NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.
This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of members of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine.
This study was supported by Grant No. NAG 5-3105 between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the organizations or agencies that provided support for this project.
Library of Congress Catalog Card Number 97–67450
International Standard Book Number 0-309-05792-2
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Cover art by Walter Smith, National Oceanic and Atmospheric Administration.
The cover is a color shaded relief image of the Earth's gravity anomaly field. Warm colors (yellow-orange-red) indicate stronger than normal gravity, whereas cool colors (cyan-blue-violet) indicate weaker than normal gravity. Highlights are illuminated from the northwest. The image combines data from EGM96 over land areas with data derived from satellite altimetry (Smith and Sandwell, 1995a) over ocean areas.
Copyright 1997 by the National Academy of Sciences. All rights reserved.
Printed in the United States of America
First Printing, September 1997
Second Printing, March 1998
COMMITTEE ON EARTH GRAVITY FROM SPACE
JEAN O. DICKEY, Chair,
Jet Propulsion Laboratory/California Institute of Technology, Pasadena
CHARLES R. BENTLEY,
University of Wisconsin, Madison
ROGER BILHAM,
University of Colorado, Boulder
JAMES A. CARTON,
University of Maryland, College Park
RICHARD J. EANES,
University of Texas, Austin
THOMAS A. HERRING,
Massachusetts Institute of Technology, Cambridge
WILLIAM M. KAULA,
University of California, Los Angeles
GARY S. E. LAGERLOEF,
Earth and Space Research, Seattle, Washington
STUART ROJSTACZER,
Duke University, Durham, North Carolina
WALTER H. F. SMITH,
NOAA Geosciences Laboratory, Silver Spring, Maryland
HUGO M. VAN DEN DOOL,
NOAA Climate Prediction Center, Camp Springs, Maryland
JOHN M. WAHR,
* University of Colorado, Boulder
MARIA T. ZUBER,
Massachusetts Institute of Technology, Cambridge
Staff
ANNE M. LINN, Senior Program Officer
VERNA J. BOWEN, Administrative Assistant
U.S. GEODYNAMICS COMMITTEE
BRADFORD H. HAGER, Chair,
Massachusetts Institute of Technology, Cambridge
DON L. ANDERSON,
California Institute of Technology, Pasadena
RICHARD CARLSON,
Carnegie Institution of Washington, D.C.
THURE CERLING,
University of Utah, Salt Lake City
RICHARD S. FISKE,
Smithsonian Institution, Washington, D.C.
GRANT GARVEN,
The Johns Hopkins University, Baltimore, Maryland
GARY A. GLATZMAIER,
Los Alamos National Laboratory, Los Alamos, New Mexico
RAYMOND JEANLOZ,
University of California, Berkeley
KENNETH C. MACDONALD,
University of California, Santa Barbara
ELIZABETH L. MILLER,
Stanford University, California
HENRY N. POLLACK,
University of Michigan, Ann Arbor
MARY LOU C. ZOBACK,
U.S. Geological Survey, Menlo Park, California
Staff
CHARLES MEADE, Senior Program Officer
VERNA J. BOWEN, Administrative Assistant
BOARD ON EARTH SCIENCES AND RESOURCES
J. FREEMAN GILBERT, Chair,
University of California, San Diego
MARK P. CLOOS,
University of Texas, Austin
JOEL DARMSTADTER,
Resources for the Future, Washington, D.C.
KENNETH I. DAUGHERTY,
E-Systems, Fairfax, Virginia
NORMAN H. FOSTER,
Independent Petroleum Geologist, Denver, Colorado
CHARLES G. GROAT,
University of Texas, El Paso
DONALD C. HANEY,
University of Kentucky, Lexington
RAYMOND JEANLOZ,
University of California, Berkeley
SUSAN M. KIDWELL,
University of Chicago, Illinois
SUSAN KIEFFER,
Kieffer & Woo, Inc., Palgrave, Ontario
PHILIP E. LAMOREAUX,
P.E. LaMoreaux and Associates, Inc., Tuscaloosa, Alabama
SUSAN M. LANDON,
Thomasson Partner Associates, Denver, Colorado
J. BERNARD MINSTER,
University of California, San Diego
ALEXANDRA NAVROTSKY,
Princeton University, New Jersey
JILL D. PASTERIS,
Washington University, St. Louis, Missouri
EDWARD C. ROY, JR.,
Trinity University, San Antonio, Texas
EDWARD M. STOLPER,
California Institute of Technology, Pasadena
MILTON H. WARD,
Cyprus Amax Minerals Company, Englewood, California
Staff
CRAIG M. SCHIFFRIES, Director
THOMAS M. USSELMAN, Associate Director
WILLIAM E. BENSON, Senior Program Officer
ANNE M. LINN, Senior Program Officer
CHARLES MEADE, Senior Program Officer
LALLY A. ANDERSON, Staff Associate
VERNA J. BOWEN, Administrative Assistant
JENNIFER T. ESTEP, Administrative Assistant
JUDITH L. ESTEP, Administrative Assistant
COMMISSION ON GEOSCIENCES, ENVIRONMENT, AND RESOURCES
GEORGE M. HORNBERGER, Chair,
University of Virginia, Charlottesville
PATRICK R. ATKINS,
Aluminum Company of America, Pittsburgh, Pennsylvania
JAMES P. BRUCE,
Canadian Climate Program Board, Ottawa, Ontario
WILLIAM L. FISHER,
University of Texas, Austin
JERRY F. FRANKLIN,
University of Washington, Seattle
THOMAS E. GRAEDEL,
Yale University, New Haven, Connecticut
DEBRA KNOPMAN,
Progressive Foundation, Washington, D.C.
KAI N. LEE,
Williams College, Williamstown, Massachusetts
PERRY L. McCARTY,
Stanford University, California
JUDITH E. McDOWELL,
Woods Hole Oceanographic Institution, Massachusetts
RICHARD A. MESERVE,
Covington & Burling, Washington, D.C.
S. GEORGE PHILANDER,
Princeton University, New Jersey
RAYMOND A. PRICE,
Queen's University at Kingston, Ontario
THOMAS C. SCHELLING,
University of Maryland, College Park
ELLEN SILBERGELD,
University of Maryland Medical School, Baltimore
VICTORIA J. TSCHINKEL,
Landers and Parsons, Tallahassee, Florida
E-AN ZEN,
University of Maryland, College Park
Staff
STEPHEN RATTIEN, Executive Director
GREGORY SYMMES, Assistant Executive Director
JEANETTE SPOON, Administrative Officer
SANDI FITZPATRICK, Administrative Associate
MARQUITA SMITH, Administrative Assistant/Technology Analyst
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Preface
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:
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evaluate the potential for using satellite technologies to measure the time-varying component of the gravity field;
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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
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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.
Jean O. Duckey
Chair