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International Network of Global Fiducial Stations: Science and Implementation Issues (1991)

Chapter: 2. Scientific Rationale For Global Networks

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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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2

SCIENTIFIC RATIONALE FOR GLOBAL NETWORKS

Introduction

Geodesy as a scientific discipline is in the enviable position of marrying fast-developing technology with rapidly expanding scientific goals. In a sense it is valid to ask whether technology development drives the science to the same extent as scientific requirements drive the technological innovations. As high-precision space-geodetic techniques become more easily accessible to the scientific community, the range of problems that geodesists can attack in earnest expands to incorporate problems previously deemed too formidable to be treated successfully. Examples abound as described in detail in several recent reviews, such as the Erice and Coolfont workshop reports mentioned earlier, and several recent reports by the National Research Council 's Committee on Geodesy (Geodesy: A Look to the Future, 1985; Current Problems in Geodesy, 1987a; Geodesy in the Year 2000, 1990a). These reports cover the opportunities offered by space-geodetic techniques and make specific recommendations concerning the areas in which to focus attention now and in the next decade or two. In this chapter we draw from these studies but adopt a somewhat more specialized point of view. Specifically, we endeavor to identify scientific problems that cannot be addressed properly in the absence of a global network, as well as applications of space geodesy that would benefit directly and substantially from such a network without requiring its existence. 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. For instance, SLR and VLBI networks (including the QUASAR network) have a global scope, although they do not provide uniform

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

coverage. More recent deployments, such as the DORIS and PRARE networks, are both global and quite uniform by design. The rapidly growing CIGNET GPS network exhibits similar characteristics, especially when considered in conjunction with other stations such as the tracking sites currently being deployed by NASA. Temporary global GPS deployments have been conducted successfully (e.g., GOTEX), and a global international campaign (GIG'91) was conducted in early 1991 under the aegis of IERS. 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) and magnetic observatories (INTERMAGNET). Largely in recognition of the value of multidisciplinary measurements, the concept of the Permanent Large Array of Terrestrial Observatories (PLATO) was originally proposed as one of the two major elements of the Mission to Planet Earth.

Scientific Goals

It is self-evident that the major purpose of global geophysical and geodetic networks is to permit the collection of observations capable of constraining models of the planet as a whole. The qualifier global takes on a dual connotation in this context. On the one hand, such data are needed to study phenomena that operate on a global scale—instead of continental, ocean basin, or any of a hierarchy of scales down to the dimensions of grains that form the rocks. On the other hand, even when the scales under consideration are much smaller than the radius of the planet, so that a regional or even local description may be adequate, global coverage is still desirable to help understand the underlying physical processes that shape the planet. Long-wavelength features are then used as a background to analyze short-wavelength components. The scientific goals of global networks should reflect this duality (see also International Association of Geodesy, 1991):

The dual scientific goals of global geophysical and geodetic networks are (1) to improve understanding of geophysical and geological phenomena that operate on global scales and (2) to provide a framework and boundary conditions to analyze phenomena that operate on smaller scales.

A simple illustration is the spherical harmonic expansions commonly used to describe global characteristics of the Earth. The geopotential and geomagnetic fields are classical examples, but this type of representation is also used for the

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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three-dimensional internal structure of the Earth determined by seismic tomography, in models of convection currents in the mantle, or even to describe the kinematic plate velocity field at the surface. In some instances, global coverage—at least at sufficiently long wavelengths—is provided by observations from satellites in low Earth orbit. This is true of potential fields. However, for small-scale features, and for all structural, kinematical, and dynamical models of the Earth that depend on surface observations, the resolution is limited by the coverage provided by surface observatories. For instance, the determination of radial and lateral inhomogeneities in seismic velocities requires a global distribution of both seismic stations and seismic sources. Because Earth is a dynamic planet, and because its surface is covered by highly mobile tectonic plates, even measuring the rotation parameters of the planet—a global characteristic par excellence—requires adequate global coverage by a network of space-geodetic observatories. 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.

With geodetic precise positioning networks, these problems are sharpened by the lack of a natural and unambiguous way to interpolate observations. We do not enjoy the benefit of having potential fields or elastic waves to define a physical averaging of pointwise properties (such as density, velocity, and Poisson's ratio) of the Earth and, therefore, must resort to using a particular mechanical and rheological model of the Earth between sites to interpolate and interpret the data. This technique is often quite successful when the intersite distances are smaller than or comparable to scales of relevant inhomogeneities. However, for spatial scales that can be realistically sampled by global networks, this approach is fraught with danger, because large-scale crustal inhomogeneities are likely to invalidate smooth strain models between sites. Geodesists therefore have deployed a hierarchy of networks in which sparser regional networks serve as fiducial sites in the survey of denser local networks. With the advent of space geodesy, we can conceive of and actually implement a global high-precision network of sites that will serve as fiducial points tied to a common reference frame with uniformly high precision. The deployment of such a network will substantially influence the various uses of space-geodetic techniques. The first steps have already been taken toward a goal that is specific to geodetic global networks, namely (see also International Association of Geodesy, 1991):

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

Geodetic measurements with a precision of approximately 3 mm or better, in both horizontal and vertical components, should be made possible at any time, anywhere on the planet, on spatial scales ranging from a few kilometers to intercontinental distances, with an achievable temporal resolution of a day or better.

Millimeter-precision geodesy is now possible even on very long baselines using VLBI (e.g., Herring, 1991). In other words, the technological know-how exists to achieve this level of precision between properly upgraded VLBI sites. The problem raised by this goal is therefore not so much to achieve very precise measurements, but rather to make them affordable and logistically feasible at unprecedented temporal and spatial densities. The equivalent strains detectable with such a deployment would be about 10-6 on local scales and better than 10-9 on global scales, with a temporal resolution of 1 day or better. On local scales such a capability obviously would neither duplicate nor supplant existing techniques such as observatory strainmeters capable of detecting strains of a few parts in 1010 over times shorter than a day (Agnew, 1987). Instead, the capability should be viewed as complementary, designed to investigate spatial patterns of rather large strains and their variability with space and time, an aim not achieved by most existing geodetic monitoring networks primarily because of their low time sampling rates.

Although our goal introduces 3 mm as a target precision, it must be recognized that in many areas of the world significant scientific progress would result even from measurements made at a precision of 10 mm. This is true, for example, of areas where crustal motions are poorly known and where any geodetic constraint would help. As an illustration, crustal deformation in the India-Eurasia collision zone, which includes the large Tibetan Plateau, is a matter of considerable current debate among geologists and tectonophysicists (England and McKenzie, 1982; England and Houseman, 1989; Molnar and Tapponnier, 1975; Tapponnier et al., 1982; Peltzer and Tapponnier, 1988). In that case, reconnaissance geodetic surveys would be most valuable to help select among the various deformation models proposed in the literature. In other instances a precision of 1 mm for daily measurements would be very desirable. This is true, for example, of areas already well instrumented, where important geological problems are likely to require analysis and interpretation of rather subtle geodetic signals. 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

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

States. Averaging observations over longer times will of course improve the detection of small signals with long time constants, but it must be borne in mind that systematic effects do not cancel in such a procedure and that the rule of thumb is that a signal-to-noise ratio improvement of a factor of three constitutes a practical limit. Furthermore, especially at the millimeter level, monumentation issues are important (e.g., Wyatt, 1982, 1989), and geological interpretation is chancier.

The goal stated above could be reached through a coordinated international effort by the middle of the decade and should be largely surpassed —in precision as well as spatial and temporal resolution—by the year 2000. As stated, it is compatible with several recommendations cited in Appendix A and incorporates some of the notions expressed in them. Although global networks are not discussed in detail in the Erice report, which focuses on geodesy, they are a centerpiece of the Coolfont report, which is permeated by the notion of global studies of the Earth. This includes not only global altimetric, geodetic, geological (e.g., soil and surface processes), and geophysical coverage (e.g., potential fields) but also studies of large-scale structures of the interior of the Earth, for which a multidisciplinary approach is essential. As an example, a recommendation of the Coolfont Plate Motion and Deformation Panel states:

In order to study plate motion we need a global distribution of geodetic stations that measure relative positions to 1 cm over one day and to 1 mm over three 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.

