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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
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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
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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.
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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.
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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.
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Representative terms from entire chapter:
global positioning
