Global Geodetic Observing System Research Articles

The crustal dynamics data information system: A resource to support scientific analysis using space geodesy

Since 1982, the Crustal Dynamics Data Information System (CDDIS) has supported the archive and distribution of geodetic data products acquired by the National Aeronautics and Space Administration (NASA) as well as national and international programs. The CDDIS provides easy, timely, and reliable access to a variety of data sets, products, and information about these data. These measurements, obtained from a global network of nearly 650 instruments at more than 400 distinct sites, include DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite), GNSS (Global Navigation Satellite System), SLR and LLR (Satellite and Lunar Laser Ranging), and VLBI (Very Long Baseline Interferometry). The CDDIS data system and its archive have become increasingly important to many national and international science communities, particularly several of the operational services within the International Association of Geodesy (IAG) and its observing system the Global Geodetic Observing System (GGOS), including the International DORIS Service (IDS), the International GNSS Service (IGS), the International Laser Ranging Service (ILRS), the International VLBI Service for Geodesy and Astrometry (IVS), and the International Earth rotation and Reference frame Service (IERS). Investigations resulting from the data and products available through the CDDIS support research in many aspects of Earth system science and global change. Each month, the CDDIS archives more than one million data and derived product files totaling over 90 Gbytes in volume. In turn, the global user community downloads nearly 1.2 Tbytes (over 10.5 million files) of data and products from the CDDIS each month. The requirements of analysts have evolved since the start of the CDDIS; the specialized nature of the system accommodates the enhancements required to support diverse data sets and user needs. This paper discusses the CDDIS, including background information about the system and its user communities, archive contents, available metadata, and future plans.

Studies of the Earth's center of mass periodical movements

At present the Earth reference coordinate frame ITRF is realized on the base of space geodesy observations in strictly secular sense with purely linear temporal changes (velocities) without any conventional models. These linear changes include primarily non modeled geophysical motions, mostly of tectonic origin. In the base of all space geodesy techniques (except of VLBI), there are precise orbits of the observable satellites, which are moving under the influence of the Earth gravity field around of the entire Earth’s center-of-mass (CM). To keep the convention of the coincidence of the terrestrial coordinate system origin with the entire Earth center-of-mass, the first harmonics (C1,1, S1,1, C1,0) of the gravity field models, used for orbital calculations, are equated to zero. Studies of the ITRF precision and stability, carried out at the different research centers, INASAN included, show that origin of the coordinate system (Geocenter) is moving relatively to the center of mass. Besides the main geophysical effects: atmosphere pressure, oceanic tides and currents, ice melting, have been taken into consideration. The annual and semiannual amplitudes of the geocenter variations are not the same for different types of measurements and fluctuate in the interval 1–23 mm for all three components of the position vector. And Z-component is always 2–3 times more than planar components. The absolute scale of the network is determined with the accuracy around of 0.5 · 10−13, that is equivalent 3 mm in the station height. The further improvement of the ITRF accuracy and stability is possible with the use of more dense and equally distributed tracking networks, equipped with the different types of modern instruments, allowed to minimize systematic errors, characteristic for every usable technique. In this connection the International Association of Geodesy arrived to a decision on the development of the Global Geodetic Observing System (GGOS), which will consist of about 40 permanent ground tracking sites, space segment of special research satellites and centers of merging, storage and analyses of the data.

The International DORIS Service (IDS): Toward maturity

DORIS is one of the four space-geodetic techniques participating in the Global Geodetic Observing System (GGOS), particularly to maintain and disseminate the Terrestrial Reference Frame as determined by International Earth rotation and Reference frame Service (IERS). A few years ago, under the umbrella of the International Association of Geodesy, a DORIS International Service (IDS) was created in order to foster international cooperation and to provide new scientific products. This paper addresses the organizational aspects of the IDS and presents some recent DORIS scientific results. It is for the first time that, in preparation of the ITRF2008, seven Analysis Centers (AC’s) contributed to derive long-term time series of DORIS stations positions. These solutions were then combined into a homogeneous time series IDS-2 for which a precision of less than 10 mm was obtained. Orbit comparisons between the various AC’s showed an excellent agreement in the radial component, both for the SPOT satellites (e.g. 0.5–2.1 cm RMS for SPOT-2) and Envisat (0.9–2.1 cm RMS), using different software packages, models, corrections and analysis strategies. There is now a wide international participation within IDS that should lead to future improvements in DORIS analysis strategies and DORIS-derived geodetic products.

