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Redalyc.ATOMIC TIME SCALES for the 21ST CENTURY Revista Mexicana de Astronomía y Astrofísica ISSN: 0185-1101 [email protected] Instituto de Astronomía México Arias, E. F. ATOMIC TIME SCALES FOR THE 21ST CENTURY Revista Mexicana de Astronomía y Astrofísica, vol. 43, -, 2014, pp. 29-34 Instituto de Astronomía Distrito Federal, México Available in: http://www.redalyc.org/articulo.oa?id=57131045008 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative RevMexAA (Serie de Conferencias), 43, 29–34 (2013) ATOMIC TIME SCALES FOR THE 21ST CENTURY E. F. Arias1 RESUMEN El Bureau Internacional de Pesas y Medidas, en coordinaci´on con organizaciones internacionales e institutos nacionales, mantiene y disemina el Tiempo Universal Coordinado (UTC). Existen tambi´enotras escalas de tiempo para satisfacer distintas aplicaciones. Este art´ıculo presenta el estado actual en la elaboraci´on de estas escalas de tiempo. ABSTRACT The International Bureau of Weights and Measures, in coordination with international organizations and na- tional institutes, maintains and disseminates Coordinated Universal Time (UTC). Other timescales exist for different purposes. This article describes the state-of-the-art in the elaboration of these time scales. Key Words: standards — time 1. INTRODUCTION traceable to UTC by participating in the relevant in- Time scales are developed for different applica- ternational comparison at the BIPM. A laboratory tions. National times (referred to as “legal”, or “k” fulfilling this condition maintains a local realiza- “official”) are maintained for providing time within tion (or approximation) to UTC, identified with the a country under the responsibility of the national acronym UTC(k). metrology institute, although in some cases astro- The system is a metrological pyramid where the nomical observatories maintain the national time. BIPM is at the top giving traceability to the SI International coordination is necessary for facilitat- units to national institutes. These disseminate the ing the interactions between the different countries. SI within their respective territories and follow the Taking advantage of well-established structures, in- mechanisms that guarantee the confidence in mea- ternational time coordination arises from the joint surements at two levels: within the frontiers, for actions of three organizations: the International Bu- supporting the societal activities, and between na- Ed. C. Allen, F. Arias, & R. Orellana reau of Weights and Measures (BIPM), in charge of tions, thus assuring confidence in measurements in the computation of the international reference time all kinds of international exchanges: financial, com- scale Coordinated Universal Time (UTC); the Inter- mercial, scientific, etc. national Telecommunication Union (ITU), respon- sible for fixing the rules for the dissemination of Time scales are constructed to fulfil particu- UTC and for establishing the procedure for its syn- lar needs. UTC provides the international refer- chronization to Earth’s rotation time; and the In- ence, with outstanding frequency stability (3 parts 16 ternational Earth’s Rotation and Reference Systems in 10 ), and high frequency accuracy (few parts 16 Service (IERS), which studies the irregularities of in 10 ); it is consequently suited for all applica- the Earth’s rotation, determines the relationship be- tions, from the most demanding, to those covering © 2014: Instituto de Astronomía, UNAM - Astronomía Dinámica en Latino América (ADeLA-2012) tween rotational and atomic times and announces the needs of civil timekeeping. The local realiza- the dates of application of the procedure for their tions UTC(k) approximate UTC with offsets ranging synchronization. from a few nanoseconds up to microseconds, and are In a process embedded in the international coor- characterized by uncertainties between 5 and 20 ns. dination for metrology, where the ultimate objective Other time scales are developed and maintained for is the traceability of physical and chemical measure- particular applications. Global navigation satellite ments to the International System of Units (SI, Bu- systems (GNSS) define internal time scales for the reau International des Poids et Mesures 2006), na- purpose of synchronizing the system and producing tional realizations of the atomic time scale become the navigation solutions. They are computed from 1 individual clock readings and are synchronized to re- International Bureau of Weights and Measures (BIPM), Time Department, Pavillon de Breteuil, 92310 S`evres, France alizations of UTC with offsets limited to tolerance ([email protected]). values specified for each system. 