1

OVERVIEW

Introduction

Geodesy, in the space age, has strived to steadily improve the precision of measurements to meet the ever more demanding metric needs of the Earth, ocean, and space sciences. These needs are most evident from the increasing number of gatherings of scientists at which the benefits of modern geodetic techniques are discussed and relied upon to help solve diverse problems in the Earth sciences (see Appendix A). Geodesy has always contributed to Earth studies on both local and continental scales. However, only since the advent of space-based geodetic techniques, the increased precision of these techniques, and the feasibility of obtaining data from a large number of sites have scientists been encouraged to apply these methods to existing global networks and to the solution of global problems. This has led naturally to the consideration of a worldwide network of interconnected fiducial stations where geodetic as well as other scientific measurements could be made. Such an international global network of fiducial stations is the theme of this report. To consider issues raised by the transition to such a network, the Panel on a Global Network of Fiducial Sites was formed by the Committee on Geodesy, National Research Council.

The Committee's supporting agencies suggested that the Panel address specific issues, including:

  1. evaluation of the scientific importance of a global network of fiducial sites, monitored very precisely, using a combination of surface-and space-geodetic techniques;

  2. examination of strategies for implementing and operating such a network, in light of the anticipated scientific return, building on existing capabilities whenever possible; and



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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES 1 OVERVIEW Introduction Geodesy, in the space age, has strived to steadily improve the precision of measurements to meet the ever more demanding metric needs of the Earth, ocean, and space sciences. These needs are most evident from the increasing number of gatherings of scientists at which the benefits of modern geodetic techniques are discussed and relied upon to help solve diverse problems in the Earth sciences (see Appendix A). Geodesy has always contributed to Earth studies on both local and continental scales. However, only since the advent of space-based geodetic techniques, the increased precision of these techniques, and the feasibility of obtaining data from a large number of sites have scientists been encouraged to apply these methods to existing global networks and to the solution of global problems. This has led naturally to the consideration of a worldwide network of interconnected fiducial stations where geodetic as well as other scientific measurements could be made. Such an international global network of fiducial stations is the theme of this report. To consider issues raised by the transition to such a network, the Panel on a Global Network of Fiducial Sites was formed by the Committee on Geodesy, National Research Council. The Committee's supporting agencies suggested that the Panel address specific issues, including: evaluation of the scientific importance of a global network of fiducial sites, monitored very precisely, using a combination of surface-and space-geodetic techniques; examination of strategies for implementing and operating such a network, in light of the anticipated scientific return, building on existing capabilities whenever possible; and

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES the assessment of whether such a network would provide a suitable global infrastructure for geodetic and other geophysical systems of the next century. To address these issues, the Panel defined (1) a core network, consisting of a relatively small number (about 30 or more) of very-high-performance stations, operating with a high degree of reliability, many (but not necessarily all) of which would be collocated with other global geodetic and geophysical instruments, thereby providing a common reference frame; (2) fiducial sites, which are defined as stations where geodetic and, if feasible, geophysical measurements are made, continuously or periodically, meeting standards and specifications established for highly precise data. The Panel believes that promising scientific potential exists in including geophysical activities, in addition to geodetic, at fiducial sites. The global network discussed in this report consists of the total of all fiducial sites, including the core network. The Panel's conclusions on the three issues, briefly, are: A global network of fiducial sites would provide an irreplaceable and vital tool for addressing several important global issues such as (1) sea-level change and postglacial rebound and (2) the monitoring of tectonic plate motion and deformation. The network also would play a critical role in providing a reference frame, monitoring Earth orientation, constraining fundamental rheological parameters of the Earth, validating models of ocean dynamics, determining Earth satellite orbits at a sustained and unprecedented level of precision, and providing essential spacecraft tracking support for scientific missions in low Earth orbit. Finally, a global network would offer a new level of support for local and regional geodetic and geophysical studies, through its reference frame and through the logistical simplification of orbit determination. As the Global Positioning System (GPS) receiver costs decline and the ease of operation and precision of measurements improve, GPS systems are proliferating worldwide. Competing systems, such as the USSR Global Navigation Satellite System (GLONASS), do not provide similar precision at this time. Furthermore, the cost for deploying systems such as Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) at core and fiducial sites is likely to be prohibitive. Therefore, the Panel concluded that GPS is likely to play a major role in global geodesy over the next decade and complement effectively existing

