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OVERVIEW AND RECOMMENDATIONS We stand on the threshold of a technological revolution in geodesy. With the introduction of space-based observational techniques over the past two decades, geodesy has undergone and continues to undergo profound changes. Between now and the end of this century, new technological advances are sure to transform the field even further, by making new generations of measurements possible, with highly increased speed and accuracy. Yet the scientific requirements, in terms of precision, spatial and temporal density of the measurements, and overall coverage of the planet, represent a formidable challenge to even the most advanced geodetic tools. The central, most important question is this: How are we, as a scientific community, to ensure that very long term geodetic observations will be carried out over the same geodetic networks using the same or comparable standards over durations of many decades? The federal system that has evolved in recent times is currently geared to cycles of funding for research and development extending over a few years at most. Moreover, because of the complexity of space technology, gathering geodetic data is no longer a simple matter of a field party making measurements and an individual serving as "computer" for the network adjustments. Instead, there is a huge variety of tasks to be done, including maintaining continuity over decades of observational techniques, monumentation, network maintenance, education, and survey practice, together with ancillary services such as technology development, data management, orbit determination, and field coordination. With so many agencies and institutions interested in various aspects of space geodesy, and with so much to be done, it is critical that careful thought be devoted to the problem of ensuring that all of these tasks are carried out efficiently and expeditiously. It is hoped that this report, Geodesv in the Year 2000, will stimulate the kind of open, wide-ranging debate within the national and international communities needed to forge a smoothly running operational system. This goal, as well as the scientific and technological issues involved, is the major challenge that currently confronts us . In the fall of 1987 the Committee on Geodesy sponsored a session at the meeting of the American Geophysical Union to highlight the opportunities the immediate future may hold for scientific and technical progress. Following the Overview and Recommendations are eight of the nine Contributed Papers prepared for that session. The ninth paper was published in 1988 (Paik et al.) The first contributed paper, an historical introduction by Rundle, is a brief account of the land-based methods previously employed in geodesy, and of how these methods often dictated the scientific problems of interest to earlier geodesists. This paper illustrates how much of the precise geodetic work pursued by the U.S. government was at one time vested in one federal agency, the Coast and Geodetic Survey. This is in contrast to the present situation where many organizations are participating in geodetic activities. The second paper, by Minster et al., 1
2 underscores the critical scientific problems that might most profitably be addressed by new space-geodetic precise positioning technology. In particular, these authors emphasize the need to focus on space-time departures from average plate motions predicted by the global, rigid-plate motion models. Much of the material in the second paper has since been incorporated in the Erice report (Mueller and Zerbin~, 1989~. The third paper, by McNutt, describes the accuracy requirements for a knowledge of the geoid in the year 2000, and what steps must be taken to achieve this and other scientific goals. Some new technology developments to attain this goal were recently described by Paik et al. (1988~. Zlotnicki, in the fourth paper, discusses the need for geodetic control in oceanographic research, focusing particularly on the need for accurate measurement of the oceanic geoid and sea level surface. The remaining papers describe developments in space-based technology currently in progress. The fifth paper, by Smith, describes a new generation of orbiting lasers for use in precise positioning by ranging to ground retroreflector networks and for use in altimetry. Lasers of this type are already in the planning stages for the Geodynamics Laser Ranging System of the Earth Observing System, and for altimetric purposes on the Mars Observer Mission. Spiess, in the sixth paper, discusses ocean-bottom geodesy, a field that will essentially be created over the next decade. Rodgers details in the seventh paper how a variety of technical improvements over the next decade are expected to improve accuracies in Very Long Baseline Interferometry observations. Finally, Melbourne, in the eighth paper, summarizes the technical improvements that will be possible in Global Positioning System receivers in the near future, leading to inexpensive and user-friendly receivers. Recommendations Both the Erice report (Mueller and Zerbini, 1989) and the papers included in the following pages demonstrate the promise of the new technology. In addition, the Erice report makes a variety of specific recommendations as to which of the various scientific problems, and which technology development alternatives, should receive the greatest priority. After consideration of these priorities, and in light of the results presented in the chapters of this volume, the Committee on Geodesy recommends the following priorities be established in support of scientific and technological opportunities in geodesy for the year 2000: 1. The U.S. government should sponsor, as a critical national priority, a vigorous coordinated program for the development and exploration of modern geodetic techniques' through aggressive pursuit of continued technological advances and through a long-term commitment to the routine acquisition' reduction archiving, and distribution of global and regional high-precision geodetic data. Observation and monitoring of high-precision networks by space- and ground- based geodetic technology address vital national needs, which cannot be satisfied on the necessary scale by private or educational sectors. Moreover, by virtue of the long periods involved, as well as the wide geographic coverage and centralized coordination necessary, federal, rather than
3 state or local leadership, is mandatory. There are critical needs for continuity in observational techniques, monumentation, network maintenance, education, and survey practice, together with ancillary services such as technology development, orbit determination, and field coordination, over periods of decades and longer. Historically, this role was filled in the United States, in large part, by the National Geodetic Survey and its predecessor agencies, which were able to provide continuity in operational procedures and standards over more than 100 years. However, many of the tasks involved in operational geodesy are now spread over a diversity of federal agencies, giving rise to problems of coordination stemming from overlapping responsibilities. Unfortunately, some agencies are no longer able to execute effectively certain programmatic objectives, due to fiscal problems arising from declining budgets. Interagency accords and financial support will therefore be essential for development and routine use of high- precision space geodetic technology. In addition, coordinated international collaboration will be necessary, given the global nature of space geodesy. 2. The federal government should organize and sponsor programs to conduct geodetic instrument research and development: maintain international geodetic observatories: strengthen global positioning networks with stations on stable plate interiors and deploy long-term, dense. frequently observed local positioning networks at sites of active crustal deformation. New and refined technologies that will enable positions to be measured frequently or continuously at the level of a few millimeters are a high priority. They will be an important component of the arsenal of techniques to be used in future studies of tectonic and volcanic phenomena as well as global climate change. To address these problems, it is crucial that federal support of geodetic observatories continue for the purposes of developing and calibrating new and existing instruments in a field environment, performing reliability tests, and comparing competing borehole, surface, and space technology systems. This program of testing and development should also include increased research to understand the origin of survey monument motion due to instabilities in the uppermost meters of the surficial layers of the earth, because such 1- to 5-mm motions will soon become a major contributor to measurement uncertainty. Seafloor observatories are also needed for testing and comparing sea floor positioning systems. Global geodetic networks, with fiducial stations sited on extremely stable monuments located in stable plate interiors, should be regularly observed as calibration points for transient deformation at plate boundaries. For example, the "Fiducial Laboratories for an International Natural Science Network" (FLINN), conceived at the Coolfont workshop, would contribute significantly to implementing this committee's recommendation. As proposed, FLINN would build on existing global space geodetic networks to achieve worldwide coverage with intersite spacing on land of approximately 1000 km, and positions to 1-cm precision over short periods (one day) and 1-mm precision over longer time intervals. Dense (0.2-2 monuments/km), highest-precision (1-3 mm), local (102-104 km2) networks confined to sites of active seismic, tectonic, or magmatic deformation with the prospect of continuous or frequently repeated observations (one per day) will be essential for short-term analyses of earthquake or volcanic activity. 3. A global topographic data set should be acquired with a vertical accuracy of about_1 m, at a horizontal resolution of about 100 m.
