International Network of Global Fiducial Stations Science and Implementation Issues (1991) / Chapter Skim
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2. Scientific Rationale For Global Networks
2. Scientific Rationale For Global Networks
Pages 22-62
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From page 22... ...
In addition, we recognize that precise global geodesy by itself is not a panacea that will produce the needed solutions, and we attempt to consider its contribution in the context of the broadly based, multidisciplinary environment advocated in Mission to Planet Earth (National Research Council, 1988~. Although space geodesy is a relatively young discipline, it tends by nature to adopt a global perspective.
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From page 23... ...
Although our focus is on geodetic networks, other disciplines dealing with solid Earth science are actively building global networks of their own. These include, for instance, seismic networks (e.g., GEOSCOPE, IRIS GSN)
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From page 24... ...
For that matter, even defining an International Terrestrial Reference Frame, on a planet where every piece of real estate is in constant motion with respect to every other piece, is a challenge (e.g., Boucher and Altamimi, 1989) , and current realizations depend on data collected at a set of globally distributed sites.
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From page 25... ...
Examples include the detection and analysis of postseismic, interseismic, and possible preseismic strains within plate boundary deformation zones such as the western United States or intracontinental seismic zones such as the New Madrid area in the central United
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From page 26... ...
As stated in the Coolfont recommendation, FLINN sites should also be tied to a geocentric reference frame with a precision of 1 mm averaged over 1 year and should serve as fiducial sites for dense regional networks (NASA, 19911.
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From page 27... ...
Such problems place very stringent demands on network operations and often require improved solutions to other problems with a global character, such as the determination of an improved geoid or the measurement of densely sampled, precise time series of Earth orientation and rotation parameters. Other objectives, no less important, are more easily defined in terms of support of various scientific endeavors: examples include the calculation of precise orbits for a variety of Earth-orbiting spacecraft and the realization of a precise global reference frame.
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From page 28... ...
For example, present sea-level change cannot be inferred correctly from tide gauge records without first adjusting observations of relative sea level for postglacial rebound. Similarly, horizontal motions associated with postglacial rebound or surface loading caused by sea-level changes are comparable to those associated with departures from the predictions of rigid plate theory.
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From page 29... ...
This is a crude, but workable characterization of a GPS core network. A 100-station network would have an average intersite spacing of ~2,200 km, and more than 400 stations would be needed to achieve an average spacing approaching 1,000 km, assuming again a globally uniform distribution.
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From page 30... ...
The problem is that these observations depend on many factors in addition to changes in the mass of water in the oceans. Factors affecting RSL across the whole range of time scales include: tides; · changes in water volume caused by changes in temperature and salinity; changes in area and shape of the ocean basins; changes in volume and spatial distribution of ice; oceanographic signals such as basin-scale circulation, local currents, and atmospheric or meteorological signals such as winds and barometric pressure changes; · elastic end viscous response of the solid Earth, including postglacial rebound and tectonic motions; · local ground effects such as compaction or changes in the water table; · solid Earth tides and ocean loading tidal responses; and · changes in the inertia tensor and Earth orientation/rotation parameters associated with changes in the Earth's center of figure relative to the center of mass.
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From page 31... ...
10-~ to 102 Deep (500 to 4,000 m) 101 to 104 Glacial Accretion and Wastage Mountain Glaciers 10i to 102 Greenland Ice Sheet 102 to 105 East Antarctic Ice Sheet 103 to 1Os _ West Antarctic Ice Sheet 102 to 104 .
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From page 32... ...
This effect is due mainly to continued viscous relaxation, or postglacial rebound, following the Pleistocene deglaciation ~ 10,000 years ago, although tectonic motions and elastic response to loading of the crust also contribute, as we shall see. Perhaps the best understood of the short-term effects is the inverse barometer effect: local sea level rises 1 cm for every 1-mbar decrease in barometric pressure.
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From page 33... ...
Gauges in tectonically active areas must at present be excluded from the analysis given the current lack of independent measurements of the motion of the land. Modeled tide gauge motions caused by postglacial rebound have been removed to obtain better long-term secular variations in eustatic sea level (see, e.g., Pettier, 1990)
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From page 34... ...
