BOLETÍN INFORMATIVO DEL SISTEMA INTERAMERICANO DE METROLOGÍA - OEA

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1 I INFO IM QUERÉTARO, MÉXICO / Dcembre 008 / December 008 BOLETÍN INFORMATIVO DEL SISTEMA INTERAMERICANO DE METROLOGÍA - OEA INFORMATIVE BULLETIN OF THE INTERAMERICAN METROLOGY SYSTEM - OAS N Ó CI A Z N A G R D O E L O S ES T AD O S A M E R I C A N O S IM SISTEMA INTERAMERICANO DE METROLOGÍA

2 IM SISTEMA INTERAMERICANO DE METROLOGÍA DIRECTORIO DIRECTORY Presdente Presdent Humberto S. Brand, INMETRO, Brazl Secretaro Ejecutvo Executve Secretary Oscar Harasc, OEA / OAS Consejero Técnco Techncal Advsor Clare Saundry, NIST, USA Coordnador del Comté Técnco Techncal Commttee Char Lus Musso, LATU, Uruguay Coordnador del Comté de Desarrollo Profesonal Professonal Development Commttee Char Ignaco Hernandez, CENAM, Méxco Coordnador del Grupo de Trabajo sobre Sstemas de Caldad Qualty Systems Task Force Char Wllam Anderson, NIST, USA Secretaro Secretary Jorge A. Paz Cruz, INMETRO, Brazl COORDINADORES DE LAS SUBREGIONES SUBREGION COORDINATORS ANDIMET Juan Carlos Castllo Vllarroel, IBMETRO, Bolva CAMET Gustavo Montel Quedo, LANAMET, Ncaragua CARIMET Theodore Reddock, TTBS, Trndad and Tobago NORAMET Jm McLaren, NRC, Canada SURAMET Ever Cabrera, INTN, Paraguay INFO IM

3 INFOSIM INDICE / CONTENTS INTRODUCCIÓN / INTRODUCTION Humberto Brand SIM INTERNATIONAL COMPARISON OF 50/60 Hz ENERGY (00 007)...7 Tom Nelson, Nen Fan Zhang, Nle Oldham, René Carranza, Sergo Campos, Maro Monge, Harold Sánchez, Ana María Franco, Lucas D Lllo, Robert Duarte, Eddy So, Carlos Sauders, Henry Postgo, Carlos Favero. THE SIM TIME AND FREQUENCY NETWORK. 15 Mchael A. Lombard, Andrew N. Novck J. Maurco López R. Francsco Jménez, Jean-Smon Boulanger, Raymond Pelleter, Rcardo de Carvalho, Raúl Solís, Carlos Donado, Harold Sánchez, Carlos Andrés Quevedo, Gregory Pascoe, Danel Pérez. NUEVAS CONSIDERACIONES SOBRE INCERTIDUMBRES Y RECTA DE CALIBRACIÓN EN QUÍMICA ANALÍTICA....6 F. Kornblt, L. Castro. PARTICIPACIÓN EN LA ESCUELA DE VERANO DE METROLOGÍA DE LA OFICINA INTERNACIONAL DE PESAS Y MEDIDAS BIPM Ana Armjos, Jessca Chavarría, Valne Smarçaro da Cunha, Francsco García. NOTI-SIM 40

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5 INTRODUCCIÓN INTRODUCTION El INFOSIM se ha convertdo en el medo de dfusón en la comundad del Sstema Interamercano de Metrología. Este número se ha aprovechado para dfundr resultados de comparacones en nuestra regón, en la que se requere acelerar aún más ejerccos de esta naturaleza. Tomamos como ejemplo la prmera comparacón nternaconal en energía eléctrca en la frecuenca de 50/60 hertz y la consoldacón de la red de tempo y frecuenca del SIM. Además ncluye una contrbucón sobre la estmacón de la ncertdumbre de las meddas cuando los resultados se obtenen medante una curva de calbracón, tema que aún se encuentra bajo estudo en la comundad nternaconal. Por otro lado, el INFOSIM contene una relatoría por la cual cuatro representantes de la comundad del SIM comparten sus experencas de la pasada Escuela de Verano que anualmente organza el BIPM. Y como sempre en esta edcón no podemos pasar por alto las actvdades de nuestra comundad, latr de nuestra vda metrológca, menconada en la seccón NOTI-SIM. Por últmo, con la fnaldad de promover una mejor comuncacón extensble a todo nteresado, el INFOSIM y otros documentos relevantes, como la versón del VIM en español, se están ponendo a su dsposcón en forma electrónca en The INFOSIM has become the means for communcaton wthn the communty of the Interamercan System of Metrology. Ths ssue ams at dffusng the results of comparsons n our regon, where exercses of ths nature should be accelerated. The results of the frst regonal comparson n electrc energy wthn a frequency of 50/60 hertz as well as the consoldaton of the tme and frequency web of the SIM are presented. Also, t ncludes a contrbuton on the estmaton of measurement uncertantes when measurement results are obtaned through calbraton curves, topc whch s stll under analyss n the nternatonal communty. On the other hand, the INFOSIM contans a secton n whch four representatves of the SIM communty share ther experences from the 007 Summer School organzed annually by the BIPM. As usual n the INFOSIM, the man actvtes of our communty, heartbeat of our metrologcal lfe, are mentoned n the secton NOTI-SIM. Fnally to offer a better communcaton, the INFOSIM and other relevant documents, as for example the VIM n Spansh, are now avalable at Humberto Brand SIM presdent Humberto Brand, Presdente del SIM 5

