The U.S. Naval Observatory Alternate Master Clock (USNO AMC) is located at Schriever Air Force Base near Colorado Springs, Colorado, on the same site as the Global Positioning System (GPS) Master Control Station.

The USNO AMC is a facility which was designed and implemented to better fulfill our responsibilities for providing the best quality and most precise time to the Department of Defense and the nation. The USNO AMC is capable of independent performance of the basic USNO Washington, DC, timing mission in any event requiring it to perform autonomously. The stated requirement at full operational status is to keep the USNO AMC within 2 to 3 nanoseconds of our primary Master Clock at the USNO in Washington, DC. The full operational specifications have been fulfilled since MJD 50250 (15 June 1996).

This installation houses three hydrogen masers , twelve cesium-beam frequency standards , their associated measurement and distribution systems, and several GPS time transfer receiver systems. Although the purpose of the AMC is to provide precise time in the event of a catastrophic failure of the USNO Master Clock in Washington, DC, its role has expanded to include providing precise time to the Global Positioning System operated from Schriever AFB.

Synchronization to the Washington Master Clock, at the nanosecond (billionth of a second) level, is achieved via Two Way Satellite Time Transfer.

The AMC is also providing time over the Internet using Network Time Protocol, over modems, and by telephone voice announcer.

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