USNO (US Naval Observatory)
The United States Naval Observatory (USNO) provides a wide range of astronomical data and products, and serves as the official source of time for the U.S. Department of Defense and a standard of time for the entire United States.
USNO products support activities in the following areas:
Integrated Image and Catalog Archive Service, star catalogs, fundamental reference frames, space astrometry.
Astronomical data calculators (e.g., rise/set times, Moon phase, etc.), information on astronomical phenomena and related topics, astronomical and navigational almanacs, software products.
Daily, weekly, and long-term EO products (e.g. IERS Bulletin A), GPS-based products (rapids, ultra-rapids, UTGPS), VLBI-based products, background information, technical and scientific publications and software.
USNO Master Clock, Network Time Protocol (NTP) servers, web-based time synchronization, GPS timing products and services, Two-Way Satellite Time Transfer, and Loran-C timing products.
USNO's library holds one of the most complete collections of astronomical literature in the world.