The interdisciplinary role of space geodesy—Revisited

In 1988 the interdisciplinary role of space geodesy has been discussed by a prominent group of leaders in the fields of geodesy and geophysics at an international workshop in Erice (Mueller and Zerbini, 1989). The workshop may be viewed as the starting point of a new era of geodesy as a discipline of Earth sciences. Since then enormous progress has been made in geodesy in terms of satellite and sensor systems, observation techniques, data processing, modelling and interpretation. The establishment of a Global Geodetic Observing System (GGOS) which is currently underway is a milestone in this respect. Wegener served as an important role model for the definition of GGOS. In turn, Wegener will benefit from becoming a regional entity of GGOS. What are the great challenges of the realisation of a 10 −9 global integrated observing system? Geodesy is potentially able to provide – in the narrow sense of the words – “metric and weight” to global studies of geo-processes. It certainly can meet this expectation if a number of fundamental challenges, related to issues such as the international embedding of GGOS, the realisation of further satellite missions and some open scientific questions can be solved. Geodesy is measurement driven. This is an important asset when trying to study the Earth as a system. However its guideline must be: “What are the right and most important observables to deal with the open scientific questions?”.

Geodetic Observations of Glacial Isostatic Adjustment: Understanding Glacial Isostatic Adjustment: A Joint DynaQlim/GGOS Workshop; Espoo, Finland, 23–26 June 2009

The phenomenon of glacial isostatic adjustment (GIA), with its unique temporal and spatial signatures, provides one of the great opportunities in the geosciences for obtaining information about dynamic Earth processes. GIA contains information about recent climate forcing, being dependent on the geologically recent loading and unloading of ice sheets. It presents a unique opportunity to study the dynamics and rheology of the lithosphere and mantle; is responsible for substantial sea level variations; and is of fundamental importance to geodesy because site positions, reference frames, Earth rotation, and gravity are all influenced by GIA.The Global Geodetic Observing System (GGOS) of the International Association of Geodesy and the International Lithosphere Program's Regional Co‐ordination Committee Upper Mantle Dynamics and Quaternary Climate in Cratonic Areas (DynaQlim) jointly organized a workshop that attracted 36 international scientists to Finland. The objectives of this workshop were to (1) review the current state of the science in modeling GIA, (2) review the use of geodetic measurements to constrain and test GIA models, (3) identify obstacles to improving GIA models, and (4) identify improvements to the global geodetic observing system that are required to better understand GIA.

AGrav—The New International Absolute Gravity Database of BGI and BKG and its benefit for the Global Geodynamics Project (GGP)

Abstract A new database for absolute gravity (AG) measurements has been implemented at BGI and BKG and is operational now for storing absolute gravity data either in the form of metadata or as detailed measurement results. The database development was proposed by the IGFS (International Gravity Field Service) and is expected to have a great importance for the GGOS (Global Geodetic Observing System) initiative. This database will provide an overview about AG stations and observations and by this improve the cooperation between gravity groups and foster the combination with other geodetic observation techniques. The international community of absolute gravimeter users is asked to contribute to this database. In addition to its primary purposes, demonstration of the global site distribution and information about available observations, the database could also provide an important contribution to the Global Geodynamics Project (GGP). Precise repeated absolute gravity measurements at the superconducting gravimeter (SG) sites are necessary for the determination of SG drift parameters and can be used for checking SG instrument calibration factors. The AGrav database is capable of storing the necessary AG observations at the SG location in detail up to the “single drop level” and provides this information for the combination with SG time series. An example for a selected station is presented. It is proposed to establish an interface between the AGrav and GGP databases.

Contributions of the French Institut Géographique National (IGN) to the International DORIS Service

DORIS is one of the four geodetic space techniques participating to the Global Geodetic Observing System (GGOS). Since the early development of this system, the Institut Géographique National played a specific and active role. Within, the International DORIS Service (IDS), IGN is in a particular position. While it is responsible for the installation and the maintenance of the DORIS ground tracking stations, it also handles one of the two IDS data center delivering DORIS data and products and has been an IDS Analysis Center for years, providing all possible IDS products, in particular the latest ignwd08 time series in preparation of ITRF2008. This paper explains the different aspects of the IGN contribution to IDS from an historical point of view, presents current activities and scientific results and provides a perspective for future activities. Recent DORIS results show a 10 mm precision or better when more than four DORIS satellites are available. Comparisons between recent DORIS solutions (ign07d02 and ign09d02) and past ITRF realizations show that errors are shared between the DORIS and the ITRF realizations. Some problems related to DORIS data processing are also discussed and possible ways to solve them in the future are discussed. In particular, we can now reject the tropospheric origin of the problem detected in the Envisat data after the software upload of October 12, 2004. A few applications in geodesy (terrestrial reference frame, Earth’s polar motion) and geophysics are also discussed as a natural extension of these service-type activities.