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES techniques. The Panel therefore examined with particular care global deployment strategies explicitly using GPS. This choice of technology influences in turn the nature of the global network. A viable strategy for implementing and operating the network is to integrate existing global networks into a common reference frame and to establish a GPS core network of about 30 or more sites, which permits the definition and realization of the global reference frame and supports the precise determination of orbits and Earth orientation parameters. The GPS core network should therefore incorporate sites presently occupied by equipment devoted to space geodetic measurements via techniques such as VLBI, SLR, Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), and Precise Range and Range-rate Equipment (PRARE). Growth of the network beyond the core stations can take place by progressive evolution of sites from a status of periodic reoccupation to a status of permanent occupation, with a higher density of sites in tectonically active areas and areas critical to the resolution of postglacial rebound problems. An analysis of the requirements for a global network capable of meeting these objectives indicates that the network should grow over time to about 200 fiducial sites including the core stations. In this respect the definition of the fiducial network adopted by the Panel is more ambitious than the one contemplated for the International GPS Geodynamics Service (IGS), which proposes that the stations in the fiducial network, other than the core stations, would be reoccupied at regular intervals and would not generally operate continuously, at least initially. The Panel 's definition agrees with the concept proposed for the Fiducial Laboratories for an International Natural science Network (FLINN) (NASA, 1991). Such a global network of fiducial sites will provide a suitable long-term infrastructure for geodetic and geophysical studies, especially if a special effort is made to foster coordination with other global deployments (e.g., seismological) and promote a multidisciplinary approach. Such fiducial sites would then evolve into terrestrial observatories, or laboratories, at which a variety of measurements would be made in concert to permit more comprehensive studies of the Earth. Even though many fiducial sites might be occupied only periodically in the early phase of the global network deployment, it is the Panel's view that, in the long term, the majority of sites would be permanently and continuously occupied and equipped with multidisciplinary batteries of instruments. The Panel therefore views the concept of a global network of fiducial sites in a favorable light, provided that its implementation proceeds along the lines of international cooperation that have historically characterized geodesy. However,

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES with the much denser temporal and spatial sampling implied by the various global network concepts recently proposed (see Appendix A), data flow, data management, data processing, and data archiving loom as much larger problems than in the past. Although solutions exist in theory, they need to be implemented, tested, and evaluated in the context of realistic experiments. Ongoing and planned services and campaigns, such as the International Earth Rotation Service (IERS), the First GPS IERS and Geodynamics Experiment (GIG'91), or the International GPS Geodynamics Service (IGS) Epoch '92, will provide much of the data needed for such tests and evaluations. This report first discusses the scientific rationale behind the concept of an extensive global network of fiducial sites. It defines the scientific goals that govern the design and operation of the network and identifies two classes of scientific objectives: (1) general geophysical objectives characterized by important scientific problems that cannot be solved without a global approach and (2) geodetic objectives that call for a global deployment of fiducial sites. These objectives are discussed in detail in Chapter 2. Chapter 3 addresses operational considerations, which include a review of existing and emerging technology, issues raised by the inevitable transition to newer technologies, and items to consider in order to mitigate the complications introduced by such transitions. Data flow and data management issues are then raised, as well as operational concerns such as standards and data formats. In Chapter 4 a plan is proposed, including a hypothetical global network that could effectively address the scientific problems discussed in Chapter 2. Special attention is given to the benefits that would accrue from a deliberate effort to coordinate global deployment and operation of seismic, magnetic, atmospheric, and environmental networks. The Committee on Earth Sciences (1989) of the White House Office of Science and Technology Policy recently published a report, Our Changing Planet: The FY 1990 Research Plan, that addresses research priorities in the U.S. Global Change Research Program and provides the framework for yearly updates in the program 's definition. That report emphasizes global observation programs that tend naturally to be more efficiently conducted from space-based platforms. However, in situ, or ground-based, observations are accorded a rather prominent place as well, and the report notes that “there are many important parameters which we can't yet measure from space” (p. 16). The report also identifies seven “interdisciplinary scientific elements” (p. 104) that together constitute the backbone of the U.S. Global Change Research Program. They are, in order of assigned priority:

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES climate and hydrologic systems, biogeochemical dynamics, ecological systems and dynamics, Earth system history, human interactions, solid Earth processes, and solar influence. All of these seven elements call for observational programs that are global, and all require a substantial ground-based component. In situ measurements play an even more important role in the report Reducing Natural Hazards: A 10-Year Research and Applications Strategy (Committee on Earth and Environmental Sciences, October 1990), in which the scientific elements deemed most important match elements 1, 3, and 6 above. This U.S. national program would be an important contribution to the International Decade for Natural Hazard Reduction, described in the report Confronting Natural Disasters (National Research Council, 1987b). In 1988 the Task Group on Earth Sciences of the Space Science Board, National Research Council, produced a seminal report entitled Mission to Planet Earth. The primary theme of the report pertains to the study of the Earth as a system, to be explored by a variety of techniques, including in particular space-based techniques. The main recommendations are couched in terms of four Grand Themes, the first of which pertains to the “structure, evolution, and dynamics of the Earth's interior and crust,” (p. 5) the space science orientation of the Task Group on Earth Sciences notwithstanding. To conduct the necessary research and to collect the required data, the Task Group recommended deployment of a Permanent Large Array of Terrestrial Observatories (PLATO) that would complement the Earth Observing System (EOS). The concept of a global network of terrestrial fiducial laboratories is, therefore, in tune with the main recommendations of Mission to Planet Earth. (The term Mission to Planet Earth, coined by the Task Group, has since been adopted by NASA to define a proposed set of orbital missions starting late in the next decade and extending into the next century. Although the Task Group's concept and the NASA proposal have many similarities, we shall use the term in this report in the sense defined by the Task Group and not as a description of a particular space mission.) At several major workshops in the past three years, global issues in geophysics and geodesy held a prominent place. The first, an international workshop on The Interdisciplinary Role of Space Geodesy, was held in Erice, Sicily, in July 1988.

