In 1996 the U.S. Naval Observatory Alternate Master Clock (USNO AMC) was installed and declared operational at the GPS Master Control Station (GPS MCS) located at Schriever AFB, CO. An ensemble of cesium-beam and hydrogen-maser clocks along with their associated measurement and distribution systems were installed at the Colorado facility. 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 and frequency to the DoD, the nation, and the world. The USNO AMC is capable of independent performance of the basic USNO Washington, DC time and frequency mission in the event of a catastrophic failure. The stated requirement at full operational status is to keep the USNO AMC MC to within 2 to 3 nanoseconds of the primary USNO Master Clock (USNO MC), located in Washington, D.C., at all times. The full operational status was reached and has been fulfilled since MJD 50250 (15 June 1996). TWSTT plays a key, and fundamental, and important role in the resulting quality, accuracy, and precise timee and frequency performance of the distributed components of the USNO MC. TWSTT is the only real-time remote time comparison system which will allow the 2 to 3 nanosecond performance to be met. Automated TWSTT experiments and associated data processing are being performed every hour comparing directly. In general, the scatter of a single point at the fundamental sampling time of one hour is +/- 250 picoseconds. Figure 1, updated occasionally, shows the hourly measured time differences between the two MCs.
Figure 1. UTC time differences between Master Clock #2 and the AMC's Master Clock #1 via TWSTT. USNO AMC (MC #1) is steered to USNO (MC #2) to maintain close synchronization between the two clocks and UTC.