NIST operates two secondary UTC(NIST) time scales at locations outside of its Boulder, Colorado campus. The first is located in Fort Collins, Colorado at the site of NIST radio stations WWV and WWVB. The second is located at the NIST headquarters in Gaithersburg, Maryland. In a hypothetical scenario of a complete failure of the Primary, Alternate, and Contingency time scales at the Boulder, Colorado site, NIST can continue to keep time at the sub-microsecond level from the two secondary time scales, as described in the following sections.
Secondary UTC(NIST) Time Scale in Fort Collins, Colorado - NIST has operated a secondary time scale at Fort Collins, Colorado since 2007. The Fort Collins site is the home of radio stations WWV and WWVB, which distribute UTC(NIST) signals to many millions of users, and also houses a node of NIST’s popular Internet Time Service. The Fort Collins time scale is the reference for each of these services and is typically controlled to be within ±20 ns of the primary UTC(NIST) time scale in Boulder.
The operation of the Fort Collins time scale is very similar to the Primary time scale in Boulder, but the clock ensemble is much smaller and does not currently include any hydrogen masers. Two redundant time scale multi-channel measurement systems (MCMS) measure an ensemble of five cesium atomic clocks every 12 minutes, and an external synthesizer is programmed to steer the output of an atomic clock to realize the UTC(NIST) secondary time scale.
Secondary UTC(NIST) Time Scale in Gaithersburg, Maryland - The NIST campus in Gaithersburg, Maryland houses a node of NIST’s Internet Time Service and the fiber-based remote time transfer calibration service. A small local time scale, consisting of two cesium clock, provides the reference for these services. No MCMS or external synthesizers are used, instead a time interval counter (TIC) monitors the time-difference between the two clocks PPS output, and a second TIC measures the difference between one clock and a GPS common-view receiver that is locked to the primary time scale in Boulder. Occasional (approximately monthly) frequency corrections are programmed directly to the primary clock with the goal of coordinating its time output to within ±20 ns of UTC(NIST). Similar, frequency corrections are applied to the alternate clock.
UTC(NIST) Distribution - When UTC(NIST) signals are distributed to users, any uncompensated signal delay between the UTC(NIST) reference plane and the user will contribute to the accuracy of the time received by the user. The accuracy of UTC(NIST) distribution systems ranges from picoseconds, for signals distributed within the NIST Boulder laboratories, to nanoseconds for NIST distribution systems such as Two-Way Satellite Time Transfer or the Time Measurement and Analysis Service (TMAS), to hundreds of microseconds or milliseconds for radio station WWVB, to milliseconds for the Internet Time Service (ITS), radio stations WWV and WWVH, and to tens or hundreds of milliseconds for the Telephone Time of Day Service or the time.gov web clock. All of these systems are referenced to the UTC(NIST) time scale, and at their various levels of accuracy, they distribute the official time for the United States to users around the nation and around the world.