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES The resulting report (Mueller and Zerbini, 1989), hereafter referred to as the Erice report, contains 11 main recommendations, several of which concern issues that call for a global point of view. A number of these recommendations were reiterated and summarized in Geodesy in the Year 2000 (National Research Council, 1990a). The second major workshop was held in July 1989 at Coolfont, West Virginia, with the specific purpose of developing a NASA program in solid earth science. From the point of view of global geodesy, this workshop recommended the deployment of a global network of permanently occupied sites, to be known as Fiducial Laboratories for an International Natural science Network (FLINN) (NASA, 1991). Finally, a workshop held in May 1990 at the Woods Hole Oceanographic Institution focused on issues associated with measuring changes in global sea level. The associated report, Towards an Integrated System for Measuring Long Term Changes in Global Sea Level (Joint Oceanographic Institutions, 1990), specifically treats the need for continued development of geodetic techniques on both global and local scales. The main recommendations from these various studies are summarized in Appendix A. Scientific Priorities Scientific priorities that clearly call for global in situ measurements are described in considerable detail in Mission to Planet Earth (National Research Council, 1988). A fundamental tenet of that report is that the extremely wide range of temporal and spatial scales of the phenomena that govern the state and evolution of the solid Earth, the atmosphere, and the oceans “require measurements by a variety of means, all requiring completeness, simultaneity, and continuity ” (p. 11). Not surprisingly, geodetic measurements will play a fundamental role in the study of many of these phenomena. However, with the advent of space-geodetic techniques, this role has evolved and grown, because precise geodetic measurements at almost all scales are now capable of detecting signals, such as tectonic strains and sea-level changes, that had heretofore only been inferred from geological evidence. Geodesy is now a major contributor of quantitative data that help Earth scientists solve geological and geophysical problems. This contribution has been recognized explicitly by Lambeck (1988), who coined the term “geophysical geodesy.”

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES Among the major topics in geophysical science that will benefit from a global network of fiducial stations, three have long been recognized as clear targets for a geodetic attack. They are (1) changes in global sea level, which are thought to be an important index of global climatic change and which require careful geodetic control to separate signals and noise sources of comparable amplitudes (e.g., Carter et al., 1989); see also Sea-Level Change (National Research Council, 1990b) and Towards an Integrated System for Measuring Long Term Changes in Global Sea Level (Joint Oceanographic Institutions, 1990); (2) postglacial rebound, a closely associated topic, which is also coupled to the viscosity structure and flow of material in the Earth's mantle; and (3) global and regional tectonic motions and deformation, which address directly the dynamic characteristics of the crust and mantle on short time scales never accessible until very recently. These topics cannot be tackled, however, until other, more purely geodetic problems are solved. These include calculation of precise ephemerides for the satellites used by space-geodetic techniques, determination of Earth orientation and rotation parameters, and realization of a precise reference frame for global studies. Interestingly, we shall see that these geodetic objectives can be achieved with a much sparser network than the geophysical ones mentioned above. This has substantial implications for the design of a global network and the attendant logistical structures, such as systems for data collection and analysis. A number of scientific missions in near-Earth orbit require very precise knowledge of the ephemerides of the spacecraft. This is particularly true of missions such as Aristoteles, ERS-1, and TOPEX/POSEIDON. It will be true of a number of missions in various stages of planning for the second half of the decade, including, in particular, GP-B and EOS. Geodetic support for such missions can be provided at the required level only if a global tracking network is available. At the same time, the availability of GPS receivers on low-orbit spacecraft would significantly strengthen a global GPS network by providing, in essence, additional network nodes at very high elevation. Mission support is therefore a clear motivation for the deployment and operation of global fiducial networks. Finally, a powerful motivation for a global network derives from the support it can afford the geodetic community at large by providing important data products such as precise orbits, a global realization of a terrestrial reference frame, and regional fiducial points to which local and regional networks can easily be tied. Partly in recognition of such benefits, the International Association of Geodesy (IAG) sponsored a symposium, Permanent Satellite Tracking Networks for Geodesy and Geodynamics, at the meeting of the International Union of Geodesy

