The United States Naval Observatory (USNO) is one of the oldest scientific agencies in the United States, with a primary mission to produce Positioning, Navigation and Timing (PNT) for the U.S. Navy and the U.S. Department of Defense. Located in Northwest Washington, D.C. at the Northwestern end of Embassy Row, it is one of the pre-1900 astronomical observatories located in an urban area; at the time of its construction, it was far from the light pollution thrown off by the (then-smaller) city center. The USNO operates the "Master Clock", which provides precise time to the GPS satellite constellation run by the U.S. Air Force.