Although this recommendation is couched in terms of a geodetic network, the concept that has been derived from it involves a set of globally distributed sites with a multidisciplinary vocation, described collectively as FLINN. 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, 1991).

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

Objectives

The primary scientific objective of a Global Network of Fiducial Sites is to provide an effective and economical means of acquiring, analyzing, and archiving the data required to solve global problems. The primary objectives are identified directly with research topics that cannot be pursued in any other way. In this category we place the study of complex multiscale problems such as global sealevel monitoring and postglacial rebound, for which global coverage is essential. A second category involves the precise measurement of tectonic motions, including plate deformation. The most interesting signals are departures from the motions predicted by the long-term geological models (e.g., Jordan and Minster, 1988a). Further, many signals of practical interest that take place on local and regional scales can only be interpreted properly if the large-scale motions are well understood. 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. Applications that do not intrinsically require a global network but that would benefit directly from it cover a wide range of geological problems, such as relative motions across specific plate boundaries and plate boundary deformation zones, earthquake and volcanic cycles, and altimetric surveys. Finally, with the advent of inexpensive, and lightweight space-geodetic techniques such as GPS, a suitably dense, carefully maintained global set of fiducial points would have a significant impact on almost all aspects of geodesy, including practical surveying considerations.

For our present purposes, geological and geophysical objectives may be discussed from the point of view that they identify certain scientific users of global geodetic data, the scientific customers of the global network. Geological and geophysical objectives are primarily applications of the capability obtained through the global network. But most of these applications require some degree of densification of the network to achieve the spatial and temporal resolutions needed to make substantial contributions.

Two major global problems are used as examples to support and focus our discussion. These are:

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×
  • global sea-level change, response to surface loads, and postglacial rebound and

  • tectonic motions and intraplate deformation.

The rates of horizontal motions from these phenomena are comparable. The main distinction is that the regional vertical motions associated with the first are typically much greater than for the second. As a result, although each of the problems of eustatic (global) sea-level change, postglacial rebound, and intraplate deformation has its own intellectual focus, these problems are very tightly intertwined from an observational standpoint. Not one of them can be solved independently of the others. 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. The challenge is to separate the deformations associated with tectonics from those associated with surface loads.

In order to meet this challenge, we must first meet specific geodetic objectives, which must subsume those listed by Knickmeyer (1990). These include:

  • determination of precise orbits for spacecraft used in geodesy and geophysics,

  • precise determination of Earth rotation and orientation parameters,

  • realization of a precise global terrestrial reference frame,

  • support of scientific orbital missions, and

  • support of local and regional studies.

The first four of these objectives might be identified with providers of information and data made available via the global network. The fifth objective pertains to end users of the global network. Because of the large community of professional users of the data products (such as GPS ephemerides) provided by the global network operation, the concept of a service is an important component of international organizations such as IERS or the proposed IGS. However, this concept affects primarily the issues of data processing, management, and distribution; it does not introduce new constraints on network design as high-quality orbits (covered by the first objective) would satisfy the vast majority of the needs of the professional community.

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

The global solid Earth scientific problems that can be addressed, at least in part, through a global geodetic network share one feature: they are characterized by a spectrum of spatial and temporal scales. As a result, one is strongly tempted to seek ever-improved resolution by densifying the network everywhere. One possible approach is to construct a large global network as an assemblage of properly connected regional networks, the operation of which must be carefully orchestrated and coordinated. This approach may well be viable in the long run, but it seems preferable first to discuss general global network design without introducing the complications of a practical implementation strategy. In the following discussion, several numbers will prove useful:

The area of the Earth's surface is 5.1 × 108 km2, with a square root of 2.3 × 104 km. This means that a 25-station global network uniformly distributed on the surface of the Earth will have an average intersite spacing of ~4,400 km. 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. Of course, such a geodetic network cannot be easily realized because of the presence of large oceanic areas without emerged lands.

Geophysical and Geological Objectives
Eustatic Sea-Level Change.

Global sea-level change is one of the most easily imagined end results in the chain of effects hypothesized to come from man's interference in the global climate system (see, e.g., Sea-Level Change, National Research Council, 1990b; also Towards an Integrated System for Measuring Long Term Changes in Global Sea Level, Joint Oceanographic Institutions, 1990). Human agricultural activity and consumption of fossil fuels are causing a possibly unprecedented increase in carbon dioxide and other greenhouse gases in the atmosphere. This is expected to lead to increased retention of solar heat and thus to global warming. Two possible consequences are more rapid melting of polar ice caps and mountain glaciers and an increase in the volume of water in the oceans because of thermal expansion. Both would cause a worldwide (eustatic) sea-level rise. Such an event, even if it were not large enough to cause widespread inundation, would be disastrous for coastal regions if only because it would increase the rate of coastal erosion. On the other hand, the climate changes

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

associated with global warming might lead to increased precipitation over ice sheets, resulting in an increase in the amount of water trapped in polar ice caps and thus a decrease in eustatic sea level.

While those who attempt to model the complex interconnection of climatic processes have generated great controversy without much predictive success, other scientists attempting to measure directly the temperature or sea-level manifestations of the indisputable increases in atmospheric carbon dioxide are also facing great difficulties. Relative sea-level (RSL) change would seem to be a fundamental observable that should be obvious over the years at any coastal location. And in fact RSL has been monitored fairly precisely at many locations. Many good tide gauge records go back more than 100 years. 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:

  • 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 tides;

  • atmospheric or meteorological signals such as winds and barometric pressure changes;

  • elastic and 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.

These factors were the subject of an extensive review in Sea-Level Change (National Research Council, 1990b). Table 1 summarizes the factors contributing to relative sea-level changes at various time scales.

Problems at shorter time scales associated with extracting a reliable rate of eustatic sea-level change from tide gauge records are illustrated by Figure 2. First, as the figure shows, annual deviations from the secular trend can be fairly large—the mean annual sea level varies at the level of centimeters. This variation is due to atmospheric and oceanographic (short-term) influences on RSL.

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

TABLE 1. Some mechanisms of sea-level change (from National Research Council, 1990b).

 

Time Scale (years)

Order of Magnitude of Change (mm)

Ocean steric (thermohaline) volume changes

   

Shallow (0 to 500 m)

10-1 to 102

100 to 103

Deep (500 to 4,000 m)

101 to 104

100 to 104

     

Glacial Accretion and Wastage

   

Mountain Glaciers

101 to 102

101 to 103

Greenland Ice Sheet

102 to 105

101 to 104

East Antarctic Ice Sheet

103 to 105

104 to 105

West Antarctic Ice Sheet

102 to 104

103 to 104

     

Liquid Water on Land

   

Groundwater Aquifers

102 to 105

102 to 104

Lakes and Reservoirs

102 to 105

100 to 102

     

Crustal Deformation

   

Lithosphere Formation and Subduction

105 to 108

103 to 105

Glacial Isostatic Rebound

102 to 104

102 to 104

Continental Collision

105 to 108

104 to 105

Sea Floor and Continental Epeirogeny

105 to 108

104 to 105

Sedimentation

104 to 108

103 to 105

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

FIGURE 2. Regional averages of annual sea level anomaly (from Barnett, 1984; see also National Research Council, 1990b, p. 17).

Second, the secular trend varies with location, with differences between local trends comparable to the amplitude of the secular trend itself. 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. This effect is important in the midlatitudes, where large pressure fluctuations over

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

periods of a few days cause sea-level changes of 10 to 20 cm. On a monthly basis, a typical standard deviation of 4 mbar leads to a 4-cm variation in RSL, translating into a 1-cm standard deviation for annual RSL. While barometric changes probably have a very small variability on a decadal basis, century-scale barometric trends have been noted at individual sites. Another local and short-term contributor to RSL is wind forcing. It is particularly significant in shallow seas, where the wind effects may be larger than the barometric contributions.

