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REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 70, NUMBER 6 JUNE 1999 REVIEW ARTICLE Introduction to time and frequency metrology Judah Levinea) JILA and Time and Frequency Division, NIST and the University of Colorado, Boulder, Colorado 80309 Received 24 July 1998; accepted for publication 22 February 1999 In this article, I will review the definition of time and time interval, and I will describe some of the devices that are used to realize these definitions. I will then introduce the principles of time and frequency metrology, including a discussion of some of the types of measurement hardware in common use and the statistical machinery that is used to analyze these data. I will also introduce various techniques of distributing time and frequency information, with special emphasis on the global positioning system satellites. I will then discuss the advantages of clock ensembles and a prototype time-scale algorithm. I will conclude with a discussion of how clocks are synchronized to remote servers using noisy and poorly characterized transmission channels. S0034-67489900106-9 I. INTRODUCTION different applications, we will see that all of them can be understood in terms of a few basic principles. I will discuss Clocks and oscillators play important roles in many ar- the details of specific applications only insofar as is needed eas of science and technology, ranging from tests of general to illustrate these basic ideas. relativity to the synchronization of communication systems I will not discuss the very large number of optical