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES and Geophysics (IUGG) held in Vienna, Austria, in August 1991 and has issued a Call for Participation in an International GPS Geodynamics Service (see Appendix C). The notion of service is crucial in this respect. Such actions constitute a clear indication of the timeliness of these ideas. Global Networks Use of the word global when characterizing a fiducial network implies a certain distribution of stations. The actual coverage and intersite spacing will vary with the requirements of the users and with the signals that must be accounted for properly. For instance, relative plate motions in the definition of a terrestrial reference frame may be accounted for through a theoretical (geological) model or through a Tisserand condition (e.g., Boucher and Altamimi, 1989). (Tisserand axes are defined by minimizing the kinetic energy of the mobile plate system and are characterized by null linear and angular momenta.) Either approach can be made unambiguous by appropriate selection of conventions and analysis methods, but the second is rather more difficult to assess in a geophysical context without conducting a detailed analysis of how the network samples the plate mosaic. It is natural for space geodesy to adopt a global perspective. This perspective has been held in the past and holds for many existing or planned techniques and networks, such as SLR, VLBI (including the Soviet QUASAR Project [Finkelstein and Yatskiv, 1989], the French DORIS system, the German PRARE system, the USSR GLONASS, and the GPS). From the point of view of a global network of fiducial sites, it is clear that GPS will play a major role in achieving global geodetic coverage. Already, the Cooperative International GPS Network (CIGNET) (Schenewerk et al., 1990; Neilan et al., 1990) and other global GPS networks, such as the NASA Deep Space Network (DSN), offer 20 to 30 permanent stations with a global, if uneven, distribution of sites. The Panel, based on the scientific goals discussed in this report, concluded that a core network of about 30 or more stations would be appropriate. Global GPS campaigns have also been conducted successfully (e.g., GOTEX, IERS GIG'91). Embryonic permanent and continuously recording GPS networks are being deployed in Japan and California (e.g., Shimada et al., 1990; Bock et al., 1990).

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES In other disciplines, global has essentially the same meaning, although colored by the particular data products the network is supposed to support. For instance, for very-long-period seismology, a global broadband network can be quite sparse, but for the International Seismological Observing Period (ISOP) a very large number of stations will participate in an unprecedented coordinated experiment. In geomagnetism the INTERMAGNET program is aiming at a global distribution of nearly 70 sites in the next few years, with real-time data transmission via satellite. Recently, E. H. Knickmeyer and C. Boucher, in an exchange of correspondence (Knickmeyer, 1990; Boucher, 1990) offered a rationale for a Common, Global, Integrated, Fundamental Network. The IERS, as well as a number of working groups of the IAG, would play a central role and provide the needed international umbrella. Knickmeyer and Boucher analyzed the implication of each qualifier in the following terms: Common: Commonality to all data users and data producers requires an international scope and an international umbrella. International groups will therefore play a central role. Global: The word is usually taken to mean uniform coverage, but the mean intersite spacing required depends on the application and on practical considerations, such as the distribution of geologically active zones. For these reasons, a slightly nonuniform distribution may be superior to a rigidly uniform one. Integrated: This modifier has several connotations: (1) A global network using a specific technology should have enough sites collocated with sites equipped with other technologies to permit the realization of a common coordinate system. (2) Other measurements (such as cryogenic or absolute gravity) should be performed at the same sites—that is, in the same reference frame—which can affect the selection of global sites. (3) Individual sites should all adhere to a common set of standards and data formats to permit exchange and analysis of both local and global data. Fundamental: The global network will be a primary network to which existing regional networks, or future densification networks, will be linked. An ever-growing proportion of the Earth sciences community is taking a global view of the planet. The point is important because much can be gained by looking beyond the straightforward applications of instrumentation deployed by scientists in any given discipline and actively seeking to cross disciplinary boundaries. If the direct scientific benefits are not always necessarily obvious, the

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES practical benefits, in terms of economies of scale, technology transfer, and logistical simplifications, will quickly be realized if they exist. State of the Art The nature and distribution of existing global geodetic networks have been determined by the needs of particular techniques, political and funding realities, and many other considerations and constraints. Some networks have evolved from early experiments into an operational mode, while others have been able to build on existing technology into an almost immediately operational system. Current space geodetic techniques can conveniently be classified into optical and radio systems. The most precise optical systems are based on the measurement of the time of flight for a laser pulse between source and detector. The most precise radio systems measure a similar time of flight in the 1- to 20-GHz microwave spectrum or the differential time delay of a signal received at two sites. The laser ranging systems had their experimental foundations in the early 1960s and reached a significant milestone in 1969 with the successful ranging to reflectors placed on the moon by Apollo astronauts. These space-geodetic systems are referred to as the Lunar Laser Ranging (LLR) and the Satellite Laser Ranging (SLR) systems. The SLR network has evolved into an operational network that regularly tracks the dedicated LAGEOS (U.S.) satellite as well as several others, such as Starlette (France), Ajisai (Japan), and the Etalon satellites (USSR). SLR is the primary U.S. tracking system for the TOPEX/POSEIDON mission (U.S./France) to be launched in 1992 (DORIS being the primary French tracking system). The LAGEOS satellite series will be augmented with LAGEOS 2 to be launched in 1992 and LAGEOS 3 planned for launch in 1994. A spaceborne version of SLR will be the tracking system for the Geoscience Laser Ranging System (GLRS), which is slated for launch on the Earth Observing System. Current ground-based systems are characterized by an instrumental precision of better than 1 cm in a single range measurement, which can be corrected, in part, for tropospheric refraction if sufficiently precise measurements of temperature, pressure, and humidity are made or if two-color range measurements are made. The radio systems can be classified according to the radio source used. The VLBI systems ordinarily use extragalactic radio sources and a differential mode of signal analysis that requires receivers at two or more sites. Other radio systems are based on satellite sources, in particular, the well-publicized and widely used GPS (U.S.) constellation of satellites and the USSR Global Navigation Satellite