Other complications contaminating the RSL record introduce variability on larger spatial and longer temporal scales. Larger regional effects are due to steady-state basin-scale flows and to oscillations on regional and global scales. Figure 3 is a world map of dynamic ocean topography derived from GEOSAT satellite altimetry. To determine dynamic ocean topography from satellite altimetry, the orbit must be determined very precisely using tracking stations (tied to the global fiducial network). Furthermore, atmospheric and ionospheric effects must be modeled, which requires knowledge of atmospheric variations, and the geoid must be removed, which again links the sea-level problem to solid Earth geophysics. Despite this need for a cascade of interdisciplinary data corrections, satellite altimetry has been very successful in observing sea surface topography over a fairly wide range of spatial and temporal scales. Data from the GEOSAT mission are still providing better insight into the basin-scale circulation (see Figure 3) and are helping to quantify regional effects such as the El Niño-Southern Oscillation (Douglas, 1991). However, resolving large-scale oscillations with periods of years to decades and correcting RSL estimates for such sources of contamination (basin-scale sloshing) will require longer observation periods (e.g., Sturges, 1987). Upcoming missions such as ERS-1, TOPEX/POSEIDON, and the Earth Observing System (EOS) will provide immense volumes of new data that will presumably help unravel the many oceanic, atmospheric, and solid Earth contributions to understanding global sea level.

To estimate eustatic sea-level changes, various corrections have been applied to correct the raw data for the many effects on tide gauge readings of RSL (e.g., Douglas, 1991). 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., Peltier, 1990) (although, for reasons discussed below, the corrections made include substantial uncertainties, with differences among models of ~1 mm/yr). These studies of corrected tide gauge records indicate a global eustatic sea-level rise of about 2 mm/yr over the past 50 to 100 years. Only about

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

FIGURE 3. Dynamic sea surface topography caused by global ocean circulation derived from 1 year of GEOSAT Exact Repeat Mission altimeter data. Contour interval is 10 cm (from Denker and Rapp, 1990).

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

10% of this rise can be accounted for by thermal expansion of the oceans, so a substantial input of water into the oceans is needed to account for the rest. Lakes, groundwater, and mountain glaciers seem to account for only about a 0.7 mm/yr increase in sea level (e.g., Meier, 1990); hence, about 1 mm/yr is unaccounted for. The only sources that have been proposed to explain this discrepancy are Greenland and Antarctica, although published—albeit controversial —estimates of mass balance for these ice sheets indicate growth rather than decay (e.g., Zwally, 1989; Zwally et al., 1989; Meier, 1990). It is apparent from the variability of estimates in the literature that we do not know either the magnitude or sometimes even the sign of specific contributions of various sources to variations in sea level. As recognized by the Committee on Earth Sciences of the White House Office of Science and Technology Policy (1989, p. 32):

Our most glaring deficiency in knowledge of the cryosphere is whether Antarctica and Greenland are gaining or losing ice because, with 99.3% of the combined volume of the world's ice, changes in their volumes have the greatest potential impact on sea level.

Careful monitoring of snow accumulation (possibly aided by satellite missions to profile ice sheet topography) combined with field surveys of ice sheet flux and satellite monitoring of iceberg calving is one approach to this problem (Thomas, 1991). However, a direct approach, using the elastic properties of the crust as a spring balance for detecting changes in the ice sheet mass, has the advantage of providing a spatially integrated response (Hager, 1991). For a uniform disk of ice with a radius of 1,000 km, the vertical displacement of the Earth's crust at the edge would be about 10% of the total change in ice thickness, with the crust springing upward in response to ice wastage and sagging in response to accumulation. Rela-tive horizontal displacements are up to two-thirds of the vertical displacements, with the edges springing outward in response to a decrease in ice sheet mass. (These motions are elastic responses to very recent loads, not long-term visco-elastic behavior such as postglacial rebound.) Thus, one important reason for deploying an accurate geodetic network in polar regions is to assess changes in ice sheets in order to constrain the net water input to or removal from the ocean basins.

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) of the Earth relative

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

to the center of mass (CM), whose trajectory in inertial space is, of course, unaffected by this redistribution of mass, since no external forces are involved. 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) would lead to a shift of the CM relative to Earth's surface of 15 mm. Table 2 lists recent estimates of ice sheet evolution and the corresponding effects on sea level, expected elastic displacements of the crust, and CF shifts. This set of simple conclusions is given primarily for illustration purposes. More extensive modeling is needed to account for all possible effects, including combinations of various sources of possible CF shifts. One important criterion of a global network of fiducial sites is that the positions of these sites should be known in a center-of-mass reference frame to a precision sufficient to support the scientific questions raised by mass redistribution. These changes in surface load also change the moment of inertia tensor, leading to polar motion (e.g., Douglas et al., 1990).

The surface loads resulting from changes in short-term nonsteric loads such as shown in Figure 3 will also cause significant elastic displacements of the crust that will be detectable by a global geodetic network. Thus, oceanographic, meteorological, and solid Earth processes are closely coupled.

To summarize the present understanding of global sea-level changes, we note that many processes acting on different spatial and temporal scales are contributing. Observations of many different quantities are necessary to unravel the interfering effects. Meteorological parameters (rainfall, surface winds, surface pressure) are relevant not only for their own direct climatological sakes but also for their indirect effect on sea-level measurements and surface displacements at the shorter time scales. If we can understand the effects of these factors within the frequency band in which they are dominant, we will be able to remove their effects over longer time periods. And if we can monitor these effects continuously, we will reduce the contamination of our results by spatially and temporally localized effects.

Postglacial Rebound.

Intimately related to RSL change is postglacial rebound; this classic problem in geophysics is gaining renewed importance for both scientific and societal reasons. Aside from the relationship to global climate change by way of the rebound signal's strong presence in most available tide gauge and geological observations of relative sea level, today the problem is of increasing interest because it can provide crucial constraints for understanding the dynamics of

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

TABLE 2. Proposed rates of change in average thickness of the Greenland and Antarctic ice sheets, with the predicted concomitant change in relative sea level, vertical displacement rate, u (positive downward), and shift of the center of figure (CF).

Ice Sheet

Radius(km)

∆ Ice (cm/yr)

∆ RSL (mm/yr)

u (mm/yr)

CF Shift (mm/yr)

Greenland (1978-1986) (Zwally, 1989)

465

21

−0.4

10

0.1

Greenland (1990-2050) (Meier, 1990)

465

−60

1.2

30

0.3

Antarctic (recent) (C. Bentley, private communication, 1991)

2,100

2

−0.8

4

0.3

Antarctic (1990-2050) (Meier, 1990)

2,100

13

−5

27

1.7

mantle convection and the driving mechanism of plate tectonics (e.g., Peltier, 1989). Several recent investigations of mantle viscosity as probed both by changes in surface loads (associated with deglaciation) and by the geoid signal from interior loads (plate tectonic slabs) have suggested that the radial and lateral variations of viscosity have more structure than had previously been believed. 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. The fundamental observations used in modeling postglacial rebound are surveys of the relative heights of geomorphological features such as raised beach terraces that can be dated reasonably well over the past few hundred to few thousand years. The rheological models that fit these observations generally predict present local vertical changes (either upward or downward, depending on location relative to the historic ice load) on the order of 0.5 to 10 mm/yr. Horizontal velocities are comparable. This means that postglacial rebound, like the elastic displacement of the Earth in response to changes in

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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surface loads, has large geodetic signatures that must be understood and modeled thoroughly if we wish to study sources of motion such as those from continental epeirogeny or intraplate deformation. Models derived by different investigators vary greatly in their predictions of present motions. The reasons include the following serious problems:

  • The geometry of the ice load is not known in detail.

