The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce Alberts is president of the National Academy of Sciences.

The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. William A. Wulf is president of the National Academy of Engineering.

The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Kenneth I. Shine is president of the Institute of Medicine.

The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce Alberts and Dr. William A. Wulf are chairman and vice chairman, respectively, of the National Research Council.

For the past three decades, it has been possible to measure the Earth's static gravity field from satellites. Such measurements have been used to address many important scientific problems, including the internal structure of the Earth and geologically slow processes such as mantle convection. In recent years, improved accuracy of satellite gravity measurements has made it possible to resolve the time-varying component of the global gravity field. These temporal variations are caused by dynamic processes that change the mass distribution in the Earth, oceans, and atmosphere, and could potentially be used to study a new class of important scientific problems.

The Committee on Earth Gravity from Space was formed under the auspices of the National Research Council's U.S. Geodynamics Committee to undertake the following tasks:

1. evaluate the potential for using satellite technologies to measure the time-varying component of the gravity field;

2. assess the utility of an improved global gravity field in general, and the time-varying component in particular, for addressing problems of interest to the earth science, natural hazards, and resource communities; and

3. determine what complementary data are needed to increase the usefulness of satellite-derived gravity data.

With regard to the first charge, the committee reviewed approximately a dozen mission concepts that were planned or envisioned by investigators in the United States and Europe. We grouped the missions into broad categories and developed a "generic" mission scenario for each category. In the course of our investigation it became clear that future technological refinements might yield more information than is currently feasible, so we expanded our list of generic mission scenarios to include future possibilities as well as current ones.

We investigated the trade-offs in orbit design that affect the spatial and temporal resolution of the mission, and chose the altitudes for our generic missions. Mission resolutions were estimated assuming a white-noise error source in the measurements and an isotropic distribution of errors over the Earth. These assumptions simplified the committee's work and left the results generic and not tied to a specific mission proposal. The white noise levels we chose were calibrated initially so that the results corresponded closely with more detailed mission simulations in the published literature (reviewed in Chapter 2). We then adjusted the noise levels to reflect what we thought was currently feasible or likely from future technology,

and asked the scientists and engineers involved in the design of current missions to review our estimates. Based on that review, we made some refinements in the white noise levels and added recommendations about ancillary acceleration and tracking data. We believe that this process has yielded results that are sufficiently generic to be broadly useful, but that also accurately reflect the capability of current technology.

The committee addressed its second and third charges by focusing on two classes of scientific problems: (1) fields of study that would be significantly advanced by a dedicated satellite gravity mission, and (2) fields that would benefit from improved gravity data but that would also require ancillary data to interpret the results. The committee also considered phenomena that move mass, but that are not currently amenable to study with satellite gravity data because of small size, high speed, or difficulty in isolating the relevant geophysical signal. A large number of natural hazards fall into this category. With a few exceptions, these phenomena are not discussed in this report

Because relatively little has been written on the application of the time-varying component of the gravity field to scientific problems, it was necessary for the committee to conduct a substantial amount of original research and to develop forward models. I would like to extend my appreciation to the committee, which undertook the demands of this study with collegial enthusiasm and a remarkable ability to work across disciplines. Each member made an important and unique contribution to the report Special thanks go to Richard Eanes, who had the time-consuming and difficult task of generalizing a dozen proposed or envisioned gravity missions into generic mission classes, and John Wahr, who modeled (and remodeled) all the time-varying phenomena discussed in this report. Although John had to leave the committee in December 1996 to honor a previous commitment, he continued, as a consultant, to complete the final model runs on behalf of the committee. The committee also expresses it sincere appreciation to the staff of the Board on Earth Sciences and Resources, and particularly to Anne Linn, Study Director of this project, for a job superbly done.

Finally, I would like to thank the individuals who made presentations to the committee or who provided input to the study. They include Knut Aagaard, Peter Bender, Srinivas Bettadpur, Bruce Bills, David Bromwich, Frank Bryan, William F. Budd, Steve Castles, Ben Chao, T.C. Chen, Tom Clark, Oscar Colombo, Ab Davis, Mike DiPirro, Danon Dong, Mario B. Giovinetto, Arnold Gordon, Brad Hager, Gordon Hamilton, K.H. Ilk, Yu Jin, Masao Kanamitsu, Günther Können, Steve Marcus, Marcia McNutt, Mark Meier, Bill Melbourne, P.C.D. Milly, Mery Molenaar, RS. Nerem, Lauri Newman, Denis O'Brien, Ho Jung Paik, Erricos Pavlis, Dick Rapp, Mark Richards, Rick Rosen, Suranjana Saha, Jae Schemm, Peter Shirron, Dave Smith, Detlef Stammer, Court Stevenson, Byron Tapley, Jim Titus, Kevin Trenberth, Michael Watkins, Raymond Willemann, and Victor Zlotnicki.