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES System (GLONASS). More recent radio systems developed for space-geodetic applications include DORIS, which will be carried on TOPEX/POSEIDON, and PRARE, which will be carried on ERS-1, an Earth Resources Satellite of the European Space Agency. The VLBI systems have demonstrated millimeter-level precision for the lengths of transcontinental and intercontinental baselines (e.g., Herring, 1991). GPS systems have been shown to achieve comparable precision for shorter baselines. Radio measurements are influenced by both ionospheric and atmospheric effects, but the ionosphere's contribution can be largely removed with dual-frequency measurements, and the dry component of the atmosphere can be appropriately modeled. The wet component of the atmosphere can be handled by using water vapor radiometer measurements or by including appropriate estimation of atmospheric parameters (e.g., Herring et al., 1990). Direct comparisons of coordinates of 16 fiducial sites common to both SLR and VLBI networks have shown agreement at the 2-cm level (e.g., Ray et al., 1991). The remaining 2 cm could be due to remaining systematic errors in either or both techniques (e.g., water vapor effects for VLBI, orbit errors for SLR, and local survey errors for all space-geodetic techniques). Insufficient data exist to gauge the effects of other geodetic parameters, such as the gravity field (static and time-dependent), since, in practice, the insensitivity of VLBI to those parameters precludes a determination. Future radio applications using, for example, a GPS receiver on a low-altitude satellite will permit independent determinations of the gravity field. Regardless of the technique, availability of a global fiducial network of stations is critical to the successful determination of such geodetic parameters. Existing and planned networks for the various techniques are described in Chapter 3 of this report. Some of the networks are based on long-standing international cooperation (e.g., SLR), while others have developed more rapidly (e.g., CIGNET). Still others have been deployed for particular missions (e.g., the DSN tracking of GPS). In many instances, special efforts were made to ensure that different networks included several common sites to eliminate the effects of unknown relative orientations. The IERS is a prominent user of the existing SLR, LLR, and VLBI networks. Not only are the data generated by the networks used to determine Earth orientation, but the establishment of conventional reference frames for a variety of other applications is an important IERS product. The mission statement of the IERS is reproduced in Appendix B. It touches on many of the issues discussed in this report. IERS is a member of the Federation of Astronomical and Geophysical Data Analysis Services (FAGS) and cooperates with the Bureau International des Poids et Mesures (BIPM) in its activities concerning the UTC (Coordinated

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES Universal Time) time scale. The IAG's recent Call for Participation in a proposed International GPS Geodynamics Service (IGS) is also likely to increase the use of existing global network data through close coordination with IERS. Recommendations Six primary recommendations have been formulated from the discussions in this report. They outline a path that, it is believed, can be followed over the next decade or so to deploy an International Global Network of Fiducial Sites and, ultimately, build a network that conforms with concepts proposed for PLATO (National Research Council, 1988) or FLINN (NASA, 1991). The premise of these recommendations is that GPS is most likely to be the technology around which the global network will be built, at least for the foreseeable future. The popularity of the technique, its affordability, and mounting evidence that it can provide the required accuracy of measurements all combine to make it an extremely attractive way to address the scientific objectives of the global network. The experience accrued worldwide over the past few years by Earth scientists using GPS to solve geological and geophysical problems provides ample proof that, as a global geodetic tool, this system effectively complements the global networks already in place, particularly the VLBI network. The Panel carefully considered the various issues raised by choosing GPS as the prime candidate technology for the global network. Some of these—such as reliable, accurate, and timely orbit determination, definition of a terrestrial reference frame; and monitoring of Earth orientation and rotation parameters —will be addressed by selecting well-conceived deployment strategies and creating the infrastructure necessary to facilitate global data flow, timely processing, and effective dissemination of data products. Selection of GPS as the primary technique for the global network also raises a variety of nonscientific questions. In particular, GPS is designed, deployed, and operated by the U.S. Department of Defense (DoD). The DoD has a stated policy that access to the Precise Positioning System (PPS) will be restricted to authorized users, whereas the Standard Positioning System (SPS) will be generally accessible to civilian users. Usage of PPS has been restricted by a mechanism called Selective Availability (SA) since March 1990. The DoD also plans tests of the Antispoofing (AS) capability of the system, which involves encryption of the signals, when the system is fully operational. The Panel found from the geodetic community's (as yet limited) experience with SA that most geodetic applications