  • The melt history is not known in detail.

  • Different a priori assumptions are made by different investigators.

  • Rebound models do not yield very good resolution of mantle structure.

  • Lateral variations in mantle rheology are likely.

These factors trade off against the details of the resulting rheological models that fit postglacial rebound—a reflection of the nonuniqueness of the inverse problem. Some of these problems can be reduced by considering viscosity constraints inferred from the geoid response to interior loads (such as descending slabs), constraints associated with the dynamics of plate motion, and constraints derived from observations of Earth rotation and nutation. 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? Different estimates of mantle rheology have come out of studies focused on different regions. There is a systematic trend in these estimates of upper-mantle viscosity as a function of tectonic age. We can rank them in order of decreasing inferred upper-mantle viscosity, as follows: (1) Laurentian (predominantly shield); (2) Fennoscandia (shield plus Paleozoic fold belt, close to ocean); (3) Australia (shield and Paleozoic fold belt, close to ocean); and (4) Pacific Islands (ocean basin).

  • What has been the amount of melting of the Antarctic ice sheet in the past 6,000 years? There is a substantial trade-off between mantle-inferred viscosity structure and ice load history. Models that assume no recent melting yield estimates with higher upper-mantle viscosities and smaller lower-mantle viscosities than those that assume recent melting. These models differ in their predictions about the current rate of collapse of peripheral bulges surrounding various ice sheets, as well as on the tilting of continental margins caused by the load from increased melt water in the ocean basins.

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Consideration of postglacial rebound will influence the optimum geometry of a global network of fiducial sites, and its contribution must be fully understood before other problems can be solved. In terms of network design, these points lead to the following general considerations:

  • Network design is a multiscale problem. The density of sites must vary as a function of distance from the center of the ice sheet, and good azimuthal coverage is needed.

  • Global coverage would help to unravel the complexities of the problem. However, at large distances from the ice sheets, the signals are small (0.5 mm/yr vertical, compared to 5 to 10 mm/yr vertical close to the ice sheets, based on disk load approximations). The problem may be best addressed with a combination of global and regional sites.

  • 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.

  • Horizontal as well as vertical motions are useful for discriminating among models of mantle rheology (e.g., James and Morgan, 1990).

Changes in the ice mass will also trigger secular motions of the pole. The centroid of the annual and Chandler (14-month) motions shifts in response to long-term redistribution of mass within and on the surface of the Earth. Historically, the pole has been shifting by approximately 3 milliarc-seconds per year toward Hudson Bay, Canada. This rate is in good agreement with the rate expected from glacial rebound. Since the motion depends on all changes in mass distribution, it cannot be ascribed to any specific cause, such as ice buildup on Greenland, but it can be used to place constraints on mass relocation and thus help test the consistency of putative changes (Douglas et al., 1990).

Tectonic Motions and Deformation.

A major conclusion from the NASA Crustal Dynamics Project is that the space-geodetic estimates of relative plate motions over the very short time of 10 years or so are in very good agreement with the million-year averages provided by geological data. Such an agreement over five orders of magnitude in time scales is of great interest to scientists who construct numerical models of mantle convection and plate tectonics. That this agreement

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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holds for essentially all the plates sampled by space-geodetic sites is illustrated by Figure 4, where the relative motions between sites on stable plate interiors, estimated by satellite laser ranging to LAGEOS, are shown to be extremely close to and well correlated with the NUVEL-1 (DeMets et al., 1990) geological estimates.

This agreement may be an indication that the motions of the interiors of the plates are steady, implying that the present plate geometry is the product of continuous motions over millions of years. However, as discussed by Smith et al. (1990) (Figure 4), the difference in the average NUVEL-1 and SLR geodesic rates is small but statistically significant and requires an explanation. They discuss three main possibilities, namely (1) plate acceleration, (2) bias in the geological time scale, and (3) geographical bias in the SLR network. Differences between the geological predictions and VLBI results also seem significant for the baseline from Westford Massachusetts to Wettzell (Germany), for which the NUVEL rate is 18.8 ± 0.5 mm/yr, while the 1985-90 VLBI rate is 14.9 ± 0.4 mm/yr, although the effect of local deformation remains an unresolved issue (T. Herring, Massachusetts Institute of Technology, private communication, 1991; see also the section on Spatial and Temporal Sampling and Figure 6 on page 58).

Space-geodetic techniques are already contributing valuable estimates of plate motions in areas where geological evidence is weak. These include relative plate motions across trench boundaries, where the lack of geological rate information leads to substantially poorer estimates (DeMets et al., 1990), and in regions where geological data are too sparse to permit reliable solutions (e.g., around the Philippine plate). Also included are areas where the ideal rigid-plate model does not adequately describe tectonic interactions, especially within the continents and along their margins. Examples include the Mediterranean (WEGENER-MEDLAS Project; Wilson, 1987) and the western United States (e.g., Minster and Jordan, 1987; Jordan and Minster, 1988b; Ward, 1990; Argus and Gordon, 1990; Smith et al., 1990; Humphreys et al., 1990). Although we do not yet enjoy the benefit of long time series, it is clear that the combination of permanent space-geodetic systems, incorporating all existing techniques (such as SLR, VLBI, GPS, DORIS, and PRARE), will help refine plate motion models in several such areas.

To eliminate the trade-off 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. Furthermore, with the Westford-Wettzell baseline mentioned above, if we decimate the original data set to one point every 90 days, the rate estimate becomes 16.5 ± 1.4

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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FIGURE 4. Comparison of SLR-determined geodesic rates with those predicted by NUVEL-1 for 54 baselines with end points well within the interiors of five plates. The slope of the line is 0.949 ± 0.019 (from Smith et al., 1990).

5mm/yr, which is not distinguishable from NUVEL-1 at any comfortable confidence level. This indicates that comparison of geodetic and geological estimates at the level where we look for temporal dependence of tectonic rates requires smaller errors. Furthermore, frequent sampling is required to elucidate the temporal character of the errors and to determine the noise spectrum. 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.

Only a few geodetic data sets have been published that involve measurements frequent enough to warrant time series analysis. Many consist of terrestrial measurements, but some baselines (e.g., Westford-Wettzell) have been measured

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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frequently enough over the past decade or so to provide estimates of the noise processes and the detectable signals. Another example, shown by Figure 5, is the time series representing the length of the Westford (Massachusetts) to Ft. Davis (Texas) baseline as a function of time. This particular baseline has been the subject of considerable debate during the Crustal Dynamics Project, since the eye-detectable excursion apparent during the approximate 1984-88 period could not easily be explained in terms of geologically plausible causes. The last data points in the series suggest that the long-period (5-year) transient may have ended recently, but the details of this recovery are missing, because of the data gap in 1989-90. The other point well illustrated in Figure 5 is that both the error bars on individual data points and the overall scatter show large variations as well as an overall decrease with time. With the global network of fiducial sites, such complications will presumably be rare, at least for a substantial subset of sites, but they cannot always be avoided in practice, so that the network design should attempt to compensate for them, in particular, by choosing a strong network geometry.

From a geophysical standpoint, even if the nominal design requirement for the global network is for reliable positioning of sites at ~3,000-km spacing, strong arguments can be made that closer spacing (~1,000 km) is highly desirable. Again, Figure 5 is a case in point. This time series shows apparent time-variable motion at the 10 mm/yr level, a surprising result for a baseline spanning a stable plate interior. It is clear that frequent measurements of intermediate locations (i.e., a denser network) would greatly aid the interpretation of this behavior.