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7 SIM INTERNATIONAL COMPARISON OF 50/60 HZ ENERGY (00-007) Tom Nelson, Nen Fan Zhang, and Nle Oldham (NIST), René Carranza and Sergo Campos (CENAM), Maro Monge (CONACYT), Harold Sanchez (ICE), Ana Mara Franco (INMETRO), Lucas D Lllo (INTI), Robert Duarte (INTN), Eddy So (NRC), Carlos Sauders (SENACYT), Henry Postgo (SNM), Carlos Favero (UTE) ABSTRACT: Results of the Inter-amercan Metrology System (SIM) nternatonal energy comparson are presented. Partcpatng countres were Argentna, Brazl, Canada, Costa Rca, El Salvador, Mexco, Panama, Paraguay, Peru, Uruguay, and the Unted States. KEYWORDS: SIM, Internatonal comparson, Energy, travelng standard 1. INTRODUCTION The Inter-amercan Metrology System (SIM) sponsors perodc nternatonal comparsons of electrcal unts mantaned at Natonal Metrology Insttutes (NMIs) n the Amercas. The frst SIM electrcal comparson was conducted n the late 1990s and used dgtal multmeters as travelng standards wthn the fve sub regons n SIM to measure ac and dc voltage and current and dc resstance [1]. At about the same tme, NIST was plotng the Consultatve Commttee for Electrcty and Magnetsm CCEM-K5 comparson of 50/60 Hz electrc power [] that ncluded NMIs n North and South Amerca. As a follow-up to the K5 comparson, NIST was selected to plot the SIM comparson of 50/60 Hz electrc energy, descrbed below. The plan s to lnk to CCEM-K5 so that more NMIs n the Amercas wll be ted to the key comparson database mantaned by the Internatonal Commttee of Weghts and Measures, CIPM [3]. 1 After consultaton wth other NMIs, NIST metrologsts decded to perform the comparson at 50 and 60 hertz. Three ponts were selected to test the ampltude and phase measurng capabltes of the NMIs: 10 volts and 5 amperes at power factors 1.0, 0.5 lead (ld), 0.5 lag (lg), where lead (capactve) ndcates that the current leads the voltage and where lag (nductve) ndcates that the current lags the voltage. A lst of partcpatng laboratores s gven n Table 1. Table 1. Lst of partcpants, NMIs, and measurement dates. Laboratory Measurement date NIST, USA August 00 May 007 ICE, Costa Rca July 003 SENACYT, Panama August 003 CONACYT, El Salvador November 003 CENAM, Mexco June 006 NRC, Canada May 007 INMETRO, Brazl February 004 UTE, Uruguay Aprl 004 INTN, Paraguay August 004 SNM, Peru Aprl 005 INTI, Argentna July 006 Two travelng standards (Radan model RM-11) were used to reduce the tme requred to complete the comparson. These nstruments are ac-powerto-frequency or energy-to-pulse converters based on the tme-dvson-multpler operatng prncple. Wth 10 V and 5 A appled at 1.0 power factor, the nomnal output frequency of the converters s Hz. Coeffcents of voltage, current, and power factor for these standards are neglgble 6 (less than 5 parts n 10 ) over a range of ±0. % of these parameters. Partcpants were asked to mantan a ±0.1 % tolerance. Temperature coeffcents are also neglgble n the range of 3 C ± 3 C. Humdty nfluences are more dffcult to measure. Partcpants were asked to record the ambent temperature and humdty. 1 NMI s used n ths document to denote the laboratory responsble for energy standards wthn each country partcpatng n the comparson. Identfcaton of commercal equpment s not ntended to mply recommendaton or endorsement by NIST, nor s t ntended to mply that the equpment s necessarly the best avalable for the purpose. 7