4 Existing topographic data are largely derived from stereo-photogrammetric techniques, whose data vary greatly in resolution, accuracy, format, and reference level. Acquisition of a coherent, global data set should constitute a major geodetic goal for the year 2000. Over the continents, these data are best obtained from space, by advanced radar and laser altimeters. Radar interferometers may also be feasible, and research into these and other technologies should be vigorously pursued. Improved topographic data are essential for interpretation of the continental gravity field, and would support a variety of tectonic, hydrological, and ecological studies. Monitoring alpine glaciers and the Greenland and Antarctic ice sheets to determine their responses to the possibility of carbon-dioxide-induced global warming is critical. Alpine glaciers serve as sensitive indicators of mean global temperature, while changes in the polar ice caps must be monitored and understood because of their potential for catastrophic impact on mean sea level. 4. The geodetic community strongly encourages the development of space- based techniques to determine variations in the earth's gravity field to a resolution of 100 km or better. with an_accuracy of several milligals. together with airborne techniques using precise Global Positioning System (GPS) tracking to obtain regional gravity fields with a resolution of 10 km at an accuracy of better than 1-mgal. Improved gravity field information is necessary for studies of compensation mechanisms, lithospheric structure, and geodynamical studies of the earth's deep interior. Error analysis studies suggest that resolution of the global gravity field to 100 km can be obtained by the use of satellite-borne gravity "radiometers, one of which is currently planned for the European ARISTOTELES mission, to fly at 200 km, with an accuracy goal of 5 mgal. Enhanced resolution can be obtained by flying a superconducting gravity "radiometer in a drag-free configuration at an altitude of 160 km, for a planned accuracy of 1-mgal. A "radiometer based upon proven airborne "radiometer technology, mounted in a conventional satellite, should be considered as a low-risk alternative. The goal of 100-km resolution may still be inadequate for many tectonic studies. Additional studies suggest that 1-mgal precision, from 300-km to 5-km wavelengths, can be achieved with an airborne "radiometer system under differential GPS control. Centimeter-level radar altimetry over the oceans, with improved space-time resolution, is of considerable importance. These data will lead to substantial improvement in our knowledge of the oceanic gravity field, have numerous oceanographic applications, and be of value in studies of bathymetry and marine tectonics. 5. To enable the study of complementary geophysical atmospheric. and oceanographic phenomena an observational program should be implemented to recover vector earth rotation data. such as polar motion and length of day, at the sub-milliarcsecond level. on time scales from cycles per day to cycles per decades.
The time variation of the earth's instantaneous rotation vector is governed by the planetary angular momentum budget, which in turn depends explicitly on a variety of factors including externally applied torques. Having both long- and short--term measurements permits study of problems including the fluid-outer and solid-inner core interaction, resonances, time variation of the earth's gravity field, and transfer of angular momentum between the atmosphere, oceans, and the solid earth. These data would also support establishment and maintenance of conventional terrestrial coordinate system, which is necessary for comparison of results obtained by different space geodetic technologies, including very long baseline interferometry, satellite and lunar laser ranging, and the Global Positioning System. J ~ NOTE _ ~_ a _ Lo The Committee on Geodesy expresses its appreciation to the authors of the eight contributed papers included in this report. However, responsibility for the Overview, including the recommendations, rests with the Committee on Geodesy. References Mueller, I.I., and S. Zerbini, Proceedings of the International Workshop. The ~nteruisc~pl~nary Role of Space Geodesy Erice Sicily. JU1Y 23-29. 1988, Lecture Notes in the Earth Sciences, Springer-Verlag, Berlin, 1989. National Aeronautics and Space Administration, Solid Earth Science Branch, Earth Science and Applications Division, Office of Science and Applications, Mayor Emphasis Areas for Solid Earth Science in the l990s: Report of the NASA Coolfont Workshop, Nov. 27, 1989. Paik, H.J., J.-S. Leung, S.H. Morgan, and J. Parker ^ ~~ ~ ~~ EOS Trans. AGU 69, an urb~t'ng Gravity Gradiometer, E_w~i~ bY 1601-1611, 1988.
