Automated Clock Comparisons and Time Scale Generation in the SIM Region
Michael A. Lombardi, J. Mauricio Lopez-Romero, Nelida Diaz-Munoz
The second, the base unit of time interval in the International System, is defined in terms of the two hyperfine states of the Cesium atom ground energy level. This definition has so far served the metrology community well, and the uncertainty of the best realization of the second has improved by a rate of about one order of magnitude per decade over the past 50 years, reaching a current level of a few parts in 1016.  This continual reduction in uncertainty has increased the level of performance expected from both time and frequency transfer systems and from the time standards maintained by national metrology institutes (NMIs). During recent years, an automated time comparison network has been developed within the Sistema Interamericano de Metrologia (SIM), a regional metrology organization. The SIM Time Network (SIMTN) allows NMIs to compare their time scales via the Global Positioning System (GPS) common-view and all-in-view time transfer techniques, and makes results available through the Internet in near real time.  The SIMTN has proven to be robust and reliable, and the uncertainty of its comparisons is similar to the uncertainty of the key comparisons published by the Bureau International des Poids et Mesures (BIPM) in its monthly Circular T. The large number of geographically dispersed clocks measured by the SIMTN made it attractive to develop a SIM time scale (SIMT), which is computed in near real time and immediately made available to the general public via the Internet. This rapid computation allows contributing laboratories to easily monitor their time scales, and to quickly detect short term fluctuations in stability and accuracy. This paper discusses both the SIMTN and the SIMT, primarily focusing on the SIMT algorithm and the results of its performance.