8 NMI x u x u x u x u x u x u Some laboratores reported results n percentage regstraton, wth uncertantes n percent of readng. Others 6 reported results as errors n ppm, ìw/w, ìwh/wh, ìj/j (all equvalent to parts n 10 ), wth uncertantes n the same unts. Some reported values n terms of readng and others n terms of full scale (appled VA). All values 6 were converted to errors and standard uncertantes n parts n 10 of readng. These normalzed results are gven n Table. 6 Table. Reported Errors x,k and Standard Uncertantes u,k (n parts n 10 of readng), where denotes the NMI and k denotes the test pont. Travelng standard Hz 50 Hz ld 0.5lg ld 0.5lg,1,1,,,3 NIST ICE SENACYT CONACYT CENAM NRC Travelng standard NIST INMETRO UTE INTN SNM INTI ANALYSIS The travelng standards were measured at NIST before the comparson began, when the comparson was completed, and at several ponts durng the comparson. Dependng on the standard and the test frequency, NIST performed up to 100 ndependent measurements at each test pont from 00 to 007. Snce two travelng standards were used and measured at dfferent NMIs, the comparson was treated as two ndependent loops wth the plot laboratory (NIST) as the common lnk. The sngle loop analyss and notaton used n ths comparson s based on that descrbed by Zhang et al. n reference 4. In ths case, for each test pont n the( j th j=1,), assume that a smple lnear regresson model holds for measurements made by NIST, 1k 1 1 k 1k we 3 X ( j) ( j) ( j) t ( j) ( j) (1) for k = 1,..., Kj. The average of { t ( j), k 1,..., K } k j s t 1 ( j) The average of X 1k s X 1. We further assume that the random error, ( j ) 1 k, has a zero mean and an uncertanty u ( j) 1 for NIST.,3,4,4,5 For the other laboratores ( 1), measurements are taken at tme t ( j) and the correspondng model s X ( j) ( j) ( j) t ( j) ( j),..., I j where the random error has a zero mean and a standard uncertanty of u ( j ) for =,... I j and I th j s the number of labs n the loop.,5 j,6,6 () Snce the Type B uncertantes of the NIST measurements are the same for all tme perods, as n reference 4, the regresson parameters for the th j artfact are estmated by: K j ( t ( j) t ( j))( X ( j) X ( j)) ( j) 1k 1 k 1 k 1 K j ( t ( j) t ( j)) 1k 1 k 1 ( j) X ( j) ( j) t ( j) 1,..., I j 3 The use of a lnear model was questoned by CENAM but based on the dstrbuton of NIST measurements, t was consdered the best compromse. (3) (4) 8

9 u u The correspondng uncertantes and the covarance terms are obtaned usng eq of reference 4. The comparson reference value (CRV) at an optmal tme t * ( j ) s, for the j th loop: I j CRV ( j) w ( j) X ( j) (5) t * ( j ) 1 I j where * t ( j) w ( j) t ( j) 1 and 1 u ( j) w ( j) I j 1 u ( j) k 1 k (6) We use the smplfed symbol CRV(1) to ndcate st the CRV for the 1 loop, usng travelng standard 5058 and CRV() to represent the CRV for the nd loop, usng travelng standard The regresson components, CRVs and ther uncertantes u CRV are gven n Table 3. Results from partcpants that obtan energy traceablty from other NMIs or laboratores were not used n the CRV computaton. Table 3. Regresson parameters, CRV, and u 6 CRV (n parts n 10 ). Standard 60 Hz 50 Hz lead 0.5 lag lead 0.5 lag á(1) â(1) CRV(1) (1) CRV á() â() CRV() () CRV From reference 4, the degree of equvalence of th th the laboratory, e.g., n the j loop ( 1, ) relatve to the CRV n the correspondng loop s the dfference * D ( j) ˆ ( j) ˆ( j) t ( j) CRV ( j) (7), CRV j 1, j The uncertanty of ths dfference s gven n reference 4 (eqs. 31 and 33). The dfferences (D) and uncertantes ( u D ) are gven n Table 4. 6 Table 4. Degrees of equvalence and the correspondng uncertantes (n parts n 10 ). 60 Hz 50 Hz CRV(1) usng travelng standard ld 0.5lg ld 0.5lg NMI D u D D u D D u D D u D D u D D u D NIST CENAM NRC CRV() usng travelng standard NIST INMETRO UTE INTI

