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 AFB in Colorado.


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

Computer Display Clocks

USNO servers provide a wide variety of web-based time synchronization products including embedded web clocks to display UTC(USNO) on other web pages.

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.

network time protocol (ntp)

Network Time Protocol (NTP) is an Internet standard that enables client computers to synchronize to USNO.  NTP runs as a client program on a computer.


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.  |  |  Navy FOIA  |  DoD Accessibility/Section 508  |  No Fear Act  |  Open Government  |  Plain Writing Act  |  Veterans Crisis Line  |  DoD Safe Helpline  |  Navy SAPR  |  NCIS Tips  |  Privacy Policy  |  Contact Webmaster
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