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This volume is the result of the dedicated effort undertaken by an international group of scientists and administrators, who have contemplated the challenge of the future of space-based earth science for the next decade. Recognizing the need for defining new milestones both in science and technology, they have developed a detailed report of what could be achieved and what challenges remain after twenty fertile years of space exploration. The reader will find a wealth of information about the role of space geodesy in the Earth Sciences of the 1990's.

In this review talk, I would like to report on the proper motion analysis, which has been recently carried out together with M. Soma and M. Yoshizawa: There has been a persistent demand in astronomy for accurate stellar positions and proper motions, which are represented by an inertial reference system constructed on the basis of a set of consistent astronomical constants. In the reference system the precessional constant plays a primary role. In a series of papers Fricke (1967a, b, 1977a, b) has deter mined the luni-solar precessional correction to Newcomb's value and the fictitious motion...

The following topics are addressed: space astrometric missions, ground based optical techniques, very long baseline interferometric (VLBI) techniques, impact of these techniques on astrophysics and on geodynamics, and issues related to celestial and terrestrial reference frames. In the area of space optical astrometry, the reduction of one year of data from the Hipparcos mission shows excellent results for more than 40,000 stars. Recent developments in ground-based optical astrometry are concentrated on CCB meridian circles and transit instruments. The VLBI technique is the most powerful...