English: The US Naval Observatory (USNO) Alternate Master Clock at Schriever Air Force Base, Colorado. This serves as an alternate to the master clock ensemble at USNO, Washington, D.C., to determine the time standard for the United States Department of Defense, and also contributes to the international atomic time standard UTC. It consists of an ensemble of caesium beam atomic clocks which are all compared with each other to keep them synchronized. It serves as a Stratum 0 time server for the Network Time Protocol (NTP).