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 method to determine the earth rotation parameters, nutation series, and baseline lengths. More accurate celestial and terrestrial reference frames are needed and recent achievements in this field are presented, for example FK5 catalogues and a comparison of VLA and Hipparcos positions.

This book on reference systems is the first comprehensive review of the problem of celestial and terrestrial reference systems and frames. Over 20 years, the importance of this problem emerged slowly as the accuracy of new observational techniques improved. The topic has already been approached in several symposia such as Stresa (1967), Morioka (1971), Perth (1973), Columbus (1975, 1978 and 1985), Kiev (1977) and San Fernando (1978). Two IAU colloquia held in Turin (1974) and in Warsaw (1980) were exclusively devoted to discuss reference systems. During this time, the problem of terrestrial and celestial reference systems has been discussed also in many astronomical and geodetic symposia, but always among other topics. Thus, a review devoted solely to the definition and practical realization of such systems was needed. It is hoped that this book, containing modern comprehensive reviews of important facets of this problem will contribute not only to a better and wider understanding of the mathematics and the physics that are behind the concepts and the realizations, but also to future development in a field that can only expand with the rapidly increasing accuracy of geodetic and astronomical observations. We are pleased to thank all the authors of the book who have enthusiastically agreed to contribute to the book in their field of competence and have gracefully accepted guidance from the editors in the definition of the subject and of the interfaces with other chapters. We thank Prof. Y.

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 of the equinox, which have been adopted in the "IAU (1976) System of Astronomical Con stants." Based on the precessional correction and the equinoctial motion thus established, the fundamental reference system, the FK5 system (Fricke et al. 1988) for positions and proper motions, has been constructed. However, for several years geodetic VLBI (McCarthy & Luzum 1991) and LLR (Williams et at. 1991) observations have been suggesting an additional correction to the luni-solar precessional constant of the IAU (1976) System. That is, these observations indicate the precessional correction of 6. p -0: '30/cent to the FK5 system. But, the observational period ofthe earth orientation is considered to be still insufficient to separate unambiguously the precessional change of the earth orientation from the nutation with the longest period of 18. 6 years."

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.