The Panel on a Global Network of Fiducial Sites was formed by the Committee on Geodesy, National Research Council, with the encouragement of its supporting agencies, to address specific issues, including:

1. evaluation of the scientific importance of a proposed global network of fiducial sites, monitored very precisely, using a combination of surface and space-geodetic techniques;

2. examination of strategies for implementing and operating such a network, in light of the anticipated scientific return, building on existing capabilities whenever possible; and

3. assessment of whether such a network would provide a suitable global infrastructure for geodetic and other geophysical systems of the next century.

The charge to the Panel developed from the rapid growth of space-geodetic systems with a global distribution of stations, including the Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) network deployed in the 1980s; the French DORIS and the German PRARE systems; and the Global Positioning System (GPS) networks, such as the Cooperative International GPS Network (CIGNET), and the proposed Fiducial Laboratories for an International Natural science Network (FLINN). Thus, various countries and a number of national and international organizations have embarked on the development of global observing programs, with varied life spans and objectives. The scientific implications of such a collection of global networks transcend the objectives of any single organization or even any single country. The potential benefits to be derived from an international, multidisciplinary research effort with a well-defined infrastructure are sufficiently important to warrant global support and participation.

The scientific aim should be the establishment of a globally distributed network of fiducial stations that would include a core of ground-based observatories to study the Earth. The dual scientific goals of such a global geodetic and

geophysical fiducial network would be (1) to improve our understanding of geophysical and geological phenomena that operate on a global scale, and (2) to provide a reference framework and boundary conditions for analyzing the phenomena that operate on smaller scales.

Therefore, the Panel opted to broaden the initial concept and to consider how a mix of space-based geodetic techniques, expanded to include other geophysical techniques, would best contribute to the solution of fundamental geodetic and geophysical problems with a global scope. Such a broad approach is not only desirable but also essential if we are to realize the full potential of global networks for solid Earth research. The scope of the tasks imposes on the scientific community an obligation to consider how a global network of fiducial sites can be built on existing structures.

Coordination of space-geodetic global deployments with ongoing global activities in other disciplines, such as global seismology, geomagnetism, gravimetry, and absolute gravity, offers exciting possibilities for comprehensive studies of the Earth, although compatibility issues remain to be resolved. An initial complement of instruments, modified as needs are identified, should be proposed by the geophysical and geodetic science communities and a determination made as to which instruments should be installed at individual sites.

The Panel met four times during 1990. In 1991 it was recognized that a broader range of expertise was needed to deal with many of the issues raised by this study. Therefore, the Committee on Geodesy joined with the Panel in preparing the final report. The members of the Panel and the Committee represent users of various geodetic techniques and many geophysical sciences such as gravimetry, seismology, geomagnetism, tectonophysics, and oceanography. This combined Panel and Committee represent a cross section of the diverse technical and scientific interests considered in the report. In addition, the Panel especially appreciates the assistance of a number of scientists, particularly, Professor Richard J. O'Connell, Harvard University, for his early discussion on issues relevant to this report; Professor Michael Bevis, North Carolina State University, for his discussions on the issues of sea-, land-, and ice-level changes; and Drs. Oscar Colombo, Werner Gurtner, Ruth Neilan, and Stephen Lichten for their many discussions of various issues raised in this report. A substantial fraction of the material on the International GPS Geodynamics Service proposed to the International Association of Geodesy was prepared by members of this Panel; not surprisingly, this report incorporates much of the same material. The Panel also drew from the International Earth Rotation Service (IERS) annual reports and associated publications. However, responsibility for the final document rests exclusively with the Panel and the Committee.

The Panel expresses its appreciation to the staff of the Board on Earth Sciences and Resources, who provided support for its activities, and to the agencies that provided funding for this study.

J. Bernard Minster, Chairman

Panel on a Global Network of Fiducial Sites