Plate boundary zones are characterized by seismic and volcanic activity, two hazards that space geodesy may help mitigate. The spatial scales involved in earthquake and volcano monitoring are small, and these problems are properly addressed in the context of regional densification. But local and regional space-geodetic networks would benefit considerably from a global fiducial network. Since frequent, or even continuous, monitoring is increasingly the norm for this type of work, the global network also should function in this mode. This requirement is extended not only to stations used primarily for orbit determination but to others as well, since these other stations will often be used as regional fiducial sites for precise relative positioning within local networks. This is not to say that the fiducial stations alone could not collect data directly relevant to earthquakes or volcanic eruptions. For example, in the great (Mw 9.5) Chilean earthquake of 1960, the Chile trench ruptured over an area of 103 km, so that even a coarse geodetic network would have had a fair chance of including a site within the region of strong deformation. With continuous operation, such a station would

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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FIGURE 5. Time series for the Westford (Massachusetts)-Ft. Davis (Texas) VLBI baseline (courtesy of W. Carter, NOAA and T. Herring, MIT, private communication, 1991).

contribute extremely valuable information. Such events are quite rare, however, and the chances of catching an earthquake with a global network are far from overwhelming.

Finally, in plate boundary deformation zones, the spatial distribution of deformation is a critical question. 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.

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Geodetic Objectives and Applications

The scientific objectives discussed above will not be achieved unless the geodetic problems they raise are solved with sufficient accuracy and reliability. The existing fiducial networks formed by VLBI, SLR, and LLR are insufficiently dense to satisfy the scientific objectives stated previously; the most cost-effective technology now available for such densification is GPS. 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.

Determination of Precise GPS Orbits.

One of the important data products generated by the global network is a set of precise ephemerides for the various spacecraft involved, in particular the GPS (and GLONASS) constellation. To support high-accuracy geophysical studies, the orbits should be globally accurate at the 10- to 20- cm level to allow relative baseline accuracy at the level of a few parts in 109. For a variety of geophysical applications (e.g., earthquake or volcano monitoring), solutions should be available within a few days after data acquisition. This does not mean that solutions could not be produced within a shorter time. However, the trade-off between the solution delay (relative to real time) and the robustness and accuracy of the orbits so obtained remains to be explored. GPS is the only current cost-effective technology with the potential for meeting the array of scientific objectives described in the previous sections. An important element of a global fiducial network is fundamental support for determination of precise GPS ephemerides (and possibly GLONASS ephemerides at some future time), which are required for precise positioning.

Under the proposed International GPS Geodynamics Service (IGS), ephemerides for support of high-accuracy scientific applications would be transmitted in a timely fashion to geodetic users, thereby simplifying the data processing task of the user community. The Department of Defense (DoD) maintains the satellites and produces ephemerides broadcast by the satellites, but these ephemerides permit only part-per-million (ppm) geodesy, rather than the required part-per-billion (ppb) precision. The ppb need is evident from the fact that, at this level of precision, a 2,000-km spacing of fiducial sites translates into about a 2-mm baseline error, a desired level based on the preceding discussion of scientific requirements. More importantly, this level is required for all three components of the baseline. By far the most effective mechanism to satisfy this need would be for

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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DoD to broadcast more precise ephemerides. The civilian community should continue a dialogue with DoD to explore this possibility.

From the geodetic standpoint, the ephemerides should be fully traceable. That is, accepted standards should be used (e.g., McCarthy, D. D., ed., 1989) and the techniques, procedures, and constants fully documented. The information should contain not only ephemerides (positions and velocities) but also parameters derived during the orbit determination process leading to the ephemerides. Furthermore, special information about the status and health of the satellites should be available.

The geodetic objective for the generation of GPS ephemerides should be orbits of the highest precision to support high-accuracy positioning anywhere in the world. Such a determination requires a fiducial network, but as discussed by Lichten and Neilan (1990), the spacing does not need to be 2,000 km. In fact, about 20 to 30 stations will more than suffice; larger numbers would enhance the network and, through redundancy, provide a check on the data and ensure that a minimum number of stations are on-line at any one time. This set of stations, referred to as the “GPS Core Network” or “GPS Core Stations,” should be selected for worldwide coverage and for collocation with VLBI and SLR stations at a sufficient number of sites to satisfy reference frame requirements but should be dedicated to continuous GPS tracking. 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. Other problems, such as multipath effects, should be minimized by adopting a set of appropriate uniform standards for the deployment and operation of the GPS core network.

The GPS stations may be augmented to provide redundancy, to enhance geographical coverage, or to meet other needs, such as enhancing ambiguity resolution using a clustering of sites (e.g., Lichten and Neilan, 1990). Further, network operation should not be significantly affected by the temporary outage of one, two, or even several core sites. Simulations should provide a quantitative evaluation of the expected network performance for any given selection of sites and help decide whether adequate redundancy is available, particularly in remote oceanic areas where coverage is more difficult to achieve.

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Precise Earth Rotation and Orientation Parameters.

The determination of Earth rotation parameters (pole position, length of day) and orientation parameters (precession, nutation, and UT1) by the IERS is based on VLBI, LLR, and SLR. However, GPS is affected by somewhat different error sources than are these techniques, which could aid in the separation of geophysical errors from other systematic errors. Nevertheless, GPS Earth rotation and reference frame applications have not yet matured to a level comparable to that of baseline determinations. As a consequence, a campaign was conducted from Jan. 22 to Feb. 13, 1991—the First GPS IERS and Geodynamics Experiment (GIG'91)—with the goal of collecting data for investigating these applications, particularly the monitoring of short-term fluctuations in the length of the day. This relatively short campaign limits the extent of the assessment, but future campaigns proposed by the IAG (e.g., EPOCH '92) will permit a more complete evaluation. Such experiments will also help identify optimum ways to use GPS data in the IERS technology mix.

As noted in the Coolfont report (NASA, 1991), the goal for determination of pole position is 0.25 mas (milliarcseconds) with temporal resolution of 6 hr and the goal for the length of day or UT1 rate is 0.1 msec/day, every 6 hr. These values correspond to subcentimeter accuracy in determination of Earth orientation. 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.

Realization of a Precise Global Terrestrial Reference Frame.

The requirements for a terrestrial reference frame have been specified by IAG Special Study Group SSG 5.123. The intention is to permit realizations of this reference frame at the millimeter level at the Earth's surface (or 10-10) without ambiguities. In this respect the deployment and operation of a global network of fiducial sites would result in a major advance. Several points deserve mention.

This objective requires, at the core network level, integration of several techniques, particularly VLBI, SLR, and GPS, but also other techniques used on either local or global scales (e.g., DORIS, PRARE), so that all measurements are reduced to the same reference frame. This also holds for a variety of other types of geodetic or geophysical measurements, in particular gravity. SLR is needed to achieve center-of-mass reference (GPS also uses a center-of-mass reference), while VLBI is needed to provide a tie to an inertial frame of reference.

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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The IERS has tackled the problem of realizing a terrestrial reference frame by incorporating a very large number of space-geodetic sites in its solutions. However, because of the large variations in the nature of sites, in the history of site occupancy, and in the equipment at each location, characterization of the reference frame depends on the weighting scheme used in combining the data sets, which is itself somewhat subjective. 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.

Defining a suitable reference frame has been so important over the years that current and/or planned deployments pay considerable attention to it. As a result, there is already substantial collocation of sites equipped with different technologies, including GPS. Current plans for the southern hemisphere, coordinated primarily through U.S. agencies such as the National Geodetic Survey and NASA, will lead to a substantially improved situation in the next few years. In this respect the need for transportable VLBI and SLR systems endures, since the permanent VLBI and SLR networks are rather sparse, and GPS accuracies for long baselines are still questionable.

Support of Scientific Orbital Missions

An important application of the global network is to support scientific missions in low Earth orbit. A comprehensive discussion is given by Melbourne (1990). The most visible advantage in the short term is the determination of an accurate orbit independent of the usual ground tracking techniques. The benefits of a global distribution of ground stations for orbitographic systems have been demonstrated with the DORIS system and will be further illustrated with the PRARE system. Given that several orbitographic systems are now deployed, there is again a need to maintain accurate ties, through collocation between the GPS global network and these other networks (SLR, DORIS, PRARE). Data from the global network can greatly simplify the implementation of precise differential GPS orbit determination of low-altitude satellites with onboard receivers and thereby contribute substantially to a variety of scientific goals of such missions (e.g., aeronomy and gravity studies). Without global network data, the operators of such satellites must set up

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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and maintain costly ground stations of their own, so that significant cost benefits would accrue from a cooperative global network.

