page_title: United States Naval Observatory
section_title: Time service
hierarchical_section_title: United States Naval Observatory / Time service
caption_attribution_description: English: US Naval Observatory's (USNO) Master Clock Ensemble, Washington D.C., made up of a large number of Symmetricom 5071A cesium beam clocks (rack in background). This provides the time standard for the US military. The large black object in the foreground is a Symmetricom (formerly Sigma-Tau) MHM-2010 active hydrogen maser.
context_page_description: The United States Naval Observatory is one of the oldest scientific agencies in the United States, with a primary mission to produce Positioning, Navigation and Timing for the United States Navy and the United States Department of Defense. Located in Northwest Washington, D.C. at the Northwestern end of Embassy Row, it is one of the pre-1900 astronomical observatories located in an urban area; at the time of its construction, it was far from the light pollution thrown off by the city center. Former USNO director Gernot M. R. Winkler initiated the "Master Clock" service that the USNO still operates, and which provides precise time to the GPS satellite constellation run by the United States Air Force. The USNO performs radio VLBI-based positions of quasars with numerous global collaborators, in order to produce Earth Orientation parameters.
  Aside from its scientific mission, a house located within the Naval Observatory complex serves as the official residence of the Vice President of the United States.
context_section_description: The U.S. Naval Observatory operates two Master Clock facilities.  The primary facility, in Washington, D.C. maintains 57 HP/Agilent/Symmetricom 5071A-001 high performance cesium atomic clocks and 24 hydrogen masers.  The alternate master clock, at Schriever Air Force Base, maintains 12 cesium clocks and 3 masers.  The observatory also operates four rubidium atomic fountain clocks, which have a stability reaching 7×10⁻¹⁶. The observatory intends to build several more of this type for use at its two facilities. The clocks used for the USNO timescale are kept in 19 environmental chambers, whose temperatures are kept constant to within 0.1 degree C and whose relative humidities (for all masers and most cesiums) are kept constant to within 1%. The timescale is based only upon the Washington DC clocks. On June 7, 2007, 70 standards were weighted in the timescale computations.
  The U.S. Naval Observatory provides public time service via 26 NTP servers on the public Internet, and via telephone voice announcements:
  +1 202 762-1401 (Washington, DC)
  +1 202 762-1069 (Washington, DC)
  +1 719 567-6742 (Colorado Springs, CO)
  The voice of actor Fred Covington (1928–1993) has been announcing the USNO time since 1978.
  The voice announcements follow the same pattern at both sites.  They always begin with the local time (daylight or standard), and include a background of 1-second ticks. Local time announcements are made on the minute, and 15, 30, and 45 seconds after the minute. Coordinated Universal Time (UTC) is announced five seconds after the local time. Upon connecting, only the second-marking ticks are heard for the few seconds before the next scheduled local time announcement
  The USNO also operates a modem time service, and provides time to the Global Positioning System.
caption: Atomic clock ensemble at the U.S. Naval Observatory
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