APPENDIXES



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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES APPENDIXES

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES APPENDIX A REVIEW OF RECENT STUDIES This appendix reproduces recommendations made earlier, which are relevant to the global fiducial network. The recommendations made in the present report are essentially consistent with the results of these previous studies. Mission to Planet Earth In 1988 the Task Group on Earth Sciences of the Space Science Board, National Research Council, produced a seminal report entitled Mission to Planet Earth. The primary research objectives of this mission are addressed in terms of four “Grand Themes,” which are: To determine the composition, structure, and dynamics of the Earth 's interior and crust, and to understand the processes by which the Earth evolved to its present state. To establish and understand the structure, dynamics, and chemistry of the oceans, atmosphere, and cryosphere and their interactions with the solid Earth, including the global hydrological cycle, weather, and climate. To characterize the interactions of living organisms among themselves and with the physical environment, including their effects on the composition, dynamics, and evolution of the oceans, atmosphere, and crust. To monitor and understand the interaction of human activities with the natural environment.

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES To address these four grand themes, the Task Group proposed a long-term program based on four classes of systems: The Task Group recommends that the centerpiece of the global observing system be a network of satellites and platforms in the following arrangement: a set of five geostationary satellites, a set of two to six polar-orbiting platforms, and a series of special missions. The Task Group recommends the development, deployment, and long-term operation of a system of in situ measuring devices—the Permanent Large Array of Terrestrial Observatories (PLATO)—to provide complementary data to the space network. Wherever applicable, the data should be transmitted in real time and integrated with observations from space. The Task Group recommends that state-of-the-art computing technology be utilized for data analysis and theoretical modeling of Earth processes. The Task Group recommends that a full and coordinated data system, which both archives and disseminates data, be established. Erice and Coolfont Workshops In recent years two major workshops were conducted at which global issues in geophysics and geodesy held a prominent place. The first, an international workshop on The Interdisciplinary Role of Space Geodesy, was held in Erice, Sicily, in July 1988. The resulting report (Mueller and Zerbini, 1989) contains 11 main recommendations, of which at least five address issues that call for a global point of view. These include, of course, determination and analysis of the geopotential fields, but also (Agnew et al., 1988): Over the next 20 years, major efforts in applying precise positioning techniques should be aimed primarily at: Continued large-scale reconnaissance surveys with station spacing on the order of 102 km, to improve our understanding of the kinematic evolution of extensive, largely unexplored zones of continental deformation. Sustained, repeated measurements of dense networks at centimeter-level accuracy, to determine the time dependence and spatial distribution of deformation within and across zones of intense tectonic activity. Measurement frequencies

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES should range from daily to annually over a decade or more, with station spacing from 3 to 30 km and network dimensions from 10 to 1,000 km. In regions of complex deformation, geodetic measurements should be complemented by comprehensive tectonic and structural studies and careful estimates of displacements and displacement rates on geologic time scales. Continued improvement of capabilities, to achieve: millimeter-level accuracy in both horizontal and vertical components for detailed subaerial studies, system calibration, and ultimately, low-cost, routine deployment. centimeter-level accuracy in both horizontal and vertical components for sea-bottom systems. In July 1989 a planning workshop was convened by NASA at Coolfont, West Virginia, with the purpose of identifing major scientific and programmatic areas for NASA activities in solid earth science for the next 10 years. The final report, Major Emphasis Areas for Solid Earth Science in the 1990s (NASA, 1991), describes five primary research areas that emerged from the workshop discussions. They are: a global geophysical network (GGN), global topographic mapping, soils and surface processes, geopotential fields, and volcanic effects on climate. The Panel on Plate Motion and Deformation formulated four recommendations. The first two pertain (1) to the deployment of a global distribution of fiducial sites whose positions are to be maintained to 1 cm over 1 day and to 1 mm over 3 months and (2) to the establishment of a capability for beginning dense, frequent geodetic surveys in areas of tectonic interest. These are summarized as follows: In order to study plate motion we need a global distribution of geodetic stations that measure relative positions to 1 cm over 1 day and to 1 mm over 3 months. In order to monitor regional and local deformation we need a terrestrial reference frame. Both of these objectives can be accomplished with a global distribution of space-geodetic observatories.

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES In order to monitor deformation in tectonically active areas where deformation is concentrated, we need a finer spacing of geodetic sites. We recommend the initiation of a vigorous long-term program of monitoring regionally dense networks deployed across tectonically active regions, to measure and analyze motion and deformation over a broad range of spatial and temporal scales. We need globally coherent topography, geoid, and gravity coverage, and we need high-resolution satellite imagery and topography and gravity coverage in local areas. We require technological development to permit a number of applications of space-based geodesy that are not possible with current capabilities. Woods Hole Workshop During May 2-4, 1990, the Global Programs office of the National Oceanic and Atmospheric Administration (NOAA) together with the Division of Ocean Sciences of the National Science Foundation (NSF) sponsored a workshop on sea level that was organized and led by the Joint Oceanographic Institutions (JOI) of Washington, D.C., and held at the Woods Hole Oceanographic Institution. The proceedings of this workshop, summarized in Towards an Integrated System for Measuring Long Term Changes in Global Sea Level (Joint Oceanographic Institutions, 1990), include a series of recommendations, grouped around the following topics: toward an integrated system, improving tide gauge networks, developing sea-level indices, continuing satellite measurements, developing the use of geodetic techniques, long-term in situ measurements, ice sheet measurements, and developing appropriate models. For present purposes, the detailed recommendations pertaining to the use of geodetic techniques are reproduced:

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES With respect to geodetic aids and techniques, we recommend that: A Terrestrial Reference Frame (TRF) accurate to millimeter level and stable on time scales of decades is an essential component of a global change/sea-level monitoring system. The International Earth Rotation Service (IERS) and National Earth Orientation Service (NEOS) provide such a reference frame. Applications include: Survey of tide gauges on a regional basis. Satellite orbits in a common reference frame. GPS orbits. Kinematic GPS fiducial stations. U.S. agencies should continue to refine VLBI/SLR/GPS technologies and share advances with other nations. U.S. agencies should vigorously participate in the IERS to improve accuracy and global coverage at a minimal cost. New technologies, such as the super-fluid gyroscope, should be explored. GPS promises to be a “cost-effective” technology for many applications, such as airborne mapping of ice masses, and surveys of tide gauges. Agencies should provide resources to advance GPS technology as rapidly as possible. DoD should remove Selective Availability (SA) during periods of normal international relationships. SA can be defeated by double-differencing techniques, but imposition would increase resource requirements and waste funds. Absolute gravity technology should continue to be developed and tested as an alternative method (less costly) to map and monitor vertical crustal motions. Kinematic GPS techniques and airborne altimeters should be developed and deployed to measure ice masses, starting with Greenland, as soon as possible.

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