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES of GPS involve sufficient dwell time to permit averaging the effects of SA (e.g., clock dithering). Consequently, at present it appears that SA does not preclude the precise global applications of GPS contemplated for the global network. It will force network operators to do considerably more data processing work than they would have to do in the absence of SA, however, which entails a definite cost that is difficult to assess at present. From the point of view of the operation of a global network, access to the P-code would make a very significant difference in terms of the processing burden. If the P-code is not accessible, it is likely that, for a given level of available resources, a much smaller network can be analyzed, with a concomitant loss of scientific return. Certain applications, such as spacecraft orbit tracking using GPS, would benefit substantially from access to PPS. The Panel made no new explicit recommendation concerning these issues, which are the subject of ongoing discussions between the DoD and various user communities and of vigorous public debate among federal agencies, civilian users, and commercial equipment manufacturers. Nevertheless, the Panel endorses the recommendation in Towards an Integrated System for Measuring Long Term Changes in Global Sea Level (Joint Oceanographic Institutions, 1990) that the DoD should remove selective availability during periods of normal international relationships. The Panel's recommendations focus on strategies for the deployment of the global network, assuming GPS to be the most likely technique of choice. These strategies are examined in the light of overall scientific applications. Recommendation 1: The Panel recommends (1) incorporating part or all of the existing global GPS networks into a core network of about 30 or more locations as the first priority in the deployment of a global network of fiducial sites, primarily to support the reliable determination of precise GPS orbits; (2) maintaining a sufficient level of collocation of different systems, particularly SLR, VLBI, and GPS, to permit realization at the required accuracy of a common reference frame; (3) maintaining a transportable capability for VLBI and SLR systems for occasional verification and validation of long baselines; and (4) planning for long-term reoccupation of sites after transition to a new technique, system, or monument. The network should then grow over time to 200 sites, all eventually equipped with permanent, continuously recording GPS or comparable tracking equipment.

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES In deploying the global network of fiducial sites, it is essential to ensure continuity of existing geodetic time series and to tie measurements by different space geodetic techniques into a common reference frame. The development and deployment of the network should proceed as a natural extension of the activities of the past decade. In particular, it is critical whenever and wherever possible to continue the time series collected over the past few years without breaking continuity. The Panel believes that proper continuity can be maintained by building the network as a two-tiered system, comprising two main categories of sites: A GPS core network, comprising about 30 or more globally distributed, very-high-quality sites with continuous, reliable operation of GPS and other equipment, and near-real-time data acquisition and transmission to processing centers. This network would contribute the data from which would be derived (1) precise orbits, including force model parameters; (2) GPS clock estimates; (3) Earth orientation information to be contributed to IERS; and (4) ties to the terrestrial reference frame through collocation at a sufficient number of sites with multiple techniques, specifically fixed and mobile VLBI and SLR equipment, DORIS, and PRARE. Preliminary results from the GIG'91 campaign are very encouraging: A globally distributed network of about 15 GPS stations appears to yield precise orbits and polar motion estimates in good agreement with the VLBI solutions (G. Blewitt and S. Lichten, Jet Propulsion Laboratory, Pasadena, California, private communication, 1991). Care must be taken to provide adequate coverage in remote areas, particularly in the southern hemisphere. The Panel's recommended core network of 30 or more sites incorporates the notion of redundancy, which is the most straightforward way to guarantee that the quality of the data products will be robust with respect to equipment failure and similar disruptions. A larger set of stations (perhaps on the order of 200) providing denser coverage of tectonic deformation zones, regions of postglacial rebound, and coastal areas near tide gauge networks. Such sites might be occupied initially at regular intervals (in particular during global campaigns) to determine secular geodetic signals, but many would be upgraded to continuous operation over time, thereby contributing data similar to those derived from the GPS core network. These stations would contribute to the solution of global geological and geophysical problems. Of special importance to this expanded network would be the capability of periodic visits with portable systems—particularly VLBI and SLR systems—especially to isolated locations tied to the rest of the network through fairly long baselines.

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES Distribution of the larger network of fiducial stations is likely to be controlled in large part by available resources and logistics constraints. The Panel's proposal of 200 sites results from arguments presented in Chapter 4. The Panel strongly recommends that some fiducial sites be located at or near tide gauges so as to elucidate some of the problems raised in Chapter 2 concerning the separation of sea-level changes, postglacial rebound, and tectonic signals. However, it is clearly premature to extend such a recommendation to all tide gauges participating in global sea-level monitoring. Instead, the Panel recommends that a manageable subset of these sites be targeted for focused, continuous measurements. Existing permanent GPS, SLR, LLR, and VLBI stations, primarily from the CIGNET and DSN networks and from global VLBI and SLR networks, can be selected as an initial set of GPS core sites, with which the critical data flow issues can be examined now. A major advantage of such an approach stems from the existing infrastructure, which offers an opportunity for efficient use of existing resources. Continued growth should take advantage of advances in receiver technology, as well as data collection, processing, and distribution techniques. Implementation of the FLINN concept will require broad international participation and will proceed at different rates in different regions, depending on local logistical conditions and availability of reconnaissance surveys and on available resources. Recommendation 2: The Panel recommends a concerted effort to continue the integration of existing global networks. This recommendation recognizes that existing or planned global networks, including VLBI and QUASAR, SLR, LLR, DORIS, GPS/GLONASS, and PRARE have the potential to satisfy many of the scientific requirements of a global network of fiducial sites. The recommendation also entails strong support for the continued operation of enough sites for each technique, distributed globally and collocated with other techniques, at enough locations to permit the realization, at a sufficient level of accuracy, of a common reference system. This point raises not only the rather obvious issue that a mere collection of stations does not a network make, so that these stations must be melded into a network, but also brings up the thornier difficulty of integrating different technologies. IERS merges data sets from several sources to produce unified data products. The procedure involves development of an assortment of weighting schemes that acknowledge the relative reliability of these different sources. The variety of data sources generated by globally distributed networks will increase. In addition, some technologies will

