Global geodetic observatories

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ScienceDirect Advances in Space Research 55 (2015) 24–39 www.elsevier.com/locate/asr

Review

Global geodetic observatories Claude Boucher a, Mike Pearlman b, Pierguido Sarti c,⇑ a Observatoire de Paris SYRTE, Paris, France Harvard-Smithsonian Center for Astrophysics, Cambridge MA 02138, USA c Istituto di Radioastronomia (IRA) – Istituto Nazionale di Astrofisica (INAF), Via P. Gobetti, 101, 40129 Bologna, Italy b

Received 15 January 2014; received in revised form 9 October 2014; accepted 11 October 2014 Available online 22 October 2014

Abstract Global geodetic observatories (GGO) play an increasingly important role both for scientific and societal applications, in particular for the maintenance and evolution of the reference frame and those applications that rely on the reference frame for their viability. The International Association of Geodesy (IAG), through the Global Geodetic Observing System (GGOS), is fully involved in coordinating the development of these systems and ensuring their quality, perenniality and accessibility. This paper reviews the current role, basic concepts, and some of the critical issues associated with the GGOs, and advocates for their expansion to enhance co-location with other observing techniques (gravity, meteorology, etc). The historical perspective starts with the MERIT campaign, followed by the creation of international services (IERS, IGS, ILRS, IVS, IDS, etc). It provides a basic definition of observing systems and observatories and the build up of the international networks and the role of co-locations in geodesy and geosciences and multi-technique processing and data products. This paper gives special attention to the critical topic of local surveys and tie vectors among co-located systems in sites; the agreement of space geodetic solutions and the tie vectors now place one of the most significant limitations on the quality of integrated data products, most notably the ITRF. This topic focuses on survey techniques, extrapolation to instrument reference points, computation techniques, systematic biases, and alignment of the individual technique reference frames into ITRF. The paper also discusses the design, layout and implementation of network infrastructure, including the role of GGOS and the benefit that would be achieved with better standardization and international governance. Ó 2014 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Global geodetic observatories; GGOS; Space geodesy; Space geodetic techniques; Co-locations; Tie vectors

Contents 1. 2. 3.

4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Historical context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Global geodetic observatories: concepts and issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Observing systems and observatories in geodesy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Geodetic networks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Co-locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Network management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Uses of global geodetic observatories in geodesy and geosciences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Some current topics of investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. Tel.: +39 051 6399417; fax: +39 051 6399431.

E-mail address: [email protected] (P. Sarti). http://dx.doi.org/10.1016/j.asr.2014.10.011 0273-1177/Ó 2014 COSPAR. Published by Elsevier Ltd. All rights reserved.

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5.1.

6.

Local surveys, tie vectors and ITRF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1. Surveying techniques and approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2. Site peculiarities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.3. Intra-site motions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.4. Tie vector computation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.5. Observation biases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.6. Tie vector alignment into the ITRF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.7. Some potential bias sources in space geodetic techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Design and implementation of network infrastructure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Standardization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. International organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Global geodetic observatories (GGOs) host groundbased infrastructure of geodetic observing systems to monitor properties of the Earth System, including its various components: solid Earth, ocean, atmosphere, cryosphere, and biosphere. These observing systems include some combination of Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Navigation Satellites Systems (GNSS), and Doppler Orbitography and Radiopositioning Integrated by Satellites (DORIS), as well as other types of instruments such as gravimetric sensors, geophysical or environmental sensors, etc. Data products derived from networks of GGOs provide data to help us to understand the nature of global phenomena, how they are linked, how they change in time and how they affect our lives. Measurements allow us to model the effects and forecast their evolution, but even more important, they form the scientific basis on which long-term actions can be planned and reasonable policies can be adopted to allow society to better contend with current trends and future conditions. The major role of GGOs is to provide connections between various global observing systems, through ground co-locations of their instruments. Each technique measures something different, but in combination, these provide very rich sources of data to promote geophysical understanding. Local tie vectors and proper instrument modeling provide the means to inter-compare and combine the results from the separate co-located systems. This allows us to characterize the systematic differences between techniques and to construct integrated data products that take advantage of the individual technique strengths while mitigating the weaknesses. The quality of the GGO data products depends on the strength of the network including the number of sites, geographical distribution, and the quality and quantity of the individual systems measurements. The GGO may also provide the ideal infrastructure to host new instruments. The International Association of Geodesy (IAG) is presently the reference international organization dealing with

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30 30 31 31 31 31 31 34 35 36 36 37 37

GGOs. The IAG’s Global Geodetic Observing System (GGOS) and its Sub-Commission 1.2 are focused on the development and improvement of the Global Reference Frames as the basis for accurately connecting metric measurement over space (thousands of kilometers), time (decades) and evolving technology (see Beutler and Rummel, 2012; Plag and Pearlman, 2009; Rummel et al., 2005). Agencies in many countries are developing national reference frames for internal benefit and as their contribution to the international activity. See for instance the recent report established by the US National Research Council (NRC, 2010). This bottom-up and best effort mechanism leaves geographic and technological gaps as well as unnecessary duplications. With proper international governance and funding these gaps and duplications could be minimized and the importance of the GGO infrastructure for observations of the Earth, as discussed in the Global Earth Observation (GEO) initiative (see: https:// www.earthobservations.org/documents/work%20plan/ geo_wp1215_rev3_140123.pdf) could be better publicized. The IAG established the Global Geodetic Observing System (GGOS) in 2003 to integrate the three fundamental areas of geodesy (Earth’s shape, gravity field, and rotation), to monitor geodetic parameters and their temporal variations in a global reference frame with an accuracy of 10-9 or better (Plag and Pearlman, 2009). GGOS is intended to provide data products and services with sufficient geodetic accuracy, consistency, and continuity to address important geophysical questions and to help us make intelligent decisions on societal needs. This includes the decisions that we make regarding our national and international resources, our populations, and our environment. GGOS is constituted mainly from the Services, ILRS, IVS, IGS, IDS (Dow et al., 2009; Pearlman et al., 2009; Schuh and Behrend, 2012; Willis et al., 2010) and the International Earth Rotation and Reference Systems Service (IERS) and, although it has a wide spectrum of interest, the main focus at the moment is on the improvement of the International Terrestrial Reference Frame (ITRF). The US National Research Council Study (NRC, 2010) found that the most stringent requirement for the ITRF

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Table 1 GGOS Inter-Agency Committee (GIAC) Membership. Details on the GIAC can be found at http://www.ggos-portal.org/lang_en/GGOS-Portal/EN/ GIAC/GIAC.html. Agency