10 The degrees of equvalence between pars of natonal measurement standards n the same th loop, e.g., the j ( j =1,), s defned as the dfferences D, k ( j, j) D ( j) D ( j), CRV k, CRV * ( j) ( j) t ( j) CRV ( j) * [ ( j) ( j) t ( j) CRV ( j)] k ( j) ( j) k (8) when k.the correspondng uncertanty s gven n reference 4 (eq.36). However, when the two non-plot labs are n two dfferent loops, e.g., the th lab n the frst loop and the kth lab n the second loop, measurements of the lnkng lab, NIST, are used. The degree of equvalence s calculated as follows: D, k (1,) D (1) D (1) [ D () D ()], CRV 1, CRV k, CRV 1, CRV D (1,1) D (,),1 k,1 (9) where D (1,1),1 and D (,) k,1 are the par-wse degrees of equvalence between the th lab and NIST for the frst loop and the kth lab and NIST for the second loop, respectvely. The standard uncertanty of D k s gven by s gven by, (1,) u u u D, k (1,) D,1 (1,1) Dk,1 (,) (10) Par-wse tables of equvalence are gven, wth uncertantes, n Appendx A, where a negatve sgn ndcates that the NMI on the left of the table s lower than the NMI on the top of the table. 3. CONCLUSIONS Results of the frst SIM nternatonal comparson of 50/60 Hz energy have been presented. In several cases the degrees of equvalence exceed the estmated uncertantes. However, problems have been dentfed and these ponts wll be rechecked va blateral comparsons to resolve the dfferences. The measurements took fve years to complete, much longer than orgnally antcpated. Shppng, customs, and testng delays were the man problems. These must be addressed before the next comparson. References [1] H. Sanchez, J. Coff, H. Laz, D. Bennett, H. Ferrera, R. Ortega, N. Oldham, and M. Parker, "SIM Comparson of Electrcal Unts," Proc. Metrologa-000 Conference, Dec 4-7, 000, Sao Paulo, Brazl, (Dec 000) [] N. Oldham, T. Nelson T, N. F. Zhang and H. K. Lu 003 CCEM-K5 Comparson of 50/60 Hz Power Metrologa 40, Tech. Suppl [3] Gudelnes for CIPM Key Comparsons 1999 (Appendx F to Mutual recognton of natonal measurement standards and of calbraton and measurement certfcates ssued by natonal metrology nsttutes) Techncal Report Internatonal Commttee for Weghts and Measures [4] N. F. Zhang, H. K. Lu, N. Sedransk and W. E. Strawderman, Statstcal analyss of key comparsons wth lnear trends, Metrologa Partcpatng NMIs and Author Contact Informaton Natonal Insttute of Standards and Technology (NIST), Gathersburg, MD, USA, thomas.nelson@nst.gov Centro Naconal de Metrología (CENAM), Querétaro, Méxco, rene.carranza@cenam.mx Consejo Naconal de Cenca y Tecnología CONACYT, San Salvador, El Salvador, maroamonge@hotmal.com Insttuto Costarrcense de Electrcdad (ICE), San José, Costa Rca, hsanchez@ce.co.cr Insttuto Naconal de Metrologa, Normalzaçao e Qualdade Industral (INMETRO), Duque De Caxas, Brasl, amfranco@nmetro.gov.br 10

11 Insttuto Naconal de Tecnologa Industral (INTI), Buenos Ares, Argentna, Insttuto Naconal de Tecnología, Normalzacón y Metrología, INTN, Asuncón, Paraguay, metrologa@ntn.gov.py Natonal Research Councl of Canada (NRC), Ottawa, Ontaro, Canada, Eddy.So@nrc-cnrc.gc.ca Secretaría Naconal de Cenca, Tecnología e Innovacón (SENACYT), Panamá, Repúblca de Panamá, csauders@senacyt.gob.pa Servco Naconal de Metrología (SNM), Lma, Perú, hpostgo@ndecop.gob.pe Admnstracón Naconal de Usnas y Trasmsones Eléctrcas, Montevdeo, Uruguay, dslomovtz@ute.com.uy Appendx A. Par-Wse Tables of Equvalence 60 Hz, 1.0 power factor (n parts n 10 6 ) Dfferences NIST ICE SENACYT CONACYT CENAM NRC INMETRO UTE INTN SNM INTI NIST ICE SENACYT CONACYT CENAM NRC INMETRO SNM INTI Standard uncertantes NIST ICE SENACYT CONACYT CENAM NRC INMETRO UTE INTN SNM INTI NIST ICE SENACYT CONACYT CENAM NRC INMETRO SNM INTI

