tainties of the measurements themselves, The resulting uncertainty of the overall GPS and the Legal process (which is at best a statistical extrapolation and not an actual measure ment) might or might not satisfy our initial Traceabilitv of Time requirements. J The uncertainty associated with basing /ud& Levine National Institute of Standards and Technology traceability on previous calibrations exists and the University of Colorado for a mechanical artifact (a voltmeter, for example) as well, but our confidence in the stability of a properly maintained and local- 4s Faster, James Gleick notes in his recent book "A man with a watch knows what time ly available mechanical artifact usually is it is. A man with two watches is never sure." From the sundial, to the water clock, to the much higher than it is for a complex system escapement, to the pendulum, to the quartz crystal, to the atomic clock, to the Global based on a remote reference standard. The Positioning System, humanity has been obsessed with knowing what time it is. But just fact that the channel between our device fike the man with two watches, how do we know whose watch or clock is correct? In and the reference is usually not under our other words, as the rock band Chicago noted in one oftheir classic hits, "Does anyone direct control does not help matters. really know what time it is? Does anyone really care?" The answer to both questions is a Applications that require traceable meas resounding 'yes." Our modem society depends on knowing the correct time with higher urements usually have documentation 2nd higher accuracies for everything from time-stamping electronic transactions to syn- requirements as well as technical ones. chronizing telecommunications to navigating spacecrafi. "Correct" means that the time Depending on the details of the application, must be technically, and in some cases legally, traceable to national or intemational these requirements might range from main- standards. In this month's column, Dr. Judah Levine discusses these standards and the taining a simple log of the calibrations to important role GPS plays in keeping the world's timepieces both technically and legally real-time oversight or auditing by a disin- synchronized. terested third party using encrypted and Dr. Levine is a physicist in the Time and Frequency Division of the National Institute digitally signed messages. Systems that can ofStandards and Technology (NI.. formerly called the National Bureau ofStandards) support these requirements can become in Boulder, Colorado. He is also an adjoint professor in the Department ofPhysics of quite complex, because they must be pro- the University of Colorado. He received a B.A. in 1960 Yeshiva College and hm M.S. tected against both outsiders and insiders. and Ph.D. degrees from New York University in 1963 and 1966 respectively. Dr. Levine's The National Institute of Standards and research involves studies ofthe statistics offrequency standards and improving the accu- Technology (NISI') (and some other nation- racy of the distribution oftime and frequency information using both satellite and terres- al laboratories) provide onlysome of these trial methods. He is also involved in the application ofprecision measurement techniques services; other services (especially those to problems ofgeophysical interest. In collaboration with colleagues at NIST, he is intended for satisfying commercial or finan- engaged in improving methods for realizing the definition ofthe second and for distribut- cial requirements) are (or will be) provid- ing accurate time and frequency information. ed by private third parties using time sig- nals that are traceable to national standards. The Treaty of the Meter The Treaty of the Meter (Convention du Metre) is the basis for all international cooperation on questions of standards and precision metrology The treaty was signed in 1875 in Paris and was ratified by the US. Senate in 1878. The treaty was modified in 1921, and the modified version was ratified by the U.S. Senate in 1923. The modifica- "traceab1e"measurementis one that for traceability The fact that a previous time tions did not make any substantial changes A can be related to national or interna- stamp from some source was found to be to the original document. There are cur- tional standards using an unbroken chain within an acceptable tolerance of a refer- rently 49 member states of the treaty of measurements, each of which has a stat- ence standard does not by itself imply The treaty established the International ed uncertainty In this article I will describe (much less guarantee) that the current one Bureau of Weights and Measures (Bureau the traceability of time signals, with a spe- will also satisfy the same requirement. lntemational des bids et Mesures or BIPM), cial focuson the legal aspects of this ques- We can address this issue statistically which is currently located in Shes,a sub- tion. As I will show below, legal traceabili- by combining the interval since the last cal- urb of Paris. The BIPM is managed by the ty is not a purely technical question -the ibration with a statistical estimate of the International Committee of Weights and legal and technical definitions of time are stability of the reference to arrive at some Measures (Cornit6 International des kids not precisely the same (at least in the confidence interval for the current meas- et Mesures or CIPM). The President of the United States at present), and this differ- urement. The result is likely to be charac- CIPM is currently Professor J. Kovalevsky, ence could be significant in practice. terized in terms of a root mean square error, who is at the Observatoire de la CGte An unbroken chain of measurements is which is a function of the interval since the d'Azur, and the U.S. representative is Dr. a necessary but not sufficient requirement last calibration as well as of the uncer- Karen Brown, the Deputy Director of NIST GfS World January 2001 www.gpsworld.com The organizational structure defined in 4.0 . thelkaty of the Meter was initially intend- ed to deal with maintaining and calibrating ar ifact standards, such as the standard mc' ter and kilogram. The responsibilities of the BIPM were expanded over time to include other standards activities. In what follows, we will discuss only the current arrangement, which dates from 1987 when the responsibility for dealing with standards of time and frequency was transferred to the BIPM from the Bureau International de I'Heure. The CIPM appoints a number of consul- I ti tative committees to provide technical advice on questions that are referred to them. The committee that is important for -1 .o this discussion is the Consultative Commit- 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2 0 Year tee onTime and Frequency (CO,which was formerly called the Consultative CURE 1 The length of the astronomical day varies because of several different Committee for the Definition of the Second henomena including tidal friction and the exchange of angular momentum (CCDS). The CCFin turn appoints a num- etween the Earth's core, mantle, and atmosphere. This plot of the annual mean ber of working groups and subcommittees ifferences between the actual length of day and a day containing exactly to deal with specific questions. Two that 6,400 seconds (the UTC day) illustrates that during the past 100 years, the day are important for this discussion are the ased on the Earth's rotation has almost always been longer than the UTC day Working Group on International Atomic iith a maximum departure of about 4 milliseconds. Time UAI) and the subgroup on GPS and GLONASTimeTransfer Standards. neWeb 1 .o page of the BIPM has additional organiza- tional details (see "Further Reading"). In 0.8 __ particular, that page describes other rele- b vant working groups such as the one that is concerned with the realization of primary frequency standards and another which deals with time and frequency transfer using non-GPS methods such as two-way satellite time transfer. nme, Frequency, and the BIPM Althou h time and frequency were origi- nally dought of as distinct quantities with independent definitions, this distinction has not been significant for about 30 years. #en frequency standards based on atom- -0.6 ic transitions were first developed in the 1993 1994 1995 1996 1997 1998 1999 2000 2001 19%, the initial plan was to use these stan- Year ~~ ~~~ ~~ dards to realize the standard of frequency ICURE 2 The longer astronomical day requires the insertion of leap seconds inti but to maintain the standard of time astre ie UTC time scale in order to keep the difference between UT1 and UTC less nomically That method proved to be very ian 0.9 second. The five leap seconds which have occurred since 1993 are cumbersome, and the realization of the learly evident in this plot of UT1 -UTC as determined by the International Earth standards for time and frequency were otation Service. Values at 0.05 year intervals are plotted up to the beginning of unified into their current configuration on le year 2000 and at five-day intervals after that. 1 January 1972. Since 1972, the length of the second has ay defined by astronomical methods;the onds is what makes UTC a'coordinated been defined using the frequency of a ifference is removed by introducing leap time scale.) The rates of TAI and UTC ar hyperfine energystate transition in the econds as needed to keep the absolute identical between leap seconds. During th ground state of the cesium atom. iagnitude of the difference less than 0.9 past 10 years or so, the length of the UT International atomic time (TAI, using the econds.