Country

Geoscience Australia Natural Resources Canada (NRC) Shanghai Astronomical Observatory - Chinese Academy of Sciences Finnish Geodetic Institute (FGI) Institut National de l’Information Ge´ographique et forestie`re (IGN) Federal Agency for Cartography and Geodesy (BKG) Italian Space Agency (ASI) National Geodesist - Land Information New Zealand Norwegian Mapping Authority (NMA) - Geodetic Institute Korea Astronomy and Space Science Institute (KASI) Institute of Applied Astronomy (IAA) - Russian Academy of Science (RAS) Hartebeesthoek Radio Astronomy Observatory - National Facility of the National Research Foundation (NRF) Subdireccio´n General de Astronomı´a, Geodesia y Geofı´sica - Instituto Geogra´fico Nacional (IGN) Federal Office of Topography Swisstopo National Aeronautics and Space Administration (NASA) National Geodetic Service (NGS) - National Oceanic and Atmospheric Administration (NOAA)

AUSTRALIA CANADA CHINA FINLAND FRANCE GERMANY ITALY NEW ZEALAND NORWAY REPUBLIC OF KOREA RUSSIA SOUTH AFRICA SPAIN SWITZERAND USA USA

comes from sea level rise currently measured by satellite altimetry to be about 3 mm/year. However, our existing knowledge of the reference frame, that is the basis for our ability to connect measurements over space, time, and evolving technology, is only at the cm level (NRC, 2010). Errors in the current reference frame could then be aliased into our estimates of sea level rates (see e.g. Beckley et al., 2007; Morel and Willis, 2005). It could be larger; it could be smaller. The requirements for a reference frame that would be commensurate with our sea level measurements as recommended by the US National Research Council Study are an “accuracy of 1 mm and stability at 0.1 mm/year over 10 years”, which is about a factor of 10–20 beyond current capability. We point out that although sea level is the main challenge identified by the National Research Council Committee for the ITRF, many other effects such as seismic hazards and hydrology are not far behind. In addition, since measurements that need to be connected over space and time will be taken all over the world, the ITRF must be accessible 24/7 worldwide, so that users anywhere on Earth can geolocate their measurements in the same, unique, reference frame. Each of the space geodetic techniques plays an important role in the establishment and maintenance of the reference frame. The origin is determined by SLR; UT1 is determined by VLBI; and scale is determined independently by both VLBI and SLR (Altamimi et al., 2011). The dense global network distribution of GNSS, and to a lesser extent DORIS, provides the global coverage. LAGEOS and LARES define the space segment for SLR, the GNSS constellations, the DORIS satellites, and quasars for VLBI. The ground segment is defined by a globally distributed network of Core Sites, with “modern technology equipment”, co-located SLR, VLBI, GNSS, and DORIS (where available), locally tied together with accurately monitored site ties. These sites are termed “Core Sites”. In addition, a much denser network of GNSS ground stations is required

to distribute the reference frame globally to the users. Colocations with other ground based instruments such as seismometers, gravimeters, tilt meters and leveling networks, tide gauges, etc. make it easier to correlate data, better understand local conditions, and build models that more closely reflect the regional dynamics. Simulations studies by E. Pavlis show that this quality reference frame can be adequately defined with 32 globally well-distributed Core Sites with proper conditions and modern, currently available technology (NRC, 2010) This is a challenging requirement, since going from the current semi-ill-formed network of anachronistic mix of technologies into a much larger well distributed network of modern systems will take many years, much resources, and considerable organization and devotion from the international community. With this in mind, GGOS has formed the GGOS Inter-Agency Committee (GIAC) to help garner support from agencies and organizations around the world. So far 17 organizations from 15 countries have joined GIAC and signed the GIAC charter (see Table 1). More details on the GIAC and its membership can be found at http://www.ggos-portal.org/lang_en/GGOS-Portal/EN/ GIAC/GIAC.html. Additional agencies are expected to join in the near future. The GGOS Bureau of Networks and Communications has issued documentation articulating the (1) desirable space geodesy site distributions for the reference frame, (2) site requirements for GGOS Core Sites, and (3) elements of an Infrastructure Implementation Plan (See Bureau of Networks and Communications section at http://www.ggos.org/). Although this and other material are available, it is very beneficial to understand the challenges and further develop the guidelines to enable the operational implementation of the infrastructure with sufficient distribution, technology, and commitment. This paper discusses the historical context, the concept of the Global Observatories, and some of the critical issues.

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A major aspect discussed is the implementation of the essential site ties to connect co-located measurements and data products to mm accuracies. 2. Historical context Early activities related to GGOs were done mainly through international organizations such as:  International Latitude Service (ILS).  Bureau International de l’Heure (BIH).  International Polar Monitoring Service (IPMS). The emergence of satellite geodesy in early 1960’s offered a comprehensive evolution by introducing various types of ground tracking networks (optical, radio or laser). Early efforts such as the National Geodetic Satellite Program (NGSP) program, COSPAR catalogue, etc. recognized the importance of combining data and results from separate techniques and carefully documenting information on the measurement systems, sites, and data processing and analysis procedures. In the 1970’s, the contribution of space techniques increased its impact on reference frame realization and Earth rotation monitoring. In 1979, the International Astronomical Union (IAU) and the International Union of Geodesy and Geophysics (IUGG) decided to undertake an extended international activity to intercompare these various techniques. The Measurement of Earth’s Rotation and Intercomparison of Techniques (MERIT) Project was established, and with the IAU Conventional Terrestrial System (COTES) Working Group, they undertook an investigation of terrestrial reference frame issues (Wilkins, 2000). BIH became the MERIT operational center. A new type of realization of terrestrial system by combination of space techniques, the BIH Terrestrial System (BTS), in accordance with the COTES recommendations, was released for the first time in 1985, as the BTS84 solution (BIH, 1985). For that purpose, it was necessary to redefine station catalogues. In particular, the Directory of MERIT Stations, or DOMES, was established and is still in use (Wilkins, 2000). When the IERS was created in 1988, as a consequence of the MERIT-COTES recommendations, replacing both BIH and IPMS, the activities initiated at BIH were taken over by the IERS Central Bureau (site identification and cataloging, collection of site local surveys). These activities are still maintained in IERS by the International Terrestrial Reference System (ITRS) product center (http:// itrf.ign.fr). An important step was the general discussion stimulated by IAG during the IUGG General Assembly of Boulder in 1995 dealing with the concept of Fundamental Reference and Calibration Network (FRCN). Despite the lack of reaction from other IUGG Associations, a FRCN Working group was established within IAG to continue the discussions. In 1997 they submitted several recommendations to the IAG to:

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– Recognize the concept of International Space Geodetic Network (ISGN). – Define specifications for an ISGN station. – Ask the Commission on International Coordination of Space Techniques for Geodesy and Geodynamics (CSTG) to continue the work in cooperation with IERS for operational issues. During the IAG Scientific assembly at Rio de Janeiro in 1997, an ISGN Working group was formally created jointly by CSTG, IERS and the International GNSS Service (IGS, formerly International GPS Service), with the specific tasks to: – Define specifications for ISGN stations. – Encourage stronger cooperation between various contributing techniques. Gravimetric issues were also considered, ISGN becoming the International Space Geodetic and Gravimetric Network, and a first inventory of candidate stations was established. A report was finally published in the CSTG Bulletin #15 (Drewes, 1999). The progressive creation of the international services and now GGOS has provided the organization and improved mechanism to better coordinate data collection and develop better data products.