12 60 Hz, 0.5 lead (capactve) power factor (n parts n 10 6 ) Dfferences NIST ICE SENACYT CONACYT CENAM NRC INMETRO UTE INTN SNM INTI NIST ICE SENACYT CONACYT CENAM NRC INMETRO SNM INTI Standard uncertantes NIST ICE SENACYT CONACYT CENAM NRC INMETRO UTE INTN SNM INTI NIST ICE SENACYT CONACYT CENAM NRC INMETRO SNM INTI Hz, 0.5 lag (nductve) power factor (n parts n 10 6 ) Dfferences NIST ICE SENACYT CONACYT CENAM NRC INMETRO UTE INTN SNM INTI NIST ICE SENACYT CONACYT CENAM NRC INMETRO SNM INTI Standard uncertantes NIST ICE SENACYT CONACYT CENAM NRC INMETRO UTE INTN SNM INTI NIST ICE SENACYT CONACYT CENAM NRC INMETRO SNM INTI

13 50 Hz, 1.0 power factor (n parts n 10 6 ) Dfferences NIST CENAM INMETRO UTE INTN INTI NIST CENAM INMETRO UTE INTN INTI Standard uncertantes NIST CENAM INMETRO UTE INTN INTI NIST CENAM INMETRO UTE INTN INTI Hz, 0.5 lead (capactve) power factor (n parts n 10 6 ) Dfferences NIST CENAM INMETRO UTE INTN INTI NIST CENAM INMETRO UTE INTN INTI Standard uncertantes NIST CENAM INMETRO UTE INTN INTI NIST CENAM INMETRO UTE INTN INTI

14 50 Hz, 0.5 lag (nductve) power factor (n parts n 10 6 ) Dfferences NIST CENAM INMETRO UTE INTN INTI NIST CENAM INMETRO UTE INTN INTI Standard uncertantes NIST CENAM INMETRO UTE INTN INTI NIST CENAM INMETRO UTE INTN INTI

15 THE SIM TIME AND FREQUENCY NETWORK (1) (1) () () (3) Mchael A. Lombard, Andrew N. Novck, J. Maurco Lopez R., Francsco Jmenez, Jean-Smon Boulanger, (3) (4) (5) (5) (6) Raymond Pelleter, Rcardo de Carvalho, Raul Sols, Carlos Donado, Harold Sanchez, Carlos Andres (7) (8) (9) Quevedo, Gregory Pascoe, and Danel Perez (1) Natonal Insttute of Standards and Technology (NIST), Boulder, Colorado, Unted States, () Centro Naconal de Metrología (CENAM), Querétaro, Mexco, (3) Natonal Research Councl (NRC), Ottawa, Canada, (4) Natonal Observatory (ONRJ), Ro de Janero, Brazl, (5) Centro Naconal de Metrología de Panamá (CENAMEP), Panama Cty, Panama, (6) Insttuto Costarrcense de Electrcdad (ICE), San Jose, Costa Rca, (7) Superntendenca de Industra y Comerco (SIC), Bogota, Colomba, cquevedo@correo.sc.gov.co (8) Bureau of Standards Jamaca (BSJ), Kngston, Jamaca, gpascoe@bsj.org.jm (9) Insttuto Naconal de Tecnologa Industral (INTI), Buenes Ares, Argentna, dperez@nt.gov.ar ABSTRACT The Sstema Interamercano de Metrologa (SIM) conssts of natonal metrology nsttutes (NMIs) located n the 34 member natons of the Organzaton of Amercan States (OAS), whch extends throughout North, Central, and South Amerca and the Carbbean regon. SIM s one of the world's fve major regonal metrology organzatons (RMOs) recognzed by the Bureau Internatonal des Pods et Mesures (BIPM). Currently about half of the 34 member NMIs mantan tme and frequency laboratores. In order for these NMIs to establsh metrologcal traceablty and to determne the uncertanty of ther measurements, t s mportant for them to partcpate n nternatonal comparsons. The SIM tme and frequency network was developed to allow NMIs to partcpate n contnuous nternatonal comparsons wth a mnmum of effort and cost. The SIM network has advanced the state of metrology n the SIM regon by allowng as many laboratores as possble to partcpate n nternatonal tme coordnaton. It provdes contnuous, near real-tme comparsons between the natonal tme and frequency standards located throughout the SIM regon, by utlzng both the Internet and the Global Postonng System (GPS). As of September 008, 1 NMIs have been sent SIM tme and frequency measurement systems. These systems have been pad for ether by the OAS, whch s the parent organzaton of SIM, or by the NMIs themselves. Nne of these laboratores are already engaged n contnuous nterlaboratory comparsons, wth the other three expected to begn soon. Four addtonal NMIs have expressed nterest and wll be added to the network as soon as resources become avalable. Ths paper provdes an overvew of SIM and a techncal descrpton of the network. It presents the results of nterlaboratory comparsons and dscusses the network's measurement uncertantes. INTRODUCTION SIM shares the same goals as ts fellow regonal metrology organzatons (RMOs); t works to ensure the unformty of measurements throughout a large secton of the world by establshng traceablty to the Internatonal System of unts (SI). RMOs realze ths goal by performng several tasks. They revew the qualty systems of NMIs, and ther calbraton and measurement capabltes (CMCs). In addton, a well functonng RMO organzes regonal comparsons, and help the NMIs of small and developng natons mantan standards at the level of accuracy that s needed to support ther economy. Fgure 1. The world's regonal metrology organzatons (SIM regon s n orange). 15

