This article explains how the National Institute of Standards and Technology (NIST) maintains accurate time standards, which are essential for global timekeeping. NIST uses a cesium fountain frequency standard called NIST-F1, which is more accurate than its predecessor, NIST-7. This standard helps provide precise time data to the International Bureau of Weights and Measures (BIPM), which coordinates global time standards. NIST generates a real-time time scale called AT1 using a combination of cesium standards and hydrogen masers, and adjusts it to align with Coordinated Universal Time (UTC) based on data from the BIPM.
UTC(NIST), the version of UTC maintained by NIST, is calculated using a formula that accounts for time and frequency offsets between AT1 and UTC. These adjustments are made regularly, usually on the first day of each month, to ensure accuracy. Leap seconds, which are added to UTC to account for Earth’s slowing rotation, are not applied to AT1. The article also lists historical data and references to scientific papers that describe the methods and accuracy of these timekeeping systems.
Keywords: frequency standards, cesium fountain, UTC(NIST)