3. Global geodetic observatories: concepts and issues 3.1. Observing systems and observatories in geodesy The term “observations” is used here to designate quantitative measurements of an object or a phenomenon, plus the descriptive information that will be required to properly interpret these measurements. We must identify the end points of measurement and the corrections for calibration of the instrument and extrapolations to the instrumental point of reference. We might also need a time history of systems changes or environmental/operating conditions in order to fully understand the data and to develop the required data products (e.g. the ITRF, ground motions, sea level, etc.). We also need to recognize that the observing systems will derive their high level data products from a combination of ground-based, space borne, and airborne sensors, and through geodetic and engineering models and processing strategies. This combined use of observing techniques to establish a more comprehensive system is designated by the concept of system of systems. For example, the IAG is presently involved through GGOS in the Global Earth Observation System of Systems (GEOSS) (see Plag and Pearlman, 2009). Observatories traditionally refer to infrastructure (technical, logistical, administrative) used to collect observations at a specific place for a specific community (e.g.

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astronomy, space geodesy, volcanology, seismology, geomagnetism, navigation). The location of such an observatory might be determined by the close proximity of the effects to be measured (e.g. volcano or regional phenomena) or to provide synoptic or worldwide coverage to observe a global effect or property (e.g. reference frame, precision orbit determination). Geodetic Observatories (GO) maybe located as: – An element of a network deployed for a global or regional observation such as a physical realization of a reference system. – A geophysical sensor at a place where a specific geophysical signal is important (such as ocean loading or post-glacial rebound). – A calibration site, where new instruments can be installed for intercomparison or calibration. – A site to host new measurement devices in a cheaper and safer way. Data from the GOs would be shared and processed by participating groups to develop standardized data products or specialty data products to users. Therefore a minimum of standards should be designed and applied to ensure interoperability and consistent data analysis. This may dictate identification of pertinent GOs for different products or at least a criteria as to how GOs will be selected, and an agreement on a basic metadata. GOs that function as part of a global network for one application or another must satisfy certain internationally accepted criteria such as: – Adhering to the GGO standards. – Hosting more than one technique (e.g. GNSS, SLR, VLBI, DORIS, absolute and/or cryogenic gravimetry, etc.). – Hosting a high quality time/frequency standard. – Performing and monitoring site ground surveys. – Monitoring environmental effects (e.g. meteorology, groundwater). – Seeking locations with stable ground conditions with accurately predictable motion. A final selection of sites for one application or another should be based on both scientific and operational (resources, geopolitics...) considerations and network optimization. One example is the reference frame application, which has been implemented by the services including the IERS. The application of a similar approach to geophysical sensors should be investigated. 3.2. Geodetic networks A geodetic network designates a set of physical points located on the topographic surface of the Earth for which quantities such as position, geoid height, deflection of the vertical or gravity intensity are estimated. During

terrestrial survey period (e.g. triangulation or leveling), the geodetic networks were populated with monumented sites without permanent measurement systems, except for astrometric instruments (astrolabes or Photo Zenith Tubes) or coastal tide gauges. Since the advent of space techniques, the instruments themselves are considered as elements of networks. Instrument may be permanent or temporary and may be colocated with other instruments or may stand alone. Each site in the geodetic network encompasses co-located instruments and local supporting infrastructure and may include a few square meters or kilometers. The implementation of new, wider-band technologies may require larger sites and placement strategies to minimize Radio Frequency (RF) interference among on-site instruments and with RF facilities outside the site. Core site strategies may require as much as 8 hectares (20 acres) of land with topographic considerations to avoid RF problems. An essential task is to determine the intersystem tie vector between proper reference points on each instrument using topometric survey techniques and/or GPS techniques plus accurate engineering modeling. These reference points will in all likelihood be inside the instrument (e.g. intersection of the axes) or at phase center some place in space in front of the instrument. These tie vectors are potentially affected by orientation uncertainties and time variability. A local network of geodetic markers within and around the site will allow us to maintain geodetic control of the site area to monitor relative motion, both within the site elements (instruments and markers), and relative to neighboring points (geodetic footprints). 3.3. Co-locations Co-locations between geophysical sensors are fundamental to ensure a comprehensive, consistent and accurate observation of the global geophysical processes and to allow us to combine different data sources into the formulation of advanced data products. Co-locations can be fully exploited and the multidisciplinary potential of the sites well preserved only if the tie vector, expressing the relative positions of the reference point of the co-located instruments, is accurately estimated and monitored. Tie vectors connect the reference points on the co-located instruments in a consistent, often local, geodetic reference frame (see Section 5.1). Co-locations also allow us to connect local reference frames to the global reference frames. Co-location sites are a key element in the connection of local, regional and global geophysical processes that provide us with the realization of a holistic view and an understanding of the phenomena that occur on the Earth. The knowledge of the state and evolution of the Earth depends on the availability of a long time series of observations that are geographically well distributed and consistently geo-referenced. Global networks of observatories with modern co-located geophysical instruments and

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accurately determined tie vectors will allow us to create interconnected nodal points of the observing systems for long term monitoring of the evolution of the planet for a wide range of scientific and societal applications. A global network of co-located space geodesy techniques, with accurately determined tie vectors, provides the most accurate global reference frame. For best results, the tie vectors must account for any geometric and temporal variations to avoid aliasing data products and corrupting results. Using these principles, the IERS, since its creation in 1988, has periodically determined updated versions of the primary realization of the ITRS (see e.g. Altamimi et al., 2011). Numerous Earth monitoring projects devoted to global change detection and other pressing environmental issues require the reference frame to interpret observation over space, time and evolving technology. As an example, the IGS Tide Gauge Benchmark Monitoring (TIGA) project establishes co-locations between tide gauges and GPS or DORIS to separate the sea level variations from the local vertical uplift due to tectonic motion (see e.g. Wo¨ppelman et al., 2009). Another example is the co-location of seismometers with GPS for the investigation of local micro-seismicity due to abrupt movements of ice sheets (Ekstro¨m et al., 2003; Nettles and Ekstro¨m, 2010) and the contextual monitoring of the ice streams and glaciers velocities, which has significant impact on the global sea level rise (Shepherd and Wingham, 2007). Other applications include co-location of GPS and Automatic Weather Stations to retrieve tropospheric water vapor and co-location of GPS at astronomical or magnetosphere observatories to accurately provide the time and location of the measurement. The development of the reference frame requires co-location of VLBI, GNNS, SLR and DORIS. The tie vectors enter the combination of the space geodetic solutions as a fifth technique that provides geodetic constraints to the solution. As such, tie vectors must be provided with a full covariance matrix in standard exchange format, with a reliable assessment of the tie vector accuracy. Section (5.1) includes an up-to-date overview on the investigations regarding tie vectors, co-location sites and combinations. The measurement techniques (SLR, VLBI, GNSS, and DORIS) are organized as IAG Services, the International Laser Ranging Service (ILRS), the International VLBI Service (IVS), the International GNSS Service (IGS), and the International DORIS Service (IDS) (see Section 3.4). Despite their role in the ITRF combination, the local tie surveys and the tie vector solutions are not presently managed at a central level by a service, or coordinated through an international mechanism to oversee tasks such as: – – – – –

Definition of a site priority list. Definition of survey guidelines. Coordination of the surveys. Optimization of the data analysis. Validation of the results.

