THE STANDARDS OF TIME AND FREQUENCY IN THE USA J. A. Barnes Time and Frequency Division National Bureau of Standards Boulder,Colorado 80302 and C.M.R. Winkler Time Service Division U. S. NavalObservatory Washington, D. C. 20390 Summary Terms of Reference The National Bureau of Standards (NBS) and the We call a time scale any system which allows the U. S. Naval Observatory (USNO) are the two organiza- unambiguousordering of events. Calendars are (rather tions chiefly involved in distributing accurate and coarse) time scales. Indeed, the daily movement of the precise time and frequency information within the USA. sun, stars, and the moon provides the Time Scale proto- The NBS is responsible for the "custody, maintenance, type, even though the standard intervals are not uniform. and development of the national standards" of frequency Uniformity is a requirement which is becoming increas- and time (interval) as well as their dissemination to ingly more important for two reasons, one scientific the general public. The mission of the USNO includes and the other operational. the "provision of accurate time" as an integral part of its work concerned with the publication of ephemerides The widescale application of manifestly non- in support of navigation and in the establishment of a uniform time scales is impractical without corrections fundamental reference system in space. which render the scales uniform. Wide applications of time scales require synchronization of clocks. Once Both agencies provide the U.S. contribution to synchronized, such clocks become the vehicle of access the Bureau International de 1'Heure (BIH) [International to all kinds of time scales. Thus, the study of syn- Time Bureau], which has the responsibility of publish- chronization would also be properly said to belong to ing definitive values of Universal Time (UT), Inter- the broader study of time in general. national Atomic Time (LAT), and Coordinated Universal Time (UTC). As generally used, a time scale is a system which allows oneto assign "dates" to events. There are Key wordsAstronomical Time, Atomic Time, astronomical time scales and clock time scales. A Frequency,International Atomic Time, Management, sensible use of the unqualified word ''time" is the use NBS, StandardTime, Time, USNO. which embodies all of these various aspects of time scales, time measurement, and even time interval (or Introduction duration). This is certainly consistent with the diction- ary definition of the word. Thus, it is misleading to say The national responsibilities for the provision of that "time" is determined only by astronomical means. standards of time and frequency (TLF) in the USA rests Indeed, there are many different time scales--astro- with two organizations of widely different background, nomical time, biological time, geological time, atomic professional traditions, and outlook. time,etc. The measurement of T&F permeates all scientific On the other hand the calendar and fraction of a observations. It is fundamental to any system of meas- day is the legal standard to which we ultimately refer urement standards and it is an indispensable element most events for "dating". in fundamental astronomy, geodesy, and navigation. The of anevent on anearth-based time scale It follows from this wide range of applications and is obtained from the number of cycles (and fractions of interfaces with all disciplines of science and technology cycles) of the apparent sun counted from some agreed- that requirements for standards of T&F can only be upon origin. Similarly, atomic time scales are obtained srtisfied in some form of compromise. by counting the cycles of a signal in resonance with certain kinds of atoms. One of the obvious differences A short discussion of principles and terminology, between these two methods is that the cycles of atomic followed by a minimum of historical accounts will clocks are much, much shorter than the daily cycles of enable us to review the present distribution of work the apparent sun. Thus,the atomic clock requires and responsibilities of the two agencies involved, the more sophisticated devices to count cycles than are National Bureau of Standards (NBS) and the United required to count solar days. The importance of this States Naval Observatory (USNO). difference is a matter of technological convenience and is not very profound. 269 It is of technological significance that atomic With the advent of railroads and hence more rapid clocks can be read with much greater ease and with communications, this "crazy-quilt" maze of individual many thousands of times the precision than the earth local times had to come to an end. The railroads are clock. In addition, the reading of an atomic clock can generally credited with unifying the various local times be predicted with about 100, 000 times better accuracy into time zones which have presented a much more than a clock defined by the rotation of the earth. workable national time system (November 1883). In the U. S. literature on navigation, satellite What one sees clearly in this historical aequence tracking, and geodesy, the word "epoch" is sometimes is that, as communications become more rapid and used in a similar manner to the word "date". There is more far-reaching, the greater are the demands for an considerable ambiguity, however, in the word "epoch" all-pervasive and unifying convention of synchronizing and we prefer the use of the word "date", if both day clocks with each other. This convention is a matter of and time of day are given for an event. convenience. There is nothing sacred or absolute about what our clocks read; it's just important that they read Clocks, then, are devices capable of generating the same time (or have a well-defined time difference and counting time interah. In order to do this in a as between time zones). most uniform manner, modern clocks derive their frequency (rate) reference from inner atomic processes, In the days when the railroads were the primary shielded as much as possible from external disturbing means of transportation across the North American influences. Such clocks must also contain counters and continent, an accuracy of a few seconds of time was displays of accumulated time intervals. Since repetitive importantand sufficient. Nowadays, with the existence phenomena are involved here, time, in one way or of sophisticated telecommunications equipment capable another, is always identified with angles whether we of sending and receiving several million alphanumeric deal with the rotating earth or with 1 MHz signals from characters each second, there are real needs for clock afrequency standard. Our conventional hours, minutes, synchronizations at accuracy levelsof a millionth of a and seconds are angular measures ("hour angle") of second and better. astronomy("Universal" Time, UT). Time Scales for Celestial Navigation and Astronomical This identification with the angular orientation of -Uses the earth is why celestial navigators require earth- basedtime signals. Recently, more sophisticated uses Time is essential for celestial navigation. Basi- ofUT have come into being as in geodetic astronomy, cally, the reason is as follows: If one knows what time star and satellite tracking, and very-long-baseline it is (i. e., solar time) at some reference point, say the radio interferometry (VLBI) which require (and can also Meridian of Greenwich, and one also knows his local provide) UT with millisecond accuracy, Since the time, say from a sundial, then one can figure his longi- rotation of the earth is not strictly uniform (variations tude simply by remembering that the earth makes one in the length of the day are of the order of a part in 10') complete revolution on its axis in about 24 hours. a prablem exists in relation to clock time which can be kept stable to a few parts in IO". For example, noon Greenwich Mean Time is 2 a.m. Hawaiian Standard Time, or 10 hours different. Additional difficulties with clock time arise if it Thus, it's easy to calculate that Hawaii is about 10j24 is to be used in the prediction of cosmic phenomena as, of the way around the world from Greenwich, England-- for example, orbital position of celestial bodies or in other words. about 150' west of the Prime Meridian. times of arrival of signals from "pulsars". These latter signals can be resolved today with a precision of If a person were to measure the actual position of better than 5 PS. the sun in the sky using. say, a navigator's sextant, then he could get a rather accurate determination of It is clear that such uses demand a clock time local solar time. The problem, then, would be to know which offers more than just means for synchronization the correct time on the Greenwich Meridian. which would be sufficient for electronic systems applications. Approximately 200 years ago, a man in England named Harrison was awarded X20, 000 for building a Time Scales for Systems Synchronization Uses chronometer which allowed the accurate determination of longitude while at sea. Until radio made its appear- Not so long ago people were content simply to let ance, navigation at sea was very dependent upon good the sun govern their lives. When the sun came up, it clocks. was time to begin work; when the sun set, it was time to stop. Nowadays, there are many standard time broad- cast stations in the world. The best known standard With the growth of commerce and city life and time broadcast stations in North America are operated the advancement of technology, a community could have by the National Bureau of Standards (USA) and the its own clock set to agree roughly with the sun. Thus National Research Council (Canada): WWV is located in developed the idea of local time and each community Fort Collins, Colorado; WWVH is now Located on the couldhave its local time.