The Cctf Working Group on the Expression of Uncertainties in Primary Frequency Standards

The Comite Consultatif pour la Definition de la Seconde* (CCDS) created in March 1996 a Working Group on the expression of uncertainties in primary frequency standards. This paper presents the main topics included in the first report of the Working Group dated April 1997. This report gives a brief review of the conditions which led to the creation of the Working Group and a summary of the discussions which took place at its first meeting held in Neuchdtel, Switzerland, on 5 March 1997, and also during the following weeks. A main objective of the Working Group is a better understanding between laboratories which evaluate the accuracy of their primary frequency standards and their clients represented by the Bureau International des Poids et Mesures (B1PM) which uses the measurements provided by these standards for assessing the accuracy of TAI (Temps Atomique International, or International Atomic Time) and UTC. An important point for this purpose is the application to primary frequency standards of the guidelines expressed in the Guide to the Expression of Uncertainty in Measurement ///. CREATION OF THE WORKING GROUP A primary frequency standard is an instrument which produces an output whose average frequency is based on the internationally accepted definition of the second, and is operated with its own independent implementation of the cesium second and a specified accuracy that does not rely on calibration to another frequency standard. Some national metrology laboratories design and operate large primary frequency standards whose main design objective is to facilitate the evaluation of their accuracy. These instruments are the most accurate devices ever made by humans. The question of quantifying and expressing the accuracy of primary frequency standards that is "the expression of the uncertainty of primary frequency standards" arose at the 13 meeting of the CCDS in March 1996 [2]: [Dr Quinn] observed that the ISO Guide to the expression of uncertainty in measurements recommends that uncertainties be expressed as a combination of statistical (type A) and others (type B) uncertainties. In view of this, he assumes that the uncertainties quoted for primary frequency standards are entirely type B, since no associated averaging times are quoted. * At its 1997 meeting, the Comite International des Poids et Mesures (CIPM) decided to change the name of the Comite Consultatif pour la Definition de la Seconde (CCDS) to that of Comite Consultatifdu Temps et des Frequences (CCTF). 85 Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE DEC 1997 2. REPORT TYPE 3. DATES COVERED 00-00-1997 to 00-00-1997 4. TITLE AND SUBTITLE The CCTF Working Group on the Expression of Uncertainties in Primary Frequency Standards 5a. CONTRACT NUMBER