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Variations in the Earth's speed of rotation and the position and orientation of the rotation axis—known collectively as Earth orientation parameters—are observed by a variety of space geodetic techniques coordinated by the International Earth Rotation Service (IERS). Observations of Earth orientation parameters provide fundamental information about the physical properties of the Earth's interior and the variability of fluids in the atmosphere, oceans, and liquid core, and on land.

Participants at a workshop held October 14–18, 1996, at the Paris Observatory, where IERS is headquartered, called for extending IERS activities [Reigber and Feissel, 1997] to include the monitoring of geodetically important properties of the oceans, the liquid core, and terrestrial water to supplement current atmospheric angular momentum variation (AAM) measurements. Given the success of the IERS, expansion of its activities into other areas is not only natural for understanding the observed Earth orientation parameters, but essential for fulfilling its mission of improving the underlying space geodetic observations.