NIST-F2

NIST physicists Steve Jefferts (foreground) and Tom Heavner with the NIST-F2 cesium fountain atomic clock, a civilian time standard for the United States.

NIST-F2 is a caesium fountain atomic clock that, along with NIST-F1, serves as the United States' primary time and frequency standard.[1] NIST-F2 was brought online on 3 April 2014.[1][2]