In return, this particular class of applications entails a possibly significant mission-dependent operational impact on the network, particularly in terms of data availability, data rates, and telemetry. 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) than most other applications of the global network. For practical purposes, a global network of fiducial sites that satisfies the requirements listed earlier will be capable of providing the data required for mission support, provided that quality control is effective.

Low Earth orbiters, such as TOPEX/POSEIDON, Aristoteles, GP-B, or the EOS platforms, equipped with an onboard receiver to track the GPS constellation at the same time as the global network, can provide additional data of considerable value to the operation of this network:

  • The orbiter is capable of simultaneously tracking GPS satellites that are only visible separately from the ends of extremely long baselines on the ground. In this fashion the orbiter may act as a temporary bridge and expand the range of distances over which differential techniques can be used. From the point of view of network design, this makes it possible in principle to determine the precise locations of stations on remote islands using differential positioning.

  • The rapid change in geometry resulting from the motion of the orbiter will strengthen the solution of all terrestrial baselines, regardless of whether a bridge is needed. Alternatively, the observation time required to achieve a specific accuracy may be shortened.

  • If we consider the orbiter a flying station that is part of the global network, the overall network will incorporate Earth-to-space baselines with large vertical (radial) components. This will improve the accuracy of the estimated vertical coordinates (typically the worst-determined ones) and help correct for tropospheric refraction, which affects ground-based data but does not corrupt the orbiter's data. Use of orbiter data will also increase the accuracy of the GPS ephemerides as part of the operation of the global network. Simulations described by Yunck and Melbourne (1990) suggest a possible doubling in accuracy for both horizontal and vertical station coordinates, with subcentimeter results for baselines up to

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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4,000 km long, after inclusion of data from each of the EOS platforms. Ephemeris accuracy could triple in all three directions (radial, along orbit, and across orbit).

A number of Earth observing missions require precise global tracking to accurately recover the trajectory and orientation of the satellite. 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 gradiometers, and other geodetic satellites in low Earth orbit, are used to measure the Earth's gravity field on regional (100 to 1,000 km) and global (1,000 to 10,000 km) scales. The accuracy and resolution of the recovered gravity field depend primarily on the accuracy, coverage, and duration of operation of a global tracking network. Finally, high-resolution imaging systems require global tracking to provide real-time control on the location of the spacecraft and to register the images precisely during postprocessing. Thus, many planned and proposed missions depend on the implementation and maintenance of global tracking networks.

At least two types of global tracking networks have been developed. Satellite-to-satellite tracking, using the GPS system, can provide global coverage at relatively low cost to NASA since the DoD maintains the GPS system. In this configuration the high-altitude (20,000-km) GPS satellites have orbits that are relatively insensitive to the poorly known higher harmonics of the Earth's gravity field. Satellites in lower orbits (~1,000 km) can be tracked continuously by the GPS constellation. As few as a dozen well-distributed GPS ground sites would suffice in principle to give orbital accuracy that meets or exceeds that currently available with the best ground-based laser tracking systems. For redundancy to protect against station outage, about 30 stations would be prudent. Given the coverage that can be physically achieved in areas mostly covered by oceans, and given the fixed ground tracks of GPS satellites, an optimal network geometry is unlikely to be achieved. As a result, a somewhat greater number of stations might be required in areas where network geometry is not flexible (as in the South Pacific). Concomitantly, a detailed analysis of each proposed network geometry should be conducted, and the need for redundant equipment at the more isolated sites should be assessed with care.

Ground-based tracking systems also used in orbitography include TRANET (TRAnsit NETwork), DORIS, SLR, and PRARE. The most accurate range measurements (~10 mm) are made with SLR, using ground-based laser telescopes and a laser retroreflector on the spacecraft. However, since SLR sites are expensive to maintain, the global network is relatively sparse. Nevertheless, the

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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high-accuracy range measurements from SLR sites are required for precise tracking of geodetic satellites such as Starlette, LAGEOS I and II, and Stella.

Satellite Altimeters

Satellite altimeters provide topographic data for a number of important scientific applications. Precise measurements of ocean topography reveal the fine-scale marine gravity field (~10 km horizontal resolution) as well as the dynamic topography associated with mesoscale and basin-scale ocean circulation. Repeat measurements of ice topography can be used to monitor the mass balance of the major ice sheets. Over land altimeters can be used to measure topography at high spatial resolution on a global basis. In each application the topography of the surface is the difference between the altitude of the spacecraft above the closest surface (water, ice, or land) and the height of the satellite above the reference ellipsoid. For the ocean and ice applications, the altimeter can achieve an accuracy of 20 to 40 mm. Depending on the application, the radial position of the satellite must be tracked with an accuracy of 40 to 200 mm. The most stringent tracking requirements come from the World Ocean Circulation Experiment (Chelton, 1988).

Many satellite altimeter missions are planned for the next decade. ERS-1 (launched July 17, 1991) and ERS-2 (1994) will measure the marine gravity field at high spatial resolution and monitor changes in ocean dynamic topography (i.e., geostrophic surface currents) for about 6 years. PRARE and SLR will be used for tracking the ERS spacecraft. The TOPEX/POSEIDON mission (mid-1992 launch) has the most stringent requirement for radial orbit accuracy (~100 mm); it will be tracked using a combination of SLR, GPS, and DORIS systems. The Earth Observing System (EOS) satellites (launches in 1998 and 2001) will carry radar altimeters for oceanographic and geodetic applications. In addition, the second EOS mission is slated to carry a Geoscience Laser Ranging System (GLRS), which will be used for precise positioning of ground-based laser retroreflectors as well as for laser altimetry over land, ice, and water. The EOS platforms will be tracked using multiple GPS receivers as well as ground-based systems such as DORIS and SLR.

A topographic mapping mission is anticipated (1999 launch) to map the topography of the land and ice at high spatial resolution (~100 m) and high vertical accuracy (~3 m). While the type of instrumentation (e.g., laser, radar,

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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or stereo-optical) has not yet been chosen, the positions and orientations of these spacecraft must be tracked quite accurately on a global basis. A discussion of imaging altimeters appears in the section titled High-Resolution Imaging Systems.

Aristoteles, Gravity Probe-B, 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). Although the primary mission of TOPEX is to monitor global ocean circulation, it will be used as a secondary experiment for gravity model improvement. The TOPEX/POSEIDON spacecraft will carry a GPS receiver to monitor its own position and trajectory with respect to the higher GPS constellation. The ground-based part of the experiment will include at least six GPS receivers (Wu and Yunck, 1991) distributed around the globe for tracking the GPS constellation. It is anticipated that the ground receivers will be positioned to an accuracy of about 50 mm. This experiment should yield continuous position estimates for TOPEX/ POSEIDON that are accurate in the radial direction to better than 100 mm. These position estimates can be used in turn to improve the global gravity field dramatically at long wavelengths (up to spherical harmonic degree 35; Schrama, 1990). Knowledge of gravity field variations at wavelengths shorter than the spacecraft 's elevation (1,330 km) will not be significantly improved.