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– Creation of an archive for raw and reduced data. – Collection of metadata. The closest thing we have at the moment is the Institute of Geographic and Forest Information (Institut national de l’information ge´ographique et forestie`re) (IGN). Although informal cooperation among groups with survey capability has made considerable progress to date, the tie vectors are still non-homogeneous in quality and standards, guidelines, and procedures have not been made uniform (see Section 5.1). In addition, the tie vector solutions with full information are not stored and managed on a central repository: raw data, models, field sheets, analysis options, results, survey reports and other ancillary information are not easily and readily available, accessible and reusable. Local surveys data and metadata should be centrally stored on a regular base to provide easy access for the whole geodetic community, similarly to what is done with data by the four IAG services (see Section 1). As a consequence, a reliable assessment of the accuracy of each individual tie vector constituting the historical set of local site ties is presently impossible. 3.4. Network management Space geodetic instruments (VLBI, SLR/LLR, GNSS, and DORIS) are presently operated as elements of the IAG measurement services. The services develop global standards/specifications and encourage international adherence to their conventions. They collect and archive data and data products for the access by their user communities. Each technique has developed its own network as different groups built, bought, deployed and operated individual systems. Many groups deployed SLR and VLBI systems in convenient locations, which may have simplified operating economics, but did not lead to global distribution. Some groups, including NASA, BKG, the Chinese Academy of Sciences/National Astronomical Observatories of China (NAOC), and The Russian Academy of Sciences/Russian Space Agency have or are in the process of deploying overseas sites to improve network global distribution. The implementation of GGOS has helped to improve the communication and integration among the Services, and to provide a forum to help develop coordination and joint planning activities. The GGOS Bureau for Networks and Communication meets biannually to allow the Services an opportunity to present an update on its progress and to discuss impeding issues and plans for moving forward. The GGOS Inter-Agency Committee has been organized to interface with international organizations to foster recognition and support for geodetic activities, most prominently the recognition of the importance of the reference frame. Through its efforts and efforts of the GGOS Executive Committee, organizations including the United Nations and the Committee on Earth Observation Satellites

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(CEOS), have recognized the International Decade for the Reference Frame, which should provide an incentive toward its fulfillment. 4. Uses of global geodetic observatories in geodesy and geosciences As described before, GGOs are a key component of geodetic observing systems. To better understand this fact, it is important to consider the historical evolution and to identify key problems, as well as possible future trends. As an example of a new observing system, the development of space borne measurements offers great promise, either through inter-satellite links using the GNSS constellations, or by co-locating several instruments onboard individual satellites. The latter was initially demonstrated by the TOPEX/Poseidon mission, and is presently the core component of new proposed space missions such as GRASP (Nerem et al., 2011; see also http://ids-doris.org/ documents/report/ids_workshop_2008/IDS08_s4_Barsever_ GRASP.pdf) and STE-QUEST (http://sci.esa.int/ste-quest/). In such systems, the combination of co-locations on the ground and in space foresees a wider interdisciplinary science vision in modern satellite missions. The evolution of processing strategies is another area where improvement is being realized. In addition to the methods initially developed based on hierarchical combinations, new approaches, based on global analysis and data assimilation, are under development. Within the IERS where the combination approach is still operationally used for product generation (terrestrial frame, Earth rotation parameters), a task force has been establish in order to investigate multi-technique processing using common analysis software (see e.g. Pavlis et al., 2006; Gambis et al., 2009). A third area is related to analysis relevant to the various information types contained in geodetic observing systems (geometry, gravity, environment). In this paper, we have discussed only specific issues related to ITRF. Other important enhancements might include: – New strategies for ITRF, including use of space co-locations, inclusion of gravity field data, new modeling techniques, application of vertical references (including colocation with tide gauges), etc. – Application to gravity field and geoid modeling. – Application to global space-time frames, with incorporation of time/frequency standards (both ground and space borne). 5. Some current topics of investigation 5.1. Local surveys, tie vectors and ITRF Tie vectors provide the relative positions, at the measurement epoch, of the conventional reference points of the two or more co-located space geodetic instruments.

The conventional definition of space geodetic instrument reference point is provided by the individual IAG services for each space geodetic instrument type (see e.g. Sarti et al., 2009). Only the conventional reference point can be accessed during the local tie surveys. Tie vectors have a fundamental role in the computation of the ITRF: they are used to link together the solutions of the different space geodetic techniques. The ITRF is the foundation for virtually all space-based and ground-based Earth observations and it must be accurately, stably and homogeneously provided (Gross et al., 2009). The ITRF strongly relies on the quality, number and distribution of the tie vectors determined locally at co-location sites (Altamimi et al., 2005). As a matter of fact, the ITRF accuracy also depends on the accuracy of the tie vectors and future ITRF improvements reside in improving the consistency between tie vectors in co-location sites and space geodesy estimates (Altamimi et al., 2011). So far, the provision of tie vectors at the 1 mm accuracy level remains a challenging goal. In order to boost the efforts of the international geodetic community, the IERS set up the working group on “Site survey and co-location” in early 2004 (http://www.iers.org/nn_10900/IERS/EN/Organization/WorkingGroups/ SiteSurvey/sitesurvey.html?__nnn=true). The working group was created after the first IERS workshop on tie vectors and co-location sites held in Matera (Italy) in October 2003 (http://www.iers.org/nn_10902/IERS/EN/Organization/Workshops/Workshop2003.html) and whose proceedings were published in the IERS technical Note No. 33 (Proceedings IERS, 2005). We will shortly review the aspects that influence the tie vector accuracy in the remainder of this sub-section. 5.1.1. Surveying techniques and approaches The tie vector accuracy depends on the type, quality and amount of measurements acquired at the co-location site. Tie vectors are surveyed adopting site-dependent strategies and their accuracy also depends on the technical capacities and skills of the local surveying crew. In this respect, one of the long-term goals of the IERS working group has been to establish surveying standards, procedures and guidelines. Progress has been impeded because of difficulties similar to those identified by the FIG Committee on the Analysis of Deformation Surveys, established in 1978, whose final report stated that: “it would be difficult to completely unify all the approaches into one general set of guidelines for practising surveyors” and “the final choice of the [surveying] approach should be left to the user” (Chrzanowski and Chen, 1986). These statements also apply to tie vector surveying and partially explain the difficulties in setting guidelines of general validity. With the purpose of quantifying the impact on the tie vector accuracy of some site-dependent aspects, Abbondanza and Sarti (2012) carried out some simulations varying surveying approach, local network geometry and observation scheme.