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES become obsolete, and a transition to new technologies will prove necessary. Further, for a number of scientific purposes, the range of data products offered through IERS will have to be expanded—to include, for instance, full covariance information—and standards must be developed for these new products. If the global network is to realize the scientific promise it holds for its nongeodetic users and for the geodetic community at large, the path to the use of new technology must be charted in a predictable way. In the Panel's view, this recommendation requires that national and international organizations aggressively seek international cooperation and coordination of efforts conducted by many countries. Recommendation 3: The Panel recommends development and implementation of (1) data standards, (2) communication paths, and (3) data processing and archiving techniques. One of the earliest priorities is to produce and disseminate standard geodetic data products, by operating coordinated data centers to ensure a smooth flow of data from the network operators to the user community, and to establish analysis centers to develop and test techniques and results that support part-per-billion three-dimensional geodesy. The creation and operation of these centers should begin concomitantly with the initiation of the GPS core network. The effectiveness of the international flow of global data should be tested and evaluated in the context of a series of global campaigns, including a mechanism for identifying and implementing necessary improvements. This recommendation is rooted in the recognition that no matter how many sites are installed in the field and irrespective of the quantity and quality of the observations, these sites cannot constitute a network unless an effective mechanism is in place to collect the data and distribute the data sets to the users along with products generated by the analysis centers. This function must include the unavoidable updates—some of which may be required after a substantial delay—of data sets already distributed, as well as the continued monitoring of data quality. The mechanisms are well known in principle, although fraught with practical difficulties in implementation. It is essential that these difficulties be identified explicitly and that solutions be developed and subjected to rigorous tests. The nature of such tests depends on the data sets and, therefore, on the discipline addressed by the network. This means that the issues associated with data flow should be addressed from the very outset of the deployment of a fiducial network. In other words, high priority should be given to the design and creation of international data centers, and the international community should initiate

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES operation of such centers, and analysis and evaluation centers, with the first data streams available—that is, with the GPS core network data. In geodesy the time-honored approach to identifying and solving data flow problems is through the execution of campaigns. The several campaigns conducted in the recent past include the very successful MERIT campaign, which resulted in establishment of the MERIT standards and ultimately creation of the IERS. More recently, the GOTEX and the GIG'91 IERS campaigns have tested the collection, distribution, and analysis of data from large global GPS networks. In the future, global networks will be large, integration of multiple techniques will be required, and demands for rapid data access and rapid data processing and analysis techniques will increase. It is therefore essential that additional global campaigns, such as the proposed IGS Epoch'92 campaign, be executed to prepare for the deployment of a global network of fiducial sites. Recommendation 4: The Panel recommends that the global network be completed in parallel with the initiation of regional densification to study local and regional geophysical problems. This process should be driven by scientific issues at both the global and regional levels. By itself, the global network will not suffice to solve some of the crucial scientific problems. Work on geological or geophysical problems that would benefit immensely from the global network also requires a higher density of sites, on a regional or local scale, than the network can provide. Densification should therefore be considered an intrinsic aspect of the global deployment that should be driven by the scientific requirements of each application. The resolution of technical issues such as data flow and analysis can be developed in the context of the global network and applied or adapted immediately to the solution of local problems. In other words, the Panel sees substantial benefits in proceeding with local densification at the same time as the global network is being deployed, at both the technical and the scientific levels. In many instances global fiducial sites might often be selected from high-quality sites (i.e., well-monumented, geologically stable) in a regional network. Recommendation 5: The Panel recommends that, whenever feasible, the deployment of fiducial sites in the global network be coordinated, in terms of site selection and network operation, with deployment of instruments used by scientific disciplines in addition to geodesy, to (1) facilitate data

