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Precise Time DepartmentReifler precision pendulum clocks, c.1910

The U.S. Naval Observatory has maintained a Time Service Department since 1880.  At right are a pair of Reifler precision pendulum clocks, photographed in their underground vault c. 1910. 

Today, USNO's Precise Time Department is charged with maintaining the DoD reference for Precise Time and Time Interval (PTTI).  That reference is UTC(USNO).  This time-scale is determined by an ensemble of dozens of atomic frequency standards, popularly called "atomic clocks", which are continuously scanned, selected, and averaged by computer to provide a day-to-day precision measured at the picosecond (10-12 seconds) level per day.  


USNO Master Clock

The DoD common time reference is the U.S. Naval Observatory Master Clock.  It is generated at USNO in Washington, D.C. and at the Alternate Master Clock Facility at Schriever Space Force Base in Colorado.


USNO monitors the Global Positioning System (GPS) constellation and provides system timing offsets to the U.S. Space Force 2nd Space Operations Squadron (2SOPS), timing data for individual GPS satellites, and time transfer services using GPS.


USNO provides authoritative network time synchronization via the Network Time Protocol (NTP).

Two-way satellite time transfer (TWSTT)

The highest precision and accuracy in time dissemination is provided through Two-Way Satellite Time Transfer (TWSTT).  USNO provides operational time transfer and calibration services for TWSTT.

Telephone Time

USNO provides both voice announcements of the time, and services to synchronize systems over telephone modems.


USNO is in the forefront of developing robust, continuously running atomic frequency standards.  Our Clock Development Division has designed, built, and deployed six rubidium fountain clocks which have been operating continuously since 2013.  The team is now investigating optical frequency standards, which will operate at frequencies some five orders of magnitude higher than the current microwave standards.


Commander, Naval Meteorology and Oceanography Command | 1100 Balch Blvd. | Stennis Space Center, Mississippi 39529

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