The United States Naval Observatory Master Clock is the official precise time reference for the Department of Defense. As of 2015, the Master Clock is a sophisticated atomic clock system used for navigation, satellites, intelligence and communication. The clock expresses time as Coordinated Universal Time.
The Naval Observatory's Master Clock includes hydrogen masers that use the vibration of hydrogen atoms to calculate a time scale. This method has high short-term stability and accuracy. Due to the sensitive nature of the equipment, the Master Clock system is kept under temperature control and magnetic shielding. In order to keep the precise atomic clock in sync with normal time, authorities occasionally add a "leap second" to compensate for slight variations in the Earth's rotation.