Synchronized Accurate Time


The U.S. Naval Observatory Alternate Master Clock at Schriever Air Force Base.  It is used to check the time being distributed by the Global Positioning Satellites.  The clock here showed 23:59:60 Saturday, January 1, 2006, reflecting the addition of the first leap second in seven years.
In the 1990s, this instrument was the most accurate clock in the country and helped keep the GPS clocks synchronized. Built and operated at the National Institute of Standards and Technology in Boulder, Colorado, it served as the primary frequency standard for the United States—that is, it determined the length of a second of time.