and link and equipment noise in the uncertainties of [UTC - UTC(k)].

We refine our estimate of the uncertainty in [UTC - UTC(k)] by taking into account the contribution of correlations between the links. Using a matrix formulation to facilitate computation, we re-analyze the link-based uncertainties and obtain the same solution as found previously. We then evaluate the site-based uncertainties and compare the results with the link-based uncertainties.

Estimating the stability of N clocks with correlations.

Estimation of an atomic clock's frequency stability, separate from its reference, is often done using a three-cornered hat procedure. A major requirement for the success of this method is that clocks be uncorrelated. If this requirement is not satisfied, the three-cornered hat procedure can lead to misleading or even negative variance estimates.