654 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 48, NO. 2, APRIL 1999 Common-View Technique Applied to the Link CENAM–LAPEM V. Molina, M. G. L´ opez, A. A. Montonya, J. M. L´ opez, H. Jard´ on, and J. M. Figueroa Abstract— In order to improve the quality of its time and frequency services, the Mexican Power Electric Company (via its Test on Equipment and Materials Laboratory), requires trace- ability to Time and Frequency Mexican Primary Standards. With this purpose, the common-view (CV) technique, using a TTR- 6A GPS receiver, has been implemented between LAPEM and the Time and Frequency Division of CENAM. In this paper we present the results of this Mexican comparison of time standards between these two national laboratories. Index Terms— Clock comparison, common-view, frequency control, global positioning system (GPS), rubidium frequency standard, time and frequency dissemination. I. INTRODUCTION C OMMON-VIEW (CV) global positioning system (GPS) time transfer is a differential method that can reduce the effects of the selective availability (SA) in a straightforward manner [1]. Receivers in two locations simultaneously listen to the same satellite and fit arrival times relative to their local time scale. Data exchange allows the relative timing of two local time scales to be compared, removing any SA clock dither as common-mode noise [2]. The CV technique, however, requires identical receivers at both ends and an optimum view schedule, which may not be convenient to both participants [3]. This method of clock comparison, which is frequently done using a cesium, rubidium, or maser clock, can now reach an uncertainty of a few nanoseconds [4], [5]. The rubidium frequency standards are compact, have a good short-term stability, cost considerably less than a cesium clock, and are long-lived. But they must be periodically adjusted to maintain long-term frequency accuracy (of the order of weeks to months). Their frequency can be adjusted smoothly and precisely through the C-field with reference to a better standard. Some laboratories are using a rubidium clock as a standalone, free-running time and frequency reference [7]. However, the performance of these rubidium clocks can be improved when they are locked to a satellite time transfer system in a controlled environment. The results of the work Manuscript received July 2, 1998. V. Molina, M. G. L´ opez, J. M. L´ opez, and J. M. Figueroa are with the Centro Nacional de Metrolog´ ıa, CENAM, Divisi´ on de Tiempo y Frecuencia, Quer´ etaro C.P. 76900, Mexico (e-mail: vmolina@cenam.mx). A. A. Montonya is with the Laboratorio de Pruebas de Equipos y Materiales de Mexico de la Comisi´ on Federal de Electricidad, LAPEM, Guanajuato A.P. 612, Mexico (e-mail: amontoya@cfe.gob.mx). H. Jard´ on is with the Centro de Investigaci´ on y Estudios Avanzados del Instituto Polit´ ecnico Nacional, Col. S. P. Zacatenco C.P. 07000, Mexico (e- mail: hjardon@mvax1.red.cinvestav.mx). Publisher Item Identifier S 0018-9456(99)02871-5. described herein provide evidence of this improvement and in our case, constitute a means for LAPEM to eventually obtain traceability to CENAM using the GPS system and a rubidium frequency standard. The TTR-6/TTR-6A GPS [6] time transfer receivers are frequently used for realizing the CV time transfer. These receivers are precision timing systems that utilize the clear access, C/A, code transmitted by the NAVSTAR (GPS) satel- lites [1], [3]. With worldwide access to the UTC/GPS time scales transmitted using the C/A code, the TTR-6/6A, single satellite, receivers perform state-of-the-art time measurements. The TTR-6 can be used in CV mode wherein time comparison between remote sites tracking the same satellite cancels the ephemeris (position) errors and enables time coordination better than 5 ns. Two TTR-6 data output formats are available, called the NIST or BIPM format. The reason for these formats is to standardize the data making comparison for CV possible [8]. Both kinds of receivers, TTR-6/6A, were used in this work. Since March 1996, CENAM has contributed to UTC [9] using a TTR-6 receiver (serial number (S/N) 247) with the BIPM format, according to the BIPM tracking schedule for international time and frequency comparisons by GPS common views. Recently, CENAM bought a new TTR-6 (S/N 448), which was calibrated at NIST. On the other hand, the Mexican Power Electric Company (Comisi´ on Federal de Electricidad, CFE) via its Test on Equipment and Materials Laboratory (Laboratorio de Pruebas y Equipos de Mexico, LAPEM), has been using a TTR-6A (S/N 284) receiver like a secondary standard in its time and frequency calibration services. The TTR-6A contains an internal rubidium source that can be corrected by GPS satellite [3], [7] to average its frequency good to approximately . It also provides corrected pulse per second (PPS) outputs, which are steered by the satellite to about 500 ns relative to GPS time. With these characteristics a TTR-6A receiver can serve as a stand-alone frequency or timing reference standard. When properly applied, the CV technique is implemented with the same type of receiver at both ends. Nevertheless, in our case, LAPEM already has a TTR-6A receiver, which is not quite suitable for this purpose. Accordingly, in order to properly apply the CV technique to the LAPEM–CENAM link, it was necessary to program the BIPM tracking schedule for international comparisons into a TTR-6A receiver. The results of the CV time transfer technique applied to this link are presented. These results are compared with the ones obtained directly from the GPS, that is, LAPEM-GPS time, and those data previously obtained by LAPEM in the LAPEM–USNO link. 0018–9456/99$10.00  1999 IEEE Authorized licensed use limited to: CINVESTAV IPN. Downloaded on February 22,2010 at 09:55:20 EST from IEEE Xplore. Restrictions apply.