This same concept for gravity field improvement has been advanced for two satellites in low-altitude orbits, Aristoteles and GP-B. Aristoteles is a gravity gradient satellite proposed by the European Space Agency (ESA) for launch in 1996-98. The onboard gradiometer is expected to provide great improvements in the short to intermediate (150 to 500 km) wavelengths of the Earth's gravity field, whereas the precise tracking system should improve gravity models significantly at wavelengths longer than 450 km (Schrama, 1990). Although the high-precision tracking system may be a GPS instrument similar to the demonstration experiment on TOPEX/POSEIDON, it is likely that accuracy requirements for gravimetric missions will be more stringent. The global network of precisely positioned fiducial stations proposed here may contribute in a large way to the success of Aristoteles. This would be particularly true if a direct ground tracking system such as PRARE or DORIS were used in lieu of (or in combination with) the more indirect GPS approach. Moreover a direct tracking system for a low orbiter such as Aristoteles would include several hundred accurately positioned (~10 mm)

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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ground stations. Another low-orbit spacecraft that may carry GPS or other precise tracking equipment for global gravity field mapping is Gravity Probe-B (GP-B). GP-B, planned for the mid-1990s, will fly at a 600-km altitude and will conduct, as its primary mission, the Stanford relativity gyroscope experiment.

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 Imaging Systems

Space-based imaging systems such as NASA's Landsat Thematic Mapper and France's Satellite pour l'Observation de la Terre (SPOT) satellite are having a major impact on scientific applications of remote sensing technology. This impact reflects the increasing spatial and spectral resolution of these instruments, increasing ease of data acquisition coupled with decreasing costs, improvements in computer technology and image processing algorithms that permit ever more sophisticated and rapid analyses, and improved models and understanding in the scientific community.

Closely related to these changes is increased demand for more sophisticated data products. One example is the need for image data with better geometric fidelity and more accurate location knowledge. This implies the need for improvements in the areas of spacecraft location and orientation (pointing). Spacecraft designers usually distinguish between control and knowledge for both location and orientation. Control requirements are generally less stringent than knowledge requirements, the latter being computed after the fact with the benefit of accurate models and additional data. However, control is required in real time, which often makes this aspect more challenging. Earth observation missions with the most stringent location and orientation requirements can be divided into two classes: high-resolution spectral imaging systems and imaging altimeters. In the first application we require geometric fidelity such that a picture element, or pixel, will be accurately associated with a given spot on the ground. Pixel location knowledge currently planned for imaging sensors in the civilian areas implies that we need to be able to locate pixels to an accuracy of 3 m. Actual performance at present—for locating 20- to 30-m pixels from sensors with high spectral resolution—is considerably worse. Location knowledge is usually improved during postprocessing and image analysis, as the image itself allows location to be

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

determined by reference to ground tie points. Nevertheless, because it is desirable to reduce processing and analysis time, accurate a priori location knowledge would be very useful for automating the analysis.

Imaging altimeters may actually have more stringent requirements; we will therefore use them to illustrate potential tracking system applications. An imaging altimeter is a high spatial resolution instrument for height measurement over continents and ice caps. The oceanographic counterpart estimates height from time-of-flight measurements of a simple radar pulse with a spherical wavefront, the echo of which can be assumed to arrive at the spacecraft from the exact nadir point. The imaging altimeter, in contrast, uses some sort of scanning or imaging system, such as a scanning laser or synthetic aperture radar imaging system. With this approach, location and pointing knowledge (and to a lesser extent control) are critical to accurate height estimation. Spacecraft location knowledge in the radial component must be better than 0.5 m. Three-dimensional location accuracy, including along-track and cross-track components, can be slightly worse but should not exceed several meters. The required pointing knowledge depends on spacecraft altitude, surface slope, and desired accuracy; for a spacecraft altitude of 500 km, surface slope of 25 degrees, and desired height precision of 1 m, pointing knowledge should be accurate to about 1 arcsec.

Most current spacecraft designs for high-precision Earth imaging rely on a combination of sophisticated star trackers or star cameras, which can be supplemented with (inertial) gyroscopes for accurate orientation information (both control and knowledge). They also use relatively crude ground tracking information and/or orbit modeling for real-time location data and more sophisticated orbit modeling for improved location knowledge. 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 high-precision tracking techniques, which rely on numerous ground-based lasers (constrained by limitations on satellite visibility from the ground stations) and extensive after-the-fact modeling. The GPS approach has additional advantages where near-real-time location accuracy is required because GPS tracking is essentially geometric, in the sense that range is measured between the orbiter, whose position may be poorly known initially, and several GPS satellites whose positions are well known. Extensive orbit modeling is not required in this geometric approach, implying that high orbit accuracy could be available in near real time.

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

The situation is somewhat different with orientation data. Current star trackers are capable of delivering approximately 1-arcsec performance. Although star tracker technology is mature, the instrumentation can be expensive. Can three GPS antennae be used to establish both the location and orientation of a spacecraft in a cost-effective manner, thus eliminating the need for expensive star trackers? Consider first two GPS antennae (separated by a baseline that might range from 1 to 10 m in length for a typical spacecraft), feeding their signals to a common receiver. The lowest achievable standard error in position estimates from GPS data in the absence of ionospheric and atmospheric errors is about 0.1 mm (based on zero-length baseline tests with advanced digital receivers). We conclude that even with a 10-m baseline, the best possible GPS orientation performance (~2 arcsec) is not quite competitive with star tracker technology. However, a case can be made that if high-precision tracking with GPS is required on a given spacecraft for other reasons, a GPS-based orientation system might be attractive to provide redundancy with star tracker orientation systems. This might be appropriate once space-based tracking with GPS becomes routine and the cost of a space-qualified GPS receiver falls. The core network described above for location information would similarly be adequate for orientation applications.

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 IAG, 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:

  • Tie local and regional nets to the global network: The main application of the global network is to provide a precise and reliable reference frame. However, for a number of applications, having two or three global sites within 103 km of the survey area will simplify processing and logistics and often result in higher accuracy. In general, however, the existence of globally accurate orbits is likely to have the greatest impact, if only in the form of much simpler and more streamlined processing of local survey data.

  • Precise local geodetic and geophysical surveys: The number of local and regional networks monitored precisely for geophysical purposes (e.g., earthquake

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

and volcano studies) is increasing. The support afforded by a globally operated network cannot be overstated. Local dense networks would typically be operated independently of the global network, although a small number of sites could well be incorporated into the global operations, thus providing an automatic high-quality tie. Note that a single global site on Kilauea would permit reliable prediction of rift events! Finally, it should be noted that current efforts toward developing ocean bottom geodetic systems depend critically on the availability of kinematic GPS techniques when tying seafloor monuments to land-based networks.

  • Other applications: For most surveying applications (e.g., cadastral, highways), the global network and associated services (e.g., IGS) would contribute primarily orbit information, in addition to providing a global set of very-well-located primary sites. The data processing could then be streamlined, although questions arise about the timeliness of orbit solutions and data accessibility. In addition, civilian services broadcasting regional corrections to Selective Availability (SA) 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. In many countries points with accurately determined coordinates in a global coordinate system (typically longitude/latitude) are rare but are necessary for many surveying operations. Surveying and cartography are therefore hindered by the lack of accurately determined points in a global network. Errors in the control used are usually noticed when maps from two countries are joined or when engineering projects extend beyond national boundaries. The inaccuracies in the control surveys are detected in such projects because modern surveying instruments are very precise compared to those used in the control surveys. Typically, it is impossible to correct the flaws discovered, so that good new data are forced to fit the existing low-precision control. More very precise control points must be established using GPS, which would be useful for new cartographic or Geographic Information System (GIS) data collections. Locations determined with GPS technology often do not fit easily into the existing network because of the errors it contains. One may argue that the precision proposed in this report is not relevant for surveying (clearly millimeter precision is not necessary for boundary location). But the justification for keeping high precision in boundary surveys and other day-to-day surveying operations is the need for long-term maintenance. Property surveys must be maintained over long periods, so that new measurements must be merged with old ones. Imprecision hinders this process and leads to costly fudging.

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

Finally, current GIS technology relies heavily on coordinate values, which in practice are often determined very imprecisely. An accurate global network could provide the control for securing national geodetic networks and tying them to an international standard, thus helping to make data from different sources more compatible—for example, for the study of phenomena on a continental scale. A detailed review of many of these matters, including the requirements for precise control, is provided in the report Spatial Data Needs: The Future of the National Mapping Program (National Research Council, 1990c).

