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U.S. Naval Observatory Earth Orientation Department

IERS Bulletin A

The Earth Orientation Department of the U.S. Naval Observatory determines and predicts the time-varying alignment of the Earth's terrestrial reference frame with respect to the celestial reference frame. The Earth Orientation Parameters (EOP) determined are the coordinates of the Earth's pole (polar motion), the rotation angle about the pole (Universal Time or UT1), precession, and nutation. Observational results are published regularly in the IERS Bulletin A. For further information, please refer to the Bulletin A web site.

Accurate knowledge of the EOP values is needed for a variety of high-precision applications (both DoD and civilian) including navigation, astronomy, geodesy, communications, and time-keeping. In addition, the EOP variations provide subtle information on the nature of the Earth system, including the bulk properties of the Earth's interior and the interactions of the solid Earth with geophysical fluids (atmosphere, oceans, surface hydrology, etc).

The programs of the Earth Orientation Department are inherently global in scope, requiring close cooperation with a variety of other agencies, both national and international. The department serves as the Rapid Service/Prediction Center for the International Earth Rotation and Reference Systems Service (IERS), providing EOP observations and predictions on a rapid turnaround basis. This service is primarily intended for real-time users and others needing the highest quality EOP information shortly after real-time.

As a participant in the National Earth Orientation Service, the department also cooperates closely with other U.S. government agencies to coordinate and collect Very Long Baseline Interferometry (VLBI) data essential for monitoring EOP variations. Closely related to this, the department participates as an Operations Center, Correlator, and supporter of observing stations within the International VLBI Service. The department is also an Associate Analysis Center of the International GPS Service, providing GPS satellite orbits and related information.

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Last modified: 31 July 2009 Approved by EO Dept. Head, USNO