16 amongst RMOs (Fgure 1), SIM s partcularly large. The SIM regon encompasses some 7 % of the world's land mass, and about 14 % of ts populaton (an estmated 90 mllon people as of 007). The northern part of SIM resdes n the largest market n the world, the regon covered by the North Amercan Free Trade Agreement (NAFTA). Wthn the SIM regon, however, there s a large varaton n both the populatons of the natons and the strength of the economes. About two-thrds of the people n the SIM regon (approxmately 600 mllon people) resde n the Unted States, Brazl, and Mexco. In contrast, 1 other SIM natons, mostly slands n the Carbbean regon, have populatons of less than one mllon. As of 007, the per capta gross domestc product (GDP) of the Unted States and Canada exceeded $38,000 USD, but ten SIM natons had GDPs of $7,000 USD or less. Ths dsparty n populaton and money drectly translates nto the relatve amounts of resources that are made avalable for metrology. For example, about 40 full-tme professonals are employed n the area of tme and frequency metrology at the Natonal Insttute of Standards and Technology (NIST) n the Unted States, but many SIM laboratores are fortunate f they have one person, even part-tme, who s free to focus on tme and frequency measurements. In spte of ther varyng levels of resources and the dfferent obstacles that they face, all SIM NMIs share the same task: they must establsh measurement traceablty to the SI. The ablty to make traceable measurements s crtcal to an NMI; wthout t they are of lttle use to ndustry n ther country. Internatonal trade requres traceablty n order for the measurements made n one country to be accepted and trusted n another country. As a general rule, an NMI cannot establsh traceablty unless t partcpates n nternatonal comparsons. In the tme and frequency communty, ths usually means that an NMI must partcpate n the BIPM key comparsons. However, not all SIM NMIs have sgned the BIPM Mutual Recognton Agreement (MRA), and some currently lack the resources, tranng, experence, and contacts that are requred to partcpate n the BIPM key comparsons. To meet the needs of all SIM NMIs, and to establsh a new sprt of cooperaton throughout the Amercas, the SIM tme and frequency comparson network was developed. DESIGN GOALS The concept of a SIM tme and frequency comparson network was frst dscussed at NIST n The plans for the network were formalzed n a meetng held n Ottawa, Canada n July 004 between representatves of the three North Amercan NMIs: the Centro Naconal de Metrología (CENAM) of Mexco, the Natonal Research Councl (NRC) of Canada, and NIST of the Unted States. The desgn goals for the network were: To establsh cooperaton and communcaton throughout the SIM regon by buldng a network that allowed even the smallest labs to compare ther standards to those of the rest of the world. To choose equpment that was low cost and easy to nstall, operate, and use, because SIM NMIs typcally have lmted resources and small staffs. To make measurements wth uncertantes small enough to characterze the best standards n the SIM regon. Ths meant that the measurement uncertantes had to be as small, or nearly as small, as those of the BIPM key comparsons. To report measurement results n near realtme, wthout the processng delays of the BIPM key comparsons. To buld a democratc network that favored no sngle laboratory or naton, and to allow all members to vew the results of all comparsons. Once the desgn goals were establshed, the development of the network quckly proceeded. SIM measurement systems were delvered by NIST to CENAM and NRC n the sprng of 005, and the frst comparsons began n May 005 [1, ]. TECHNICAL DESCRIPTION The SIM network s based on common-vew observatons of the Coarse / Acquston (C/A) codes transmtted by the GPS satelltes on the L1 carrer frequency of MHz. Ths technque was frst used to compare remote clocks and oscllators shortly after the frst GPS satellte was launched [3], and remans the most common comparson technque used for the dervaton of Coordnated Unversal Tme, or UTC [4].

