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2 Basics of and Frequency Standard 2-1 Definitions of Time and Frequency Stan- dard

MORIKAWA Takao

The definition of most basic concepts in the time and frequency standards field, time and timescale, depends on the and technology level of the and is never immutable. In this paper, the historical change in the definitions of time and timescales such as International Atomic Time and Coordinated are reviewed. Keywords Time and frequency standard, Standard time, International atomic Time, Coordinated universal time, International system of units

1 of time and frequency role in many high-technologies, such as satel- standards lite positioning technology (exemplified by the GPS), in high-data-rate Time―together with its inverse quantity, digital communication networks, and preci- frequency―stands as one of the most basic sion measurement applications. physical quantities of daily life, included among quantities such as length and mass. 2 Establishment of standard time Since ancient times, humans have attempted to systems measure time more and more precisely, rely- ing on a variety of physical phenomena Humans have developed precise time- observed in nature. It is well known that the measurement technologies by applying the was developed in ancient Egypt best scientific knowledge available in each through celestial observations, a result of the . Meanwhile, the definition of time itself need for precise time in agriculture, the basic has grown in importance since the 19th centu- industry of the time. As overseas trade devel- ry, with improvements in measurement accu- oped in Europe in the 17th and 18th , racy and the evolution of economic activities. the establishment of a technology to determine In Europe in the 18th , particularly in precisely, needed for safe and effi- Britain, mechanical became popular cient , became one of the greatest among the middle class. At that time, there challenges for -fairing nations. Major was no such thing as standard time; as a refer- European countries set up astronomical obser- ence, was determined as the the vatories, including the Observatory crosses the meridian in a given local area. and the Paris Observatory, and encouraged the This method posed no real problems, as development of accurate clocks, in part by human activities were generally limited to offering generous rewards for success. The individual local areas. However, in the 19th was one of the products century, as railways developed and people and of such policies［1］. In today's advanced scien- goods moved more quickly over long dis- tific world, the atomic plays a crucial tances, time differences among local areas

MORIKAWA Takao 3 became a problem. In response, the the Convention of the (Convention du and Northwestern Railway determined a stan- Métre) of 1875. The Comité International des dard time in 1848, based on the time in Green- Poids et Mesures (International Committee of wich, England. As this standard railway time Weights and Measures, or CIPM) was formed gained acceptance, Greenwich Time became below the governing organization Conférence adopted by law in 1880 as British standard Générale des Poids et Mesures (the General time［2］. At the same time, discussions were Conference on Weights and Measures, or taking place regarding an international stan- CGPM, which met every four ). The dard time, and in October 1884, the Interna- Bureau International des Poids et Mesures tional Meridian Conference was held in Wash- (International Bureau of Weights and Mea- ington D.C. in the to discuss sures, or BIPM) was then established to con- world standard time. The following points duct various metrological activities under the were agreed upon［3］［4］. control of the CIPM. After 1875, a number of ① A single for all nations metrological units, including time, were would be adopted in place of multiplicity of defined internationally within the framework initial meridians. of the Convention of the Metre. In Resolution ② The adoption of the meridian passing 12 of the 11th CGPM, 1960, the International through the center of the transit instrument at System of Units (SI unit) was established, and the Observatory of Greenwich would be pro- remains in use to this ［5］. SI units can be posed to member countries as the initial classified into basic units and derivative units, meridian for longitude. combinations of basic units. This method of ③ From this meridian longitude shall be count- dividing units into two classes is not uniquely ed in two directions up to 180 degrees, east determined by physics but is in fact somewhat longitude being plus and west longitude minus. subjective. Nevertheless, from viewpoints of ④ A universal day would be adopted. This international exchange, education, and would not interfere with the use of local times. research, there were many advantages to the ⑤ The universal day would be a mean solar establishment of a single, practical, interna- day, begin for all the world at the moment of tional unit system. Considering these advan- mean midnight of the initial meridian, and be tages, the CGPM has adopted seven clearly counted from zero up to twenty-four . defined units (the meter, the kilogram, the sec- ⑥ The astronomical and nautical days should ond, the ampere, the Kelvin degree, the can- start at midnight. dela, and the mol, the last of which was added ⑦ Angular and time should be expressed to the original basic units in 1971) and 27 by the decimal system. derivative units (including frequency). How- Based on these resolutions, Japan issued ever, although the official definitions of SI Imperial Edict No.51, entitled "Initial Meridi- units are authorized by the CGPM, these defi- an Longitude Calculation Method and Stan- nitions are not immutable. They reflect the dard Time" on July 13, 1886［4］, the first estab- progress of science and technology in each era lishment of a standard time in Japan. At this and are subject to corresponding revision. In time, the was defined as 1/86,400 of a this context, in 1927 the CIPM established a mean solar day. committee to discuss a number of issues, including revisions to the definition of the var- 3 The Convention of the Metre ious units. In particular, the group thus and definition of the second formed―the Consultative Committee for Time and Frequency (CCTF)―convened to discuss In the late 19th century, an important the definition of the second. As mentioned took place with respect to the measurement of earlier, the second was initially defined as length, a physical quantity as basic as time: 1/86,400 of a mean solar day. However, astro-