34 Band r Gus to to ~ — en to to or O — ED CO 0 0 C)
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From page 35... ...
In addition to this regional elastic response, redistribution of mass caused by ice sheet melting or growth will affect the Terrestrial Reference Frame. Redistribution of surface loads by the melting of ice sheets and resulting transfer of mass to the oceans leads to a change in the center of figure (CF)
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From page 36... ...
Measuring this offset provides a powerful means of monitoring these large-scale changes. For example, melting of an average of 1 m of ice from the Antarctic ice sheet (sufficient to change sea level by 40 mm)
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From page 37... ...
Constraining the variation of mantle strength with depth is crucial for understanding the dynamics of mantle convection and the driving mechanism of plate motions. More importantly from a societal perspective, in order to measure the changes in ocean mass and volume associated with global change, it is crucial to separate out this "noise" from deformation of the solid Earth in order to recover the "signal" of global sea-level change.
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From page 38... ...
Among the scientific questions driving research in the general area of postglacial rebound, mantle rheology, and changing sea level are the following: . What are the lateral variations in mantle rheology?
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From page 39... ...
· Sampling rates, including continuous recording, are not mandated by the postglacial rebound signal itself: if the signal were large, the time scales involved could be sampled through occasional campaigns. However, high sampling rates are desirable to reduce noise in the rate estimate, to avoid temporal aliasing, and to improve noise modeling, particularly since fundamental questions about mantle viscosity will require resolution of rather subtle characteristics in the data.
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From page 40... ...
, will help refine plate motion models in several such areas. To eliminate the trade-o~ between horizontal and vertical motions that exists for isolated long baselines or sparse networks, this analysis should be done on a global basis, and the network geometry should reflect plate geometry.
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From page 41... ...
In addition, redundancy—through the survey of a geodetic footprint surrounding fiducial sites and analysis of a complete network is needed to detect contamination by local effects (e.g., subsidence and monumentation) , which may mask interesting geological signals or, worse, masquerade as such signals.
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From page 42... ...
The spatial scales involved in earthquake and volcano monitonog are small, and these problems are properly addressed in the context of regional densification. But local and regional spacegeodetic networks would benefit considerably from a global fiducial network.
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From page 43... ...
It must be resolved to reach an improved understanding of the mechanics of deformation, particularly in large regions of the world such as Tibet, where even reconnaissance surveys are still lacking. In such instances, the global network would play an important supporting role by simplifying logistical issues of reconnaissance surveys and by providing the necessary reference frame.
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From page 44... ...
As stated previously, a global fiducial network supports interdisciplinary science, but it should serve specific geodetic objectives as well. Below we discuss these geodetic objectives and the concomitant design considerations for a global fiducial network.
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From page 45... ...
The global distribution of stations is required to ensure high-accuracy geodesy everywhere in the world, as well as to support such geodetic applications as determination of Earth rotation and establishment and maintenance of a GPS reference frame. To facilitate the timely generation of ephemerides, a standard GPS receiver and antenna is required for the GPS core stations to eliminate potential problems arising from mixing dissimilar instrumentation.
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From page 46... ...
It is expected that the GPS core network, as a by-product of the GPS precise orbit determination process, can support the determination of Earth rotation parameters. To what extent the Coolfont goal can be attained remains to be investigated and is one of the challenges of the global network.
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From page 47... ...
The incorporation of GPS data in the context of the global network will require reexamination of these points. However, as data quality becomes more uniformly high, it can be anticipated that the realizations of the reference frame will improve as well.
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From page 48... ...
This impact may be felt at only a subset of the global network, which for present purposes can be identified with the GPS core network introduced above. In addition, mission support requires faster reactions and usually has a much smaller time constant (defined by the mission lifetime)
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From page 49... ...
For example, satellite altimeters require radial orbit accuracies of 20 to 40 mm to monitor slow variations in sea level; these accuracies must be maintained for at least 10 years. Satellite "radiometers, and other geodetic satellites in low Earth orbit, are used to measure the Earth's gravity field on regional (100 to 1,000 km)
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From page 50... ...
