NIST Time Scales
NIST has two major time scales. The core scale, AT1, is a weighted average of commercial clocks, dominated by active hydrogenmasers. The mostused output is UTC(NIST), a realtime realization of the international standard, Universal Coordinated Time.
 AT1 produces a stable frequency from the twelve minute measurement cycle out to several months. AT1 is generated every twelve minutes as a weighted average of several commercial atomic frequency standards. Dominated by about six active hydrogen masers, there are a number of commercial cesium beamtube standards in the scale.
 UTC(NIST) is a realtime realization of the international standard for time, Universal Coordinated Time, a weighted postprocessed time scale. The time standard, UTC, is agreed by international treaty to be generated as a weighted average of clocks around the world. As such, it only exists after the fact. Any realtime version must be a prediction of what the defined value will be when it is published in circularT.