4 Journal of the National Institute of Information and Communications Technology Vol.50 Nos.1/2 2003 nomical measurements had indicated that the "International atomic time (TAI) is the irregularity of the Earth's rotation affected the time reference coordinate established by the accuracy of time measured in this manner. Bureau International de l'Heure on the basis of Thus the 11th CGPM, 1960 adopted the follow- the readings of atomic clocks operating in var- ing definition of the second［5］, based on the ious establishments in accordance with the solar as defined by the International definition of the second, the time Astronomical Union. of the International System of Units." "The second is the fraction 1/31,556,925. The definition of TAI was revised as fol- 9747 of the for 1900 January 0 at lows in 1980 based on the 12 hours time." ［5］. This definition, however, posed a problem "TAI is a scale defined in in that long- astronomical observations a geocentric reference frame with the SI sec- would be required to provide accurate meas- ond as realized on the rotating geoid as the urements of the second. Meanwhile, the Cs scale unit." atomic frequency standard, developed in the We cannot ignore the effects of relativity rapidly evolving field of microwave spec- when considering precise time in the context troscopy, was found to be able to determine of modern science and technology. The the length of a second with a much higher effects of relativity on space-time have been degree of accuracy. From 1955 to 1958, the discussed in detail elsewhere［7］. Note that British NPL and the American USNO jointly TAI is designated as starting at 0 sec 0 min 0 measured the frequency of a Cs frequency hr on January 1, 1958 of UT1. Until 1969, standard based on ［6］. Using TAI was provided by averaging the times of the results of this measurement, the definition the atomic clocks in what was then the Bureau of the second was revised as follows by the International de l'Heure. Since 1969, TAI has th 13 CGPM in 1967-1968［5］. been provided by averaging the atomic clocks "The second is the of 9,192,631,770 of research institutes in many countries, in periods of the radiation corresponding to the order to improve the reliability and short-term transition between the two hyperfine levels of stability of TAI［8］. Most of the clocks used to the ground state of the cesium 133 atom" determine TAI are commercial Cs atomic The above definition assumes that all the clocks with accuracies of approximately 10-12. Cs atoms are in the ideal state, free of any Thus to increase the accuracy of TAI, frequen- physical disturbance. This definition was cy calibration using a primary frequency stan- -making in the sense that for the first dard becomes very important. As a case in time in the history humans adopted the atomic point, in 1976, all the primary Cs frequency time in stead of astronomical time, the latter standards of the U.S. NBS (now NIST), the having served as the reference for time since Canadian NRC, and the German PTB indicat- pre-Egyptian eras. In practice, however, many ed that the determined TAI frequency was national time and frequency standard authori- higher than the defined value by a margin of ties had adopted atomic time scale prior to the approximately 10-12. A frequency correction of 13th CGPM. To bestow official authorization －1×10-12 was therefore made to TAI on Janu- on the use of atomic time, it was requested in ary 1, 1977. Resolution 1 adopted at the 14th CGPM 1971 Currently time is no longer based on astro- that the CIPM define International Atomic nomical time but rather is described in terms Time (TAI) and that it take the necessary steps of atomic time. However, since we continue to effect the worldwide adoption of TAI, in to live on Earth, universal time based on the cooperation with associated international Earth's rotation is still widely used. This uni- organizations. Based on this request, the versal time, however, is irregular and tends to CIPM defined TAI as follows. result in delay in the long term, as it is based

