NIST Time Scale Data ArchiveNotes on NIST time scale, primary standards and services Return to Archive index 2003 2004 2005 2006 2007 2008 2009 2011 2012 2013 Current Year
Primary frequency standards developed and maintained by NIST are used to provide accuracy (rate) input to the BIPM. NISTF1, a cesium fountain frequency standard, serves as the United States primary frequency standard. The uncertainty of NISTF1 is less than 1 part in 10^{15}. The AT1 scale is run in real time using data from an ensemble of cesium standards and hydrogen masers. It is a freerunning scale whose frequency is maintained as constant as possible by choosing the optimum weight for each clock that contributes to the computation. UTC(NIST) is generated as an offset from our realtime scale AT1. It is steered in frequency towards UTC using data published by the BIPM in its Circular T. Changes in the steering frequency are usually made at 0000 UTC on the first day of any month, and the change in frequency in any month is limited to ±2 ns/day. The frequency of UTC(NIST) is kept as stable as possible at other times. UTC is generated at the BIPM using a postprocessed timescale algorithm and is not available in realtime. The parameters that we use to generate UTC(NIST) in realtime are therefore based on an extrapolation of UTC from the most recent data available. The table lists the parameters that are used to define UTC(NIST) with respect to our realtime scale AT1. To find the value of UTC(NIST)  AT1 at any time T (expressed as a Modified Julian Day, including a fraction if needed), the appropriate equation to use is the one for which the desired T is greater than or equal to the entry in the T_{0} column and less than the entry in the last column. The values of x_{ls}, x, and y for that month are then used in the equation below to find the desired value. The parameters x and y represent the offset in time and in frequency, respectively, between UTC(NIST) and AT1; the parameter x_{ls} is the number of leap seconds applied to both UTC(NIST) and UTC as specified by the IERS. Leap seconds are not applied to AT1.
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