29 30 ARIAS The composite timescales, that is, those that are of the solar system, tests of theories, geodesy, geo- constructed on the basis of an ensemble of clocks, physics, environmental studies, etc. obey algorithms whose characteristics depend on the Coordinated Universal Time (UTC) is calculated requested quality of the time scale. The basic infor- monthly at the BIPM by applying to TAI the num- mation for the establishment of such a time scale is ber of leap seconds accumulated at the moment of the difference between the clocks in the ensemble. its computation (35 until 1 January 2014). It has the The devices used for evaluating the clock differences same metrological qualities as TAI, but presents one- depend on the location of the clocks, either within a second discontinuities at the moment of insertion of room in a laboratory or in different geographical lo- a leap second to compensate for the irregular rate of cations. UTC is the extreme case where the clocks in rotation of the Earth. It is represented by clocks in the ensemble (more than 400) are spread world-wide. the laboratories that contribute data to its mainte- nance, and has been adopted as the world reference 2. SOME BASIC CONCEPTS for time coordination. It is defined in a recommenda- tion of the International Telecommunication Union In time metrology, a time scale is characterized (ITU 2002), including the procedure for insertion of by the stability and the accuracy of the underlying leap seconds. This procedure limits the offset be- frequency. The frequency stability of a time scale tween UT1 and UTC to a maximum value of 0.9 s. represents its capacity to maintain a fixed ratio be- BIPM Circular T is the monthly publication tween its unitary scale interval and its theoretical that provides at five-day intervals, the values [UTC- counterpart. The frequency accuracy of a time scale UTC(k)] for each contributing laboratory k. It gives represents the aptitude of its unitary scale interval to traceability to the SI second and to UTC to national reproduce its theoretical counterpart. After the cal- time standards from which legal times in many coun- culation of a time scale on the basis of an algorithm tries are defined. conferring the required frequency stability, the fre- quency accuracy can be improved by comparing the 3. CLOCK COMPARISONS frequency (rate) of the time scale with that of pri- The calculation of a time scale on the basis of the mary frequency standards (PFS), and by applying, readings of clocks located in different laboratories if necessary, frequency (rate) corrections. requires the use of methods of comparison of distant The Free Atomic Scale (EAL) is the first step in clocks. A prime requisite is that the methods of time the calculation of UTC; it is the weighted average transfer do not contaminate the frequency stability of the contributing clocks treated by an algorithm of the clocks; in the past, they were often a major especially designed for mid-term frequency stability limitation in the construction of a time scale. (about 3 parts in 1016 over intervals of 20 to 40 days). UTC is built with the contribution of 72 labora- Ed. C. Allen, F. Arias, & R. Orellana Its unitary interval is not constrained to be close to tories spread over the planet; therefore a strategy for the SI second. It is calculated monthly at the BIPM the clock comparison, consistent with the designed on the basis of about 420 industrial atomic clocks algorithm needs to be well defined. Based on the in 72 national institutes and observatories spread principle of non-redundant comparisons, the BIPM world-wide. establishes a network of international time links. International Atomic Time (TAI) is a continu- The uncertainty of clock comparisons ranges be- ous and uniform time scale whose unitary interval tween a few tens of nanoseconds and a nanosecond agrees with the SI second by a few parts in 1016. By for the best links, a priori sufficient to allow a com- comparison of the unit interval of EAL to the dura- parison of the best atomic standards over integration © 2014: Instituto de Astronomía, UNAM - Astronomía Dinámica en Latino América (ADeLA-2012) tion of the SI second realized by primary frequency times of a few days. This assertion is strictly valid standards in a small number of laboratories, it may for frequency comparisons, where only the denom- result that a correction to the frequency is needed. inated Type A (statistical) uncertainty affects the This process, called “steering of EAL” provides ac- process. In the case of time comparisons, the sys- curacy to TAI. TAI has no tie to the Earth’s rotation tematic uncertainty (Type B), coming from the cal- time. It is not represented by clocks, it is not broad- ibration, should also be considered. In the present cast; it provides a reference for frequency only. It situation, calibrations contribute an uncertainty that has the stability of EAL, better than that of any of surpasses the statistical component, and which can the clocks used for its computation. TAI is the basis reach 20 ns for uncalibrated equipment (§ 6 of BIPM of the realizations of time scales used in dynamics, Circular T). It can be inferred that repeated equip- for modelling the motions of artificial and natural ment calibrations are indispensable for clock com- celestial bodies, with applications in the exploration parison.
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