In view of these general considerations, progressive densification of the global network beyond the GPS core network is required in the medium to long term. 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). Redundancy of measurements is crucial, both to reduce noise and to help eliminate systematic errors through recognition and editing of data that are suspect for either geodetic (e.g., instrument malfunction) or geophysical (e.g., site instability) reasons. On the other hand, financial resources are finite, so it is important to recognize the point of diminishing returns. We discuss below a rationale for determining optimal sampling strategy, both in time and in space.

Because the forces acting on GPS satellites are much more difficult to model than those acting on a simpler satellite such as LAGEOS, continuous operation is necessary for GPS orbit determination. This means that the GPS core network must be operating continuously with high reliability. The core network also can adequately handle the determination of polar motion and Earth orientation, including high temporal resolution objectives.

For most of the other scientific problems we have talked about, the geophysical processes are expected to occur on time scales of years to decades. The frequency of observations should be determined by the trade-off between geodetic accuracy and economic/logistic considerations. The more frequent the measurements, the

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

more accurate the (averaged) values, up to some point. For example, several measurements per year clearly give more reliable determinations of crustal motion over decades than do yearly measurements, but it is not clear that daily measurements give more reliable determinations than weekly measurements. On the other hand, the more frequently measurements are made, the more financially attractive continuous monitoring becomes. For example, weekly measurements could be extracted more economically from continuously operating stations than from field campaigns, but it is not obvious that the same is true of quarterly measurements. Similar comparisons can be made with spatial sampling: if most of the resources are spent traveling between sites, a permanent instrument equipped with a data transmission system (telephone or telemetry) may be more attractive. Finally, leaving a permanent installation in the field largely eliminates certain sources of error or noise associated with installation and removal of geodetic equipment.

Assuming that high accuracy is the overriding consideration, how often should measurements be taken for optimum operation? Although our experience is still relatively limited, some relevant data are available from VLBI experiments. For example, Figure 6 shows the history of length determinations on the trans-Atlantic baseline from Westford, Massachusetts, to Wettzell, Germany, from 1984 to 1991. Individual length determinations scatter by up to ~50 mm from the best-fitting line (14.9 ± 0.4 mm/yr), even though the outliers otherwise appear to be “good” experiments from the point of view of their residuals. An unfortunate choice of individual measurement points could obscure the entire linear trend, which has a change of ~80 mm over the time shown. Averaging over 90-day windows reduces the scatter by an order of magnitude, but there is still nearly 5 mm of scatter.

Examination of Figure 6 also reveals evidence of systematic variations at the noise level (~10 mm) in the original data, some of which survive the 90-day averaging and some of which do not. These variations are as yet poorly understood, although recent solutions indicate the presence of seasonal/annual effects. In addition, if the noise in the data averages followed the classical vN law, the 90-day averages shown would have had a root mean square (rms) scatter of 2.7 mm instead of the 3.2 mm shown. Furthermore, if we decimate the original data set to one point every 90 days (taking the closest raw data points to the middle of the 90-day windows), the VLBI rate estimate becomes 16.5 ± 1.4 mm/yr, which is not resolvable from NUVEL-1. 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

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

FIGURE 6. Evolution of the Westford-Wettzell VLBI baseline length, 1984-1991 (courtesy of T. Herring, Massachusetts Institute of Technology, 1991).

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

probably correlated on time scales of 2 to 5 days, so little might be gained by computing solutions more frequently than about weekly.

Another example of the potential benefits of dense temporal sampling are the 8 year records along three baselines in the San Francisco Bay area shown by

FIGURE 7. 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. The solid line is simply a smoothed version of the data (from Lisowski et al., 1990).

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

FIGURE 8. Top panel: Laser strain record from Piñon Flat Geophysical Observatory, compared with Geodolite (open squares; courtesy of W. Prescott, J. C. Savage, and M. Lisowski, U.S. Geological Survey) and 2-color EDM (solid squares; courtesy of J. Langbein, U.S. Geological Survey) measurements of geodetic arrays near PFO. Geodolite array dimension is 15 km, 2-color EDM baselines are 4 km, and laser strainmeter length is 0.732 km. The dashed line gives the long-term strain rate found from the Geodolite network over 15 years (0.03 ± 0.013 µe/yr), in good agreement with the NW-SE strainmeter rate (0.03 µe/yr). The residual strain record highlights abrupt changes due to earthquakes and locally induced deformation. This series is formed by removing Earth tides from the monument- and laser-frequency-corrected strain record. The zero level for all series is arbitrary. Bottom panel: Comparison of absolute gravity (converted to elevation changes) and VLBI. An interesting correlation is evident in 1987, but since there is no anomaly in the PFO strain or tilt data, this may be a coincidence of instrumental effects (courtesy of D. Agnew, H. Johnson, F. Wyatt, and M. Zumberge, University of California, San Diego).

Lisowski et al. (1990) to contain signals possibly precursory to the 1989 Loma Prieta earthquake. Lisowski et al. document what they label a “marginally convincing” strain anomaly approximately 1 year before the event, over spatial scales of 30 to 50 km. 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.

Figure 8 shows a comparison of strains recorded continuously at the Cecil and Ida Green Piñon Flat Geophysical Observatory, California, with several types of geodetic measurements by both ground-based and space-geodetic techniques. Although the various trends agree reasonably well, it is abundantly clear that occasional geodetic occupations will not tell the whole story. But given the apparent scatter in the geodetic solutions, increasing the frequency of occupations to the point of continuous presence may not be scientifically justified.

The other side of this issue is, of course, the economics. The economics of continuously operating stations versus occupation by field campaigns depends on the number of campaigns per year, the number of days per campaign, the cost of collecting data continuously, and the cost of returning data to the central archive (Appendix D). For the cost estimates in Appendix D, it was assumed that 20% of

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
×

the expense of maintaining dedicated laboratories can be attributed to the global network; the break-even point is about six campaigns per year, or bimonthly occupations. As discussed by Melbourne (1990), the cost of GPS receivers is decreasing rapidly and the newer generations of receivers are very likely to require much less maintenance in the field than earlier instruments. The result is that the cost of running the global network will be dominated by data processing and communications costs.

In this respect, participants in the development of the global network will face the choice of either occupying network sites during temporary campaigns or equipping such sites with permanently installed receivers with a telemetered data stream. Both modes can be scientifically valuable, although we have argued that, at least in some cases, very frequent sampling should be a design constraint.

Clearly, the choice of operation mode must depend in part on the characteristics of the site (e.g., availability of power, communications, personnel). However, with a substantial drop in the price of the equipment itself, labor-free, unattended operations with permanently installed equipment will become more and more economical at many sites. The obvious trade-off with the desired sampling rate yields a crossover point where permanent occupation is more cost effective than a temporary campaign that is rapidly shifting such that even fairly infrequent measurements might justify a permanent installation. On the other hand, from the point of view of site selection, the requirements for sustaining data logging and telemetry become correspondingly more important. Considerable economies could result from coordination with other activities with a global perspective, as we shall argue.

Spatial sampling requirements are governed primarily by the scientific goals of the network. 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.

Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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Suggested Citation:"2. Scientific Rationale For Global Networks." National Research Council. 1991. International Network of Global Fiducial Stations: Science and Implementation Issues. Washington, DC: The National Academies Press. doi: 10.17226/1855.
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The advent of highly precise space-based geodetic techniques has led to the application of these techniques to the solution of global earth and ocean problems. Now under consideration is a worldwide network of interconnected fiducial stations where geodetic as well as other scientific measurements can be made.

This book discusses the science rationale behind the concept of an extensive global network of fiducial sites. It identifies geophysical problems that cannot be solved without a global approach and cites geodetic objectives that call for a global deployment of fiducial sites. It concludes with operations considerations and proposes a plan for development of the global network.

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