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Their results showed that these aspects impact the tie vector at the one millimeter level. 5.1.2. Site peculiarities The length of the tie vector between co-located instruments as well as the local ground control network design can impact the type and amount of measurements required. When the separation does not exceed few hundred meters, a topometric approach is usually adopted (terrestrial triangulation, trilateration and high precision spirit leveling). For these baselines, pure GPS-based methods have also been tested (e.g. Abbondanza et al., 2009a; Kallio and Poutanen, 2012). The performances of topometric- and GPS-derived tie vectors in ITRF-like combinations were evaluated by Abbondanza et al. (2009b) who showed that the former perform better and are characterized by a higher level of accuracy. When the separation between the instruments exceeds 1 km, the accuracy of topometric techniques rapidly decreases, and the GPS technique is used as an alternative to or in combination with terrestrial observations. In this latter case, the deflection of the vertical and/or the geoid undulation must be known locally to an accuracy compatible with the desired measurement objective. In addition, the values of the deflection of the vertical are strictly necessary in areas where the local topography induces abrupt variations of the local plumb line over short distances (more than 0.5 arcseconds over a few hundred meters). When local geoid undulations are present, ad hoc local surveys should be performed. Accurate values of the components of the deflection of the vertical are also required to align the tie vector into the ITRF (see Section 5.1.6), though they are seldom available. In summary, the deflection of the vertical might have a non-negligible impact locally on the tie vector accuracy and its significance in the ITRF computation, however quantitative studies are still missing. 5.1.3. Intra-site motions The local geological setting and the extent of local intrasite motions impact the tie vector stability and determine the frequency at which it should be surveyed. The local geodetic infrastructure must meet the requirements posed by the local geological features to ensure long-term stability of the tie vectors and the surrounding geodetic markers. This is a crucial issue for all ITRF co-location sites. Sarti et al. (2013a) analyzed a set of topometric observations spanning ten years to determine the long term stability of the tie vector at Medicina (Northern Italy), which is a site with very challenging geological conditions. There is a significant amount of anisotropic motion between the markers of the local ground control network, with rates up to 16 mm/a over baselines of a few tens of meters. There, the tie vector length change rate, computed over a 8 years span, amounts to (0.4 ± 0.4) mm/yr, suggesting a request for a regular monitoring of the vector every e.g. second year. In such cases, a permanent terrestrial monitoring

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system might help spotlighting seasonal, short and long term changes that request thorough local tie re-surveys. A layout of such a system is currently being investigated by NASA’s Space Geodesy Project (see Section 5.2) and might be a valid solution for some problematic sites, to detect and store series of intra-site motions on a regular base. The geological setting at Medicina is, with no doubt, extremely unfavorable; though it might be common to other co-location sites. Medicina shows the stringent necessity to suit the local geodetic network to the local geological conditions to ensure the highest level of stability of the local geodetic monuments. 5.1.4. Tie vector computation The reference points of the various space geodetic instruments are surveyed adopting different approaches and the tie vectors are computed by means of different in-house software whose performance have not been thoroughly compared (e.g. Johnston and Dawson, 2004; Sarti et al., 2004; Proceedings IERS, 2005; Leinen et al., 2007; Loesler, 2009). Only Dawson et al. (2007) compared the performances of two different in-house software packages for tie vector estimation, evaluating the effect of a varying degree of conditioning in the post-processing phase. They showed that the difference in the results from the software packages on the final tie vector estimate may be as large as a few tenths of millimeter and that there is a dependency (up to 3.5 mm) in the Up component of the estimated conventional VLBI reference point upon the geometric conditions applied to the indirect observations in the post-processing phase. 5.1.5. Observation biases Several systematic errors may affect the measurements that are used to compute the tie vector. Biased topometric observations were used in a simulation performed by Abbondanza and Sarti (2012). The authors show that the formal precision of the tie vector is not affected by the presence of systematic errors, but the accuracy of the tie vector can be degraded by several millimeters. The use of calibrated instrumentation and ad hoc correction models are mandatory to reduce the systematic errors to a minimum and to preserve the accuracy of the tie vector to the millimeter level. 5.1.6. Tie vector alignment into the ITRF The tie vectors determined with topometric observations must be accurately aligned to the ITRF before they can be used in the ITRF computation. The alignment phase is crucial as this procedure may alias the intrinsic accuracy of the tie vector by several millimeters. The alignment from the local topocentric frame into the global frame can either be pursued using a similarity transformation over common points (e.g. also surveyed with GPS) or by taking into account the deflection of the vertical and its components (see e.g. Vanı´cˇek and Krakiwsky,

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Fig. 1. ITRF2008 residuals of the tie vectors between GPS and the other space geodetic techniques. The tie vector local geodetic coordinates East, North, Up and length are identified by squares, circles, triangles and asterisks, respectively. The dashed lines identify a ± 5 mm boundary. The larger ordinate range of the bottom plot reflects the higher residual values associated to the GPS-DORIS tie vectors, in particular those of the Up component, identified by triangles.

1982). When GPS is used to determine common points, their number and quality may impact and degrade the accuracy of the tie vector (see e.g. Ray and Altamimi, 2005). An alternative to the use of common points is based on the knowledge of the local deflection of the vertical, whose accurate determination usually requires ad hoc local surveys with different techniques (see e.g. Ceylan, 2009; Hirt et al., 2010). A very important by-product of the ITRF combination is the set of post-fit position residuals (see e.g. Collilieux et al., 2007); the most recent combination residuals can be found at the following link: http://itrf.ensg.ign.fr/ ITRF_solutions/2008/. The ITRF2008 residuals of the tie vectors between GPS and the other space geodetic techniques (Z. Altamimi, personal communication) are shown in Fig. 1. From top to bottom, the three plots show the residuals over the tie vector local geodetic components East, North and Up and its norm for the GPS-VLBI, GPS-SLR and GPS-DORIS co-locations, respectively. Their magnitude is considerably higher than the 1 mm accuracy level desired for the tie vectors and also notably higher than the formal precisions associated to both the tie vectors and the space geodetic solutions. These discrepancies spotlight the presence of systematic errors in either the tie vectors, the space geodetic solutions, or both. The residuals of the tie vectors between GPS and VLBI techniques are less scattered than the others. Out of 35 GPS-VLBI tie vectors used in the ITRF2008 computation, only one shows residuals exceeding the ± 5 mm boundary