MORIKAWA Takao 5 on the Earth's rotation. As a result, UT will determined as one ahead of UTC, and gradually fall behind TAI if left uncorrected. has appointed the PTB as the organization in Coordinated Universal Time (UTC) is a time charge of realization of this time scale. Refer- scale generated by applying TAI the so called ence［9］describes the details of the ways in " adjustment," which adds or which UTC and TAI are internationally deter- removes one second to TAI as necessary in mined and the history of leap . order to reduce the discrepancy between TAI In addition to TAI and UTC, the GPS time and UT within 0.9 sec. Individual national has emerged as an important element in the time and frequency standard authorities pro- time and frequency standard field. GPS time vide UTC or the standard time offset from commenced at a UTC time of 0 hr, January 5, UTC by an integral number of hours through 1980 (USNO). This time is regulated to syn- standard frequency and time signals emis- chronize UTC (USNO) within a margin of sions. UTC is therefore defined as follows, error of 1μsec, although the leap seconds used not by the CGPM but rather in accordance in UTC are not employed［10］. with Recommendation ITU-R TF460 of the International Telecommunication Union (ITU) 4 New trends in unit definitions in charge of general telecommunications, including standard radio. The definition of a physical quantity gains "UTC is the time scale maintained by the the practical meaning for the first time when it BIPM, with assistance from the IERS, which is physically realized. Such definitions thus forms the basis of a coordinated dissemination depend significantly on the level of available of standard frequencies and time-signals. It science and technology, and must be revised corresponds exactly in rate with TAI but dif- as necessary. As is well known, the definition fers from it by an integral number of seconds. of length, previously based on the wavelength The UTC scale is adjusted by the insertion or of radiant light from Kr atoms, was modified deletion of seconds (positive or negative leap in the 17th CGPM, 1983, at which point a seconds) to ensure approximate agreement meter came to be defined as "the length of the with UT1." path traveled by light in during a time As UTC prevailed in terms of international interval of 1/299,792,458 of a second." This use, the 15th CGPM, 1975, Resolution 5 rec- modification was a result of improved accura- ommended the use of UTC as follows［5］. cy in the measurement of standard time; the "The 15th Conférence Générale des Poids new definition thus offered greater accuracy et Mesures, considering that the system called than available under the Kr standard. "Coordinated Universal Time" (UTC) is wide- Currently, the accuracy attained by the Cs ly used, that it is broadcast in most radio trans- is 10-14 to 10-15, the highest level missions of time signals, that this wide diffu- of accuracy in the measurement of any physi- sion makes available to the users not only fre- cal quantity. However, the current definition quency standards but also International Atom- of time is already about 40 years old, and with ic Time and an approximation to Universal the remarkable recent progress in laser tech- Time (or, if one prefers, mean ), note nology, it appears possible that the degree of that this Coordinated Universal Time provides uncertainty in the standard frequency for light -18 the basis of , the use of which is will be reduced to the order of 10 or so［11］. legal in most countries, judges that this usage Accordingly, the time and frequency can be strongly endorsed." definition, relying on the Cs atom, may in the Today the UTC has gained wide accept- be redefined in terms of the frequency ance as a reference for time in a range of soci- of light. Any research on time and frequency etal applications; Germany, for example, standards must be pursued in view of this and adopts middle European time as "legal" time, other trends.

6 Journal of the National Institute of Information and Communications Technology Vol.50 Nos.1/2 2003 References 1 Dava Sobel, "Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time", Penguin USA (Paper); Reprint edition, 1996. 2 S. Tsunoyama, "Tokei no Syakaishi", Chuokoron-Sha, 1984. ( in Japanese) 3 http://millennium-dome.com/info/conference-finalact.htm 4 S. Aoki, "Toki to Koyomi", University of Tokyo Press, 1982. ( in Japanese) 5 Bureau International des Poids et Mesures, "The International System of Units (SI)", 6th edition, 1991. 6 W. Markowitz, et al., "Frequency of cesium in terms of ephemeris time", Phys. Rev. Lett., 1, 3, p.105, Aug. 1958. 7 M. Hosokawa, "Four Dimensional Space-Time and Reference Frame", Review of the Communications Research Laboratory, Time scales and Frequency Standards Special issue, pp.3-18, Vol.45, No.1/2, March/June 1999. ( in Japanese) 8 B. Guinot, P.K.Seidelmann, "Time scales: their history, definition and interpretation", Astron. Astrophys., 194, pp.304-308, 1988. 9 M. Imae, "Atomic Time Scale and Frequency Standards", Review of the Communications Research Labora- tory, Time scales and Frequency Standards Special issue, pp.19-26, Vol.45, No.1/2, March/June 1999. (in Japanese) 10 W. Lewandowski, C. Thomas, "GPS Time Transfer", Proc. IEEE Vol.79, No.7, Jul. 1991. 11 Th. Udem, R. Holzwarth & T. W. Hänsch, "Optical frequency ", Nature, Vol.416, 14, Mar. 2002.

MORIKAWA Takao Research Supervisor, Applied Research and Standards Division

MORIKAWA Takao 7