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES integration, (2) achieve economies of scale, and (3) realize the scientific benefits of multidisciplinary studies of the Earth. It is clear that global geophysical networks are of great interest not only to geodesy but also to other disciplines of the Earth sciences, including seismology, geomagnetism, gravity, oceanography, and volcanology. There is of course no reason that the list should stop here; other types of measurements that should be considered include atmospheric, geochemical, meteorological, and environmental. Partly in recognition of this situation, the concept of FLINN as a set of globally distributed sites with a multidisciplinary vocation was proposed as a logical and desirable generalization of the initial geodetic network concept. This is why “Laboratories” is used in the acronym, which honors the late E. A. Flinn, who led the NASA Crustal Dynamics Project and helped develop it into an international collaboration. In this sense FLINN might really be thought of as the necessary ground component of the Mission to Planet Earth—that is, the core of the Permanent Large Array of Terrestrial Observatories (PLATO) originally proposed as an integral element of the mission. Such a coordination, if effective, could have a major positive impact on the Mission to Planet Earth and may in fact be the best way that the ground segment of this mission can be developed to its full scientific potential. Precise space-geodetic control is of course an essential ingredient of FLINN and is the basis for the long-term goal the Panel set for the global network of fiducial sites. But a coordinated program in telemetry and data management without question could provide real economies of scale: As the seismological community is developing adequate technological solutions to its part of the problem, seismologists are likely to use the lion's share of available bandwidth. Therefore, it seems probable that other disciplines will benefit from a multidisciplinary approach. Recommendation 6: The Panel recommends (1) continuing the evaluation of new observing techniques and investigation of the role of GPS in complementing VLBI and SLR, (2) continuing VLBI and SLR observations at key sites for the foreseeable future, (3) obtaining parallel data sets where major changes in instrument systems are made at critical sites, (4) continuing observations with no change in technique and no gap in data or discontinuity in the time series or its first derivative where there are discrepancies between existing systems, and (5) maintaining mobile VLBI/SLR capability to allow reoccupation of VLBI/SLR sites.

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES In this era of rapid technological development, observing systems that are more precise and more economical to operate inevitably will evolve. However, we must not lose sight of the value of geodetic time series in evaluating long-term change. It is therefore incumbent on system and network operators, to the extent possible, to minimize disruptions in these time series by changes in technology, observing techniques, and analysis procedures. Further, until it is demonstrated, within the limits of accuracy of the data, that a significant discontinuity in a time series or its derivative does not exist, the ability to collect additional data compatible with the conditions preceding the change remains very valuable and should not be abandoned. These considerations obviously apply to changes in the fundamental technology, such as the transition from VLBI to GPS monitoring at a site, as well as to the situation where a mundane change of monument takes place at a given location. It must be noted that space-geodetic techniques involve many corrections, such as those for ionospheric and tropospheric effects, water vapor, clock corrections, relativistic terms, and, of course, survey ties to a ground monument. As experience is acquired with each system, geodesists' confidence in the results increases. Nevertheless, as demands for precision and accuracy continue to push the limits of every technique, new sources of errors, both random and systematic, must be carefully assessed. It is essential to continue the systematic comparison of results obtained by different techniques along the same baseline. This comparison will be critical to the integration of GPS with the existing networks, particularly VLBI and SLR, and is the principal method we have for recognizing and eventually eliminating sources of systematic errors. Possible Network Configuration After examining many existing global geodetic and geophysical networks (see Chapter 3), the Panel undertook to map a network configuration with two purposes: (1) to develop a general feeling for the magnitude of the task of establishing a network and gauge the extent to which existing sites might be used to start the deployment in the near future and (2) to obtain a general idea of the appearance of a global network of fiducial stations. This map (Figure 1) is intended to illustrate the type of network being considered by the Panel but not to

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES FIGURE 1. Possible configuration of a global network of fiducial stations. Core sites are taken primarily from existing permanent sites; others are selected according to the arguments outlined in Chapter 4 of this report. Shaded areas indicate regions of recent or current tectonic activity.

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INTERNATIONAL GLOBAL NETWORK OF FIDUCIAL STATIONS: SCIENTIFIC AND IMPLEMENTATION ISSUES depict a specific site selection process. The GPS core network is derived from existing CIGNET and DSN sites with a few additions in locations where other countries have said they intend to run permanent GPS stations (e.g., Tahiti, where the French intend to collocate GPS and DORIS equipment). The additional sites are drawn from existing and planned campaigns and from existing networks such as those discussed in Chapter 3. Sites shown in Siberia, Tibet, and Africa are purely imaginary and are distributed so as to meet the scientific goals mentioned above, as are a number of island sites. The criteria leading to this network configuration are discussed in some detail in Chapter 4. This exercise highlighted the fact that existing and planned sites with a global flavor are already remarkably numerous. The robustness and reliability of GPS orbits determined from the core network in Figure 1 has yet to be fully quantified, so that it is not yet clear that coverage is adequate and robust in all areas. (It might be necessary to deploy redundant equipment at certain island sites to achieve the desired reliability.) Collocation of different geodetic techniques and of geodetic instruments with other scientific instruments makes considerable sense in areas where the density of existing deployments is low. This is the case in most of Africa, Antarctica, and large areas of Asia. In such instances, where any measurement would in essence break new scientific ground, the multidisciplinary site concept would offer extraordinary scientific benefits. Finally, in vast oceanic areas the global networks are unavoidably sparse. Many disciplines have been developing ocean bottom instrumentation and exploring ways to retrieve the data reliably and conveniently. This is also true of geodesy, and, as the technology becomes available over the next few years, it will become desirable and in some cases imperative to add sea bottom fiducial sites to the global network.