for all the three local geodetic components and the tie vector length (see Fig. 2, red box in the first column). This boundary value is assumed as representative of a satisfactory agreement between tie vectors and space geodetic solutions at co-located sites, also taking into account the way tie vectors are introduced in the ITRF computation (Altamimi et al., 2011). Higher residuals certainly spotlight problems of different origin that should be carefully investigated. As a matter of fact, neither the components nor the norm of twelve GPS-VLBI co-locations exceed the ± 5 mm boundary (see the green box in the first column of Fig. 2), showing the highest percentage of agreement of all techniques (Table 2). On the opposite side stand the GPSDORIS co-locations: these latter do not only exhibit the largest residuals, often exceeding several centimeters, but are also associated with the highest percentage of tie vectors outside the ± 5 mm boundary (Table 2). Indeed, the bottom plot in Fig. 1 required a larger range for the ordinate axis values to entirely show the residuals scatter. The agreement of the three types of tie vectors and the space geodetic solution can be further analyzed focusing on their single components and norm. In particular, Table 2 further elaborates the data plotted in Fig. 2, and sorts the local geodetic coordinates and the tie vector length for each type of co-location also reporting the number of outliers using percentage values. Again, it is clearly visible that the overall performance of the GPS-DORIS co-locations is worse than the other tie vectors. More than half of them exceed the ± 5 mm boundary, with identical performance on the two

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Fig. 2. Outlier occurrences at the co-location sites. Red identifies the number of tie vectors where all components and length have residual exceeding the ± 5 mm boundary; cyan, magenta and blue respectively denote vectors with 3, 2 or 1 residual exceeding the boundary. Green identifies vectors with no residual exceeding the boundary.

Table 2 Percentages of tie vectors whose residuals (sorted in East, North, Up and Length) exceed 5 mm.

GPS-VLBI GPS-SLR GPS-DORIS

East

North

Up

Length

31 26 53

26 26 53

43 74 62

23 32 49

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horizontal components (53%) and a further degradation on the vertical (62%). Almost half of the GPS-DORIS tie vector norms (49%) differ from the combined space geodetic solution by more then 5 mm. As for the GPS-SLR co-locations, the values contained in Table 2 show that only 1=4 of them exceed the boundary limit. This is certainly not the case for the Up components that surprisingly show a triple amount of outliers than those associated to the North and East components. This strongly suggests the presence of errors in the estimated SLR heights whose origins and causes are most probably related to technique specific biases (cf. Section 5.1.7). It is worth highlighting that the residuals shown in Fig. 1 do not apparently depend on the age of the tie vectors. This is somehow surprising because tie vectors taken most recently should, in fact, have higher accuracies and smaller residuals since: (i) Non linear local motions and monument instabilities are likely to degrade the accuracy of the tie vectors with time (cf. Sarti et al. 2013a). (ii) Local surveying approaches and methods used to estimate and align the tie vectors have improved with time and (iii) Instrument precisions have also been augmented, thus leading to more accurate estimates (cf. Sarti et al. 2013b). In the ITRF2008 computation, no age-dependent weighting was introduced in the combination but the aforementioned reasons suggest the identification and test of some sort of age-related weighting criteria.

Fig. 3. The magnitude of the ITRF2008 residuals associated to the tie vector length is plotted as a function of the length itself. The horizontal dashed lines mark the ± 5 mm boundary limit.

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Similar considerations hold for the residuals over the tie vector length when plotted against the length itself (Fig. 3). Longer tie vectors should be less accurate than shorter ones, especially those surveyed with topometric methods (Section 5.1.2); as a consequence, one might expect larger residuals for longer tie vectors. However, Fig. 3 shows that the norm of the tie vectors exhibit a great variability, ranging from a few meters up to a few tens of kilometers, but the residuals do not clearly increase with the norm. This might be related to the re-distribution effect of the least squares and the re-weighting of the tie vectors during the combination. Indeed, it is legitimate to question the actual ITRF colocation site selection and possibly identify an upper limit on the separation of the space geodetic instruments. One kilometer appears as a reasonable upper limit for the ITRF co-location sites, being a distance over which terrestrial techniques can be easily exploited to their utmost accuracy level and eventually compared or integrated with GPS, whose accuracy is certainly less sensitive to the extension of the network. Adopting a more stringent distance selection criterion, several tie vectors in Fig. 3, most of which connecting GPS and DORIS instruments, would be discarded. A site-specific analysis of local discrepancies is complicated by the redistribution effect of the least squares solution. Large residuals certainly reflect the presence of possible biases either affecting the space geodetic solutions, the tie vectors or both and, as Fig. 1 clearly shows, can easily exceed 1 cm, this value being considerably larger than the formal precision of both solutions (usually at the submillimeter or millimeter level). The origin of these discrepancies must be investigated with a technique specific approach, with the purpose of identifying the relevant observation biases (see Section 5.1.7) that make up the discrepancies observed site by site. This action requires a close cooperation between the IAG Services (Section 2.4), the IERS Combination Centers and the IERS “Site survey and co-location sites” working group described in Section 5.1. This cooperation would certainly favor other decisions such as the prioritization of the tie vector surveying (or re-surveying), currently entrusted to the IERS ITRS Center, based on the residual values, their age and several site dependent peculiarities and effects. An example of an effective re-surveying, driven by the high values of the combination residuals, is that of Fortaleza, Brazil, site (Ray et al., 2007). 5.1.7. Some potential bias sources in space geodetic techniques The position residuals of the combination reflect the discrepancies between the space geodetic solutions and the tie vectors, whose formal precisions can easily be ten times less than the residual values. It certainly is an indication of the presence of errors, mostly systematic, affecting the positions. A number of potential error sources possibly

