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The long-term accuracy achievable by modern cesium atomic clocks (the most common type) is better than one second per one million years. Hydrogen atomic clocks show a better short-term (one week) accuracy, approximately 10 times the accuracy of cesium atomic clocks. Therefore, the atomic clocks have increased the accuracy of time measurement about one million times in comparison with the measurements carried out by means of astronomical techniques.
On the basis of this computed time a clock reference system can be steered to produce clock signals which serve as the USNO Master Clock. The clock reference system is driven by a hydrogen maser atomic clock. Hydrogen masers are extremely stable clocks over short time periods (less than one week). They provide the stability, reliability and accessibility needed to maintain the accuracy of the Master Clock system.
Prize motivation: "for the invention of the separated oscillatory fields method and its use in the hydrogen maser and other atomic clocks.