Time and frequency activities at the U.S. Naval Observatory

The U.S. Naval Observatory (USNO) has provided timing for the Navy since 1830 and, via DoD Directive 4650.05, is the sole source of timing for the Department of Defense. In cooperation with other institutions, the USNO also provides timing for the United States and the international community. Its Master Clock (MC) is the source of UTC (USNO), USNO's realization of Coordinated Universal Time (UTC), which has stayed within 5 ns rms of UTC since 1999 and within 4 ns rms in 2010. The data used to generate UTC (USNO) are based upon 69 cesium and 26 hydrogen maser frequency standards in four buildings at two sites. USNO disseminates time via voice, telephone modem, Network Time Protocol (NTP), GPS, and Two-Way Satellite Time Transfer (TWSTT). To meet space limitations, this document will describe only the timescale algorithm and GPS time transfer precision. Further details and explanations of our services can be found online at http://www.usno.navy.mil/usno.