embedded in the tie vector estimation process were discussed in Section 5.1 and the Sub-sections therein. In addition, biased observations and systematic errors in the data analysis can substantially contribute to the final degradation of the accuracy of the space geodetic solutions, thus concurring significantly to the picture summarized by the residuals shown in Fig. 1. The biases are technique- and component-specific, as can be inferred by the differences shown in Fig. 1 as well as the varying values reported in Table 2, thus making the contributions from the space geodetic technique error substantial. One very critical aspect that might degrade the consistency of the tie vectors and the space geodetic solutions is related to the dichotomy embedded in the definition of the space geodetic instrument reference point and their difficult connection (Sarti et al., 2009). The geodetic observable is acquired at the “electronic” reference point of the instrument (antenna/feed horn phase center or photodetector) but the space geodetic solution is conventionally given at the instrument conventional reference point, whose nature is strictly geometric. Tie vectors may only express the relative positions of the conventional reference points, and cannot directly access and locate the point where the observable is acquired. The “electronic” point is mapped onto the conventional reference point during the space geodetic data processing by means of correction models e.g. the phase centre variation files for GPS (Schmid et al., 2007) and for VLBI, thermal correction models (Nothnagel, 2009). Similarly, DORIS and SLR techniques may require analogous models. Recent studies on the stability of the phase center in DORIS STAREC beacons suggested the introduction of phase center correction models for the 2 GHz frequency to account for a bias of several millimeters on the relative position between the “electronic” and conventional reference points (Tourain and Auriol, 2013). As for SLR technique, the large number of outliers associated to the Up component suggests the presence of systematic errors that specifically impact that coordinate and might be originated by e.g. inaccurate range biases modeling (see e.g. Appleby et al., 2008). The availability and accuracy of the correction models directly impacts the accuracy of the positions. The definition of proper, instrument-dependent correction models certainly is a central, up-to-date investigation topic. Any improvement of those models will increase the consistency of space geodetic solutions and the accuracy of the ITRF. In addition, residual site- or instrument-dependent effects might introduce further technique-related errors in the space geodetic solutions; when disregarded, near field effects, observation mismodeling and local monument instabilities can degrade the position accuracy (see e.g. Dilssner et al., 2008; King and Williams, 2009; Sarti et al., 2011; Steigenberger et al., 2013). The combination of space geodetic solutions, the integrated use of co-located instruments and the establishment of efficient GGOs relies on our ability to coordinate the tie vector production with the space geodetic technique services and the centers

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Fig. 4. Map showing current established core sites, core sites in process, and sites currently under consideration by NASA (denoted by circles) as either new sites or as site upgrades.

Table 3 Responses to the GGOS Call for Participation. Details can be found at http://www.ggos.org/News/News.html. Agency (Country)

Sites

BKG/FESG (Germany) NERC (UK) IRA (Italy) OSO (Sweden) FGI (Finland) IGN (Spain) SPC (Poland) SHAO (China)

Wettzell Herstmonceux Medicina, Noto, Sardinia Onsala Metsahovi Yebes Borowiec Shanghai, Beijing, Changchun, Wuhan, Kunming, Urumqi, Sanyo, (San Juan) Yarragadee, Mt. Stromlo, Katherine, Hobart Toro GSFC, Westford, Kokee Park, Monument Peak, Fortaleza, McDonald, Mt. Haleakala, Hartebeesthoek, Papeete, Arequipa Pecny Hartebeesthoek, Matera Riyadh (SALRO) Ny Alesund Svetloe, Zelenchukskaya, Badari DORIS Network

GA (Australia) NASRDA (Nigeria) NASA (US)

RIG (Czech Republic) NRF (South Africa) ASI (Italy) KACST (Saudi Arabia) NMA (Norway) RAS (Russian Federation) CNES

performing the combination of solutions, with the purpose of reducing systematic errors. 5.2. Design and implementation of network infrastructure NASA’s Space Geodesy Project is currently building and testing a prototype Core Site at GSFC as a model to

be subsequently replicated at several other sites worldwide, as NASA’s contribution to international space geodesy programs and GGOS (Fig. 4). These sites would replace legacy equipment in some cases, while some will be located at new sites that would augment or fill gaps in the current network geographically. Fig. 4 shows the site at GSFC equipped with its legacy SLR (MOBLAS -7) and Next Generation SLR (NGSLR), both the legacy VLBI and the new technology VLBI, i.e. the VLBI Global Observing System (VGOS), DORIS, and several legacy and new multi-constellation GNSS receivers. Fig. 4 shows other sites with the three or four techniques. In general, broadband reception by VGOS is sensitive to RF broadcasts from SLR radars and from DORIS beacons, and therefore careful selection of location and layout will be critical for new technology sites. In addition, great care must be taken to avoid incoming and outgoing interference with external systems such as commercial communication and media transmissions and broadcasts. A map with current core sites, sites planned for upgrade to core status, and sites under consideration for Core status are shown in Fig. 4. The circles denote sites that are under consideration for upgrade or establishment by NASA through partnerships. Many of the Proposed/Suggested Sites were submitted through a GGOS “Call for Participation” offering partnerships and involving 18 agencies (Table 3). The map in Fig. 4 does not include six SLR sites that the Russian Space Agency (ROSCOSMOS) and the Russian Academy of Science plan to deploy at sites outside of Russia to enhance tracking on the GLONASS and other GNSS constellations and to support GGOS. The process of designing the network will start with a conceptual network design and then by examining general

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areas for candidate sites. To the extent possible, the process should start by examining current space geodesy sites for geographic location, site conditions and infrastructure, and available technology and expertise. Many of the current legacy core sites and sites with less than a full complement of systems will be good candidates. We will need to locate additional candidates in new areas. We also have to realize that many of the sites will be at locations where the total necessary resources and expertise are not available and partnerships will have to be established. Some groups will be able to cost share in the hardware, some will not. Some will be able to shoulder most or all of the operations, some will not. Our choices may be based on compromises between site quality and available local resources. It’s going to be a global community effort. The Services are developing the specification for the space geodetic systems and GGOS has written a Site Requirements Document (discussed later). Design studies are underway by several groups on station layout, but this too will depend on the local site, topography and conditions. The requirements for a core site are discussed in “Site Requirements for GGOS Core Sites” (see http://cddis.gsfc. nasa.gov/docs/2011/GGOS_SiteReqDoc.pdf). The document explains the Core Site, why we need it, why we need the Reference Frame, why we need a global network, what is the current situation and what we need to implement. The document discusses conditions for an ideal site: – – – – – – – – – –

– – – – – –

Global Consideration for the Location Geology and Site Stability Site Area (reservation) Weather and Sky Conditions Radio Frequency and Optical Interference Horizon Conditions Air Traffic and Aircraft Protection Communications Land Ownership Local Ground Geodetic Networks s Local Station Network s Regional Networks Site accessibility Local infrastructure and Accommodations Electrical Power Technical and personnel support, etc Site Security and Safety Local Commitment