17 and take the dfference among them. Delays that are common to both paths dsa and dsb cancel out, but delays that are not common to both paths contrbute uncertanty to the measurement. The result of the measurement s (Clock A - Clock B) wth an error term of d SA d SB. Thus, the basc equaton (Eq. 1) for common-vew GPS measurements s (Clock A GPS) (Clock B GPS) = Clock A Clock B + (d SA d SB). (1) After the components that make up the systematc d SA d SB error term are measured or estmated, they are ether appled as a correcton to the measurement or are accounted for n the uncertanty analyss. The systematc d SA dsb error term ncludes not only delays from the satellte to the recevng antennas, but also delays that take place after the sgnal s receved. Therefore, a key to a successful measurement s for every SIM system to have well characterzed delays that are obtaned through calbraton. All SIM systems are calbrated at NIST pror to shpment to the host NMI. Each calbraton lasts for 10 days and s performed usng the commonclock method [1, ]. Fgure. Common-vew GPS Measurements. Common-vew GPS comparsons use one or more GPS satelltes as the common-vew reference (Fgure ). The objectve s to use GPS as a transfer standard so that tme standards located at remote locatons can be compared. The commonvew method nvolves a GPS satellte (S), and two recevng stes (A and B), each contanng a GPS recever, a tme nterval counter, and a local tme standard. The satellte transmts a tme sgnal that s nearly smultaneously receved at A and B, and a measurement s made every second at both A and B that compares the receved GPS sgnal to the local tme standard. Thus, the measurement at ste A compares the GPS sgnal receved over the path dsa to the local clock, S - Clock A. Ste B receves GPS over the path dsb and measures S - Clock B. The two recevers then exchange data Fgure 3. The SIM Measurement System. The SIM measurement system (Fgure 3) records the common-vew measurements and sends them to a central web server for processng. The system conssts of an ndustral rack-mount computer that contans a tme nterval counter wth sngle shot resoluton of less than 0.1 ns, and an eght-channel GPS recever. The recever s connected to an aperture coupled slot array 17

18 antenna desgned to mtgate the recepton of multpath sgnals. Ths pnwheel type antenna s smaller and lghter than a choke rng antenna, but rejects multpath sgnals equally as well [5, 6]. The SIM system accepts ether a 5 or 10 MHz reference sgnal as the counter's external tme base, and a one pulse per second (pps) sgnal from the local tme standard. An Ethernet card connects the system to the network, and partcpatng laboratores must provde an alwayson Internet connecton. Measurement data are transmtted by use of the fle transfer protocol (FTP). Passve mode FTP s used at most stes to avod problems wth frewalls, and the fle transfers have been very relable. The SIM system measures the tme dfference between GPS and the local standard every second, and both 1-mnute and 10-mnute averages are recorded for as many as eght satelltes. The system records fles that nclude the current system settngs (ncludng antenna coordnates and cable delays), followed by a matrx contanng the measurement data. The 3 column numbers match the pseudo-random nose (PRN) codes of the GPS satelltes. The 144 rows represent the number of 10 mnute segments n one day. The SIM data submtted to the network are n a format that s ncompatble wth the Consultatve GPS and GLONASS Tme Transfer Subcommttee (CGGTTS) format used by the BIPM [7]. However, software that converts SIM data to the CGGTTS format has been developed to assst NMIs that need ths capablty. The natve SIM format has the advantage of collectng about 3 % more data than the CGGTTS mult-channel format, as shown n Table 1. The web-based software processes up to 00 days of data at once. It algns the tracks where two laboratores smultaneously measured the same satellte, and performs the common-vew data reducton. The results are graphed as ether onehour or one-day averages, and the tme devaton, ó x ( ), and Allan devaton, ó y ( ) [8], of the entre data set are dsplayed. In addton, 10-mnute, one-hour, or one-day averages can be vewed n tabular form and, f desred, coped nto a spreadsheet or other applcaton for further analyss. The web ste of the SIM Tme and Frequency Metrology Workng Group ( ncludes a real-tme grd (Fgure 4) that shows the most recent tme dfferences between SIM NMIs. The grd receves new data every ten mnutes, and refreshes automatcally every fve mnutes. If a user clcks on one of the tme dfference values dsplayed on the grd, a phase plot of the comparson for the current day wll appear n ther web browser. The real-tme reportng of results allows all partcpants n the network to nstantly compare ther tme standards to each other. Ths benefts all SIM NMIs, ncludng the fve (CENAM, CENAMEP, NIST, NRC, and ONRJ) that currently send data to the BIPM for the computaton of UTC. The UTC contrbutors can now vew ntercomparson data wthout watng for the BIPM's monthly Crcular-T [9], whch ncludes results that are typcally from two to seven weeks old at the tme of publcaton. Another advantage s that the shortest reported averagng tme ( 0 ) s equal to 600 s for the SIM data, as opposed to 5 days n the case of the Crcular-T data. Ths makes t easer to dentfy short-term fluctuatons, and allows measurement problems to be solved more quckly. Method Daly Tracks Track Length Satelltes Daly Mnutes CGGTTS snglechannel CGGTTS typcal multchannel SIM max Table 1. Comparson of common-vew data formats. 18