Repeat measurements of ice topography can be used to monitor the mans balance of the major ice sheets. Over land altimeters can be used to measure topography at high spatial resolution on a global basis.
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From page 51... ...
and Other Gravity Field Missions A number of planned or proposed gravimetric satellite missions require accurate tracking on a global basis. The first is the GPS experiment to be demonstrated on-board the TOPEX/POSEIDON spacecraft (mid-1992 launch)
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From page 52... ...
Regardless of which high-precision system is used GPS, PRARE, or DORIS these upcoming or proposed satellite missions to map the global gravity field will be enhanced by a global network of fiducial stations. High-Resolution Unaging Systens Space-based imaging systems such as NASA's Landsat Thematic Mapper and France's Satellite pour l' Observation de la Terre (SPOT)
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From page 53... ...
Clearly, one of the applications of a global tracking network based, for example, on GPS would be to improve the quality of spacecraft location data, in terms of both control and knowledge. Moreover, this approach is relatively cheap in comparison to existing highprecision tracking techniques, which rely on numerous ground-based lasers (constrained by limitations on satellite visibility from the ground stations)
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From page 54... ...
Support of Local and Regional Studies Although support of local and regional studies is perhaps the primary objective when defining the need for a service, such as the International GPS Geodynamics Service initiated by the JAG, it probably has only a relatively minor impact on the design of a global fiducial network. On the other hand, applications of the global network data are many, and the following are merely possible examples: · De local and regional nets to the global network: The main application of the global network is to provide a precise and reliable reference frame.
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From page 55... ...
would clearly benefit from the global network. A global network of fiducial sites would not only benefit the global sciences but would also have positive effects on local and regional surveying operations and thus contribute to other scientific and engineering operations.
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From page 56... ...
An expanded network, compatible with and tied to the core network, would consist of many more sites, possibly numbering in the hundreds, but not all of them would have to be monitored continuously. Spatial and Temporal Sampling The scientific matters sea-level change, postglacial rebound, tectonic motions that the global fiducial network is designed to address call for highly accurate determinations of radial and horizontal velocities (~0.1 mm/yr over ~ 1,000 km)
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From page 57... ...
Thus, frequent measurements offer a clear benefit, primarily in the ability to apply statistical techniques and time series analysis to the data. On the other hand, the errors from atmospheric effects are
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From page 58... ...
- , . , 1984 Difference between ~.~' NOVEL-1 and VLBI^~/ rate: 3.9 mm/yr f.~ ~ SCIENTIFIC RATIONALE FOR GLOBAL NETWORKS Original data set i 1 WRMS seer about Slope 11.3 mm from 443 estimates .
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From page 59... ...
Lengths for three geodolite lines in the San Francisco Bay area, showing possible anomalies precursory to Loma Prieta about 1 year before the event. The break in the plots in mid-1984 corresponds to the coseismic offset for the Morgan Hill earthquake.
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From page 60... ...
60 SCIENTIFIC RATIONALE FOR GLOBAL NETWORKS NW-SE Strain 2or 1 8 1 6 1 4 1 2 ._ ~ 1 . O tin 0.8 0.4 02 ~1~ ,1 1 lil,llll, ,11,' 1 1lll 11,1 , ,1 1985.0 1986.0 1987.0 1988 0 1989 0 Geodetic Strain _: ~ _~_ _ _ .1 Daily Precipitation _?
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From page 61... ...
The time series in this study are shown in Figure 7. It seems believable that there is more to these time series than just random noise superposed on a steady trend, and it seems clear that, without the dense set of points shown, the possible anomalies would certainly be missed.
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From page 62... ...
A number of the objectives we have set forward can be met with a GPS core network of approximately 30 sites, but other objectives, such as the identification of deformation patterns, require a mean intersite spacing small enough to avoid spatial aliasing. The answer is problem and region dependent, but, again, economic considerations rapidly become the main issue, since the number of sites is a quadratic function of the wavelengths to be resolved.
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