Although the systems at a core site operate for the most part independently of each other, there are common subsystems and infrastructure, and synergy of labor. Local clocks, frequency standards, meteorological stations, communications links, and control systems, test equipment, engineering space, etc may be used in common. GNSS and DORIS operate autonomously, but need on-site monitoring and occasional maintenance and repair. The VLBI

and SLR are currently much more labor intensive, but this situation is changing. The new generation VLBI2010 is automated and several of the newer SLR systems (Zimmerwald, Wettzell, Graz and the new NASA NGSLR) support autonomous operation for considerable periods of time (see e.g. Pearlman et al., 2009; Petrachenko et al., 2012; Sun et al., 2014). Co-located newer generation systems should operate with less dedicated personnel and share on-site manpower for repair and maintenance. We expect to see more centrally controlled operations either at the site level or even at the sub-network level. The site at Wettzell, Germany is and example of the current trend toward a centralized site control system that might be the basis for use at other locations. GGOS has established a Working Group on Communication and Automation to help share some of these ideas. One area that will involve all of the systems at a core site is the accurate measurement of the tie vectors with topometric techniques required to link all of the space geodesy measurements to the common reference frame (see Section 5.1). Transition from the current network to the ultimate core network needs to be done seamlessly to avoid discontinuity in the evolution of the reference frame, which depends on the data time series. The total evolution will probably take 10–20 years. The evolution needs to be simulated as it progresses so that the impact will be understood. In the meantime, co-location sites with less than the full Core complement will continue to be an essential complement in the network and maintenance and upgrade will be vital to the improvement of the reference frame. We also have to realize that the ultimate network realization will be a compromise between our “requirements” and the practical world. In all likelihood most if not all of the sites will have some shortcoming that will have to be overcome through smart use of the data, better models, and better analysis techniques. 5.3. Standardization As mentioned earlier, several aspects would be greatly simplified if more standardization were adopted and internationally recognized. Standardization could also include items such as site cataloguing, site identification or related metadata files. Such capability should be built as much as practicable on activities and systems already developed by various organizations or projects such as GGOS and the Services, IERS Conventions, DOMES numbering, SINEX format, or NASA/CDDIS activities (Noll, 2010). Such standards should also help the interoperability of activities developed by the organizations actually contributing to the realization of these infrastructures. At this point, it is important to agree on a strategy on standardization and in particular to participate in the newly established project on geodetic references (ISO TC 211/19161) in the frame of the International Standardization Organization (ISO).

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It is expected that GGOS will be a key participant in the standardization process. 5.4. International organization GGOS and the Services (IGS, IVS, ILRS, IDS, IGFS, and IERS) through their members and participants provide the data and data products to fulfill users requirements. Each of these entities is international in scope and each interfaces with the broader international community. As mentioned earlier the GIAC and the GGOS have developed interfaces with international organizations such as GEO, CEOS, the International Council for Science (ICSU), and the United Nations to expand visibility of geodesy and garner support for geodetic products. Through the GIAC, GGOS is expanding its participation to a broader community of government agencies; to date 16 agencies from 15 countries have membership in the GIAC and several more are anticipated shortly. The GGOS organization is made up from a mix of technical institutions and government agencies, and the GIAC within GGOS has begun the process of outreach to help increase international awareness of the networks needs. Considering the societal impact of the geodetic infrastructure and the importance to making intelligent decisions that will have both national and international consequences, it may be advantageous to establish an intergovernmental governance to better guarantee the resources to implement and maintain the network infrastructure and the processes for the development of new and improved data products. GGOS has already made inroads to GEO and CEOS. Some other examples of organizations that might be brought into this process include: the International Association of Meteorology and Atmospheric Sciences (IAMAS), The World Meteorological Organization (WMO), the International Association for the Physical Sciences of the Oceans (IAPSO) or oceanography (IAPSO and IOC). A key question is the extension of the communities to be covered by such an intergovernmental umbrella beyond geodesy: seismology, volcanology, geomagnetism, tectonics, geology, etc. 6. Conclusions Core and co-located ground stations will play an everincreasing role in addressing scientific questions and issues of societal importance. The networks of stations is currently expanding in global coverage and in the adoption of modern technology to address the required improvement in the reference frame, but this expansion will probably take 10 – 20 years. Although the network is current focused on SLR, VLBI, GNSS and DORIS, we should recognize the potential benefit of these networks to a broader range of measurement instruments and synergies that could provide economic benefit. Even more important however is the scientific benefits that could be attained by having

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co-located measurements of similar and related phenomena seen in different ways. The development, implementation and maintenance of the networks is a complicated process requiring significant international participation and partnerships. The Services are striving to foster standardization in their processes; workshops, documentation and Service coordination spread information among participants on a Service-wide basis, but we still need to better address standardization and best practices on a total community basis. Fundamental to co-location where it be for the reference frame or other applications are the local site ties which connect the co-located techniques to allow us to integrate their measurements and data results for more comprehensive data products that will give us greater insight into the Earth and its environment. Both ground survey and GNSS space techniques point to point measurements play a central role in this process, but the extrapolation to the instrument reference point through specialized surveys, accurate engineering specification and engineering modeling measurements are essential to completing the tie vector determination. Some multi-techniques space concepts also look promising. Global network of Geodetic Observatories will require time and money, and organizations like the GIAC have begun programs of outreach to national agencies and institutions help expand understanding and build commitment. It might also be beneficial to develop a mechanism for international governance at sometime in the future. References Abbondanza, C., Altamimi, Z., Sarti, P., Negusini, M., Vittuari, L., 2009b. Local effects of redundant terrestrial and GPS-based tie vectors in ITRF-like combinations. J. Geod. 83 (11), 1031–1040. http:// dx.doi.org/10.1007/s00190-009-0321-6. Abbondanza, C., Sarti, P., 2012. Impact of network geometry, observation schemes and telescope structure deformations on local ties: simulations applied to sardinia radio telescope. J. Geod. 86 (3), 181– 192. http://dx.doi.org/10.1007/s00190-011-0507-6. Abbondanza, C., Vittuari, L., Negusini, M., Sarti, P., 2009a. VLBI-GPS eccentricity vectors at Medicina’s observatory via GPS Surveys: reproducibility, reliability and quality assessment of the results. In: Drewes, H. (ed.), Geodetic Reference Frames, IAG Symp. Series, 134, Munich, October 9–14, 2006, Springer-Verlag, Berlin Heidelberg. Altamimi, Z., Boucher, C., Willis, P., 2005. Terrestrial reference frame requirements within GGOS perspective. J. Geod. 40, 363–374. http:// dx.doi.org/10.1016/j.jog.2005.06.002. Altamimi, Z., Collilieux, X., Me´tivier, L., 2011. ITRF2008: an improved solution of the International Terrestrial Reference Frame. J. Geod. 85 (8), 457–473. http://dx.doi.org/10.1007/s00190-011-0444-4. Appleby, G., Wilkinson, M., Luceri, V., Gibbs, P., Smith, V. 2008. Attempts to separate apparent observational range bias from true geodetic signals. In: Schillak, S. (Ed.), Proceedings of the 16th International Workshop on Laser Ranging, Pozna´n, Poland, October 13–17, 2008, Space Research Centre, Polish Academy of Sciences. Beckley, B.D., Lemoine, F.G., Luthcke, S.B., Ray, R.D., Zelensky, N.P., 2007. A reassessment of global and regional mean sea level trends from TOPEX and Jason-1 altimetry based on revised reference frame and orbits. Geophys. Res. Lett. 34 (14). http://dx.doi.org/10.1029/ 2007GL030002. Beutler, G., Rummel, R. 2012. Scientific Rationale and Development of the Global Geodetic Observing System, Geodesy for Planet Earth. In:

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