19 Fgure 4. The SIM Real-Tme Measurement Grd. Country NMI Member of SIM Network Natonal Standard CURRENT AND FUTURE MEMBERSHIP As of September 008, 1 NMIs have been sent SIM measurement equpment, whch allows them to partcpate n the network. Four addtonal NMIs have formally expressed nterest n jonng the network, and wll be added when resources become avalable (Table ). Argentna INTI Yes Cesum Brazl ONRJ Yes Tme Scale Canada NRC Yes Tme Scale Chle INN Future Rubdum Colomba SIC Yes Cesum Costa Rca ICE Yes Cesum Guatemala LNM Yes Rubdum Jamaca BSJ Yes Cesum Mexco CENAM Yes Tme Scale Panama CENAMEP Yes Cesum Paraguay INTN Yes Rubdum Peru INDECOPI Future Rubdum St. Luca SLBS Future Rubdum Trndad / TTBS Future Rubdum Tobago Unted NIST Yes Tme Scale States Uruguay UTE Yes Cesum Table. Current and Future SIM Network Members. 19

20 A map of the SIM regon showng the current and known future members of the network s provded n Fgure 5. We antcpate that other SIM NMIs wll also be nterested n establshng a tme and frequency laboratory, and that addtonal requests to jon the network wll eventually be receved. As shown n Table, four SIM NMIs operate tme scales composed of an ensemble of cesum oscllators and/or hydrogen masers as ther natonal standard. Sx operate a sngle cesum oscllator. The remanng NMIs wll use rubdum oscllators, at least ntally, as ther natonal standard. We expect that many SIM NMIs wll upgrade ther tme and frequency standards and mprove ther measurement capabltes as more resources become avalable. Some laboratores that begn wth rubdum oscllators wll obtan a cesum oscllator, and then eventually obtan the multple cesum oscllators needed to buld an ensemble tme scale. Ths progresson has already begun. Three SIM laboratores have purchased cesum oscllators n 008: SIC n Colomba, INTI n Argentna, and ICE n Costa Rca. Fgure 5. A SIM map showng the locatons of the current (lght) and future members (dark) of the network. MEASUREMENT UNCERTAINTIES Estmatng the uncertantes of the SIM network measurements nvolves evaluatng both the Type A and Type B uncertantes as descrbed n the ISO standard [10]. Uncertantes are combned wth the root sum of squares method, where k s the coverage factor (Eq. ): U k U a U c b () To evaluate the Type A uncertanty, we use the tme devaton, ó x ( ), at an averagng tme of one day. The tme devaton [8] s a metrc calculated automatcally by our web-based software that estmates the amount of tme transfer nose. For most SIM baselnes, x() at 1 day s typcally about 1.5 ns. For the 471 km baselne between NIST and NRC, ó x ( ), at 1 day was less than 0.7 ns for the approxmate 8-month nterval shown n Fgure 6. The tme devaton wll probably never exceed 5 ns f each of the two laboratores nvolved n a gven comparson has a cesum oscllator (for comparsons nvolvng rubdum oscllators, ó x ( ), s lkely to be domnated by oscllator nose and can be much larger). Fgure 6. Tme devaton of NIST-NRC lnk for the perod from January through August, 007. To evaluate the Type B uncertanty, we have dentfed seven components that can potentally ntroduce systematc errors n the tme measurements between SIM standards. The Type B uncertantes are dscussed below and summarzed n Table 3. 0