A review of the GGP network and scientific challenges

The Global Geodynamics Project (GGP) started on July 1, 1997 and is now in its 11th year of operation. It has a relatively small number of stations (24), compared to seismic (GSN) or geodetic (GPS) networks, but it is the only database that is accumulating relative gravity measurements worldwide. As any scientific organization matures, there is a change in the culture of the project and the people involved. To remain viable, it is necessary not only to maintain the original goals, but also to incorporate new ideas and applications on the science involved. The main challenges within GGP are to ensure: (a) that the instruments are properly calibrated, (b) that data is being recorded with the highest accuracy, and with appropriate hydrological instrumentation, and (c) that the flow of data from all recording stations to the ICET database continues as agreed in within the GGP framework. These practical matters are the basis for providing high quality recordings that will extend the usefulness of the network into the future to meet new challenges in geosciences. Several new stations have been brought into operation in the past few years, but the data availability from some of these stations still leaves room for improvement. Nevertheless, the core group of stations established more than 10 years ago has been able to maintain the high standards of the original concept, and much research has been published using network data in areas as diverse as hydrology, polar motion, and Earth’s normal modes. GGP will also participate in some of the scientific tasks of the Global Geodetic Observing System program, at least initially by providing relative gravity measurements for collocation with other high precision geodetic measurements.

지구측지관측시스템(GGOS) 연구동향

현대 측지학은 전례 없는 정밀도로 지구의 물리 기하학적 형상과 운동, 특히 이의 변화를 감시하고 있다. 이는 우주시대의 지구관측을 위한 정확하고 통일된 기준좌표계를 제공할 뿐만 아니라, 학제적 연구를 통해 지구형상과 운동의 변화를 일으키는 원인을 규명함으로써 궁극적으로는 지구시스템의 정확한 이해에 기여하고 있다. 국제측지연맹에서는 다양한 측지학적 도구들을 통합하고자 지구측지관측시스템(GGOS)을 구축하고, 전지구관측시스템(GEOSS)의 주요 파트너로서 지구환경변화에 대응하려는 인류의 노력에 기여하고 있다. 여기서는 지구측지관측시스템을 중심으로 측지학이 전지구관측시스템 및 지구과학의 여러 영역과 어떤 관계를 가지는지를 살펴보고, 학제적 연구의 동기 부여를 촉진하고자 한다. The Modem Geodesy monitors physical and geometrical shape and motion of the Earth and, more importantly, its temporal variations with unprecedented precision. It provides accurate and stable reference frames for Earth observations in the Space era. Furthermore, with an aid of interdisciplinary approaches, it also traces the causes of the variations in shape and motion of the Earth and eventually contributes to a better understanding of the Earth system. The International Association of Geodesy (IAG) has established the Global Geodetic Observing System (GGOS) to integrate the multitude of geodetic tools and tried to contribute to the management of global environmental changes as a partner of the Global Earth Observation System of Systems (GEOSS). Here we introduce the contribution of geodesy to the various fields of Earth Science by focusing on GGOS and encourage interdisciplinary researches.

New Mathematical Model for Reference Point Determination of an Azimuth-Elevation Type Radio Telescope

The observations of geodetic very long baseline interferometry (VLBI) measurements refer to an invariant point of the radio telescope. This point is often called the reference point. For the interpretation of VLBI results it is necessary to know the position of this invariant point and its possible variation with time. Only the knowledge of the reference point makes it possible to combine VLBI results with results from other colocated techniques and in the framework of the global geodetic observing system (GGOS). Within the GGOS an automated monitoring of the reference points and therefore of the local-tie vectors at colocation stations is discussed earlier in other studies. In this paper a new mathematical model is described to allow such an automated monitoring of the reference point of an azimuth-elevation type radio telescope.

The International GNSS Service in a changing landscape of Global Navigation Satellite Systems

The International GNSS Service (IGS) is an international activity involving more than 200 participating organisations in over 80 countries with a track record of one and a half decades of successful operations. The IGS is a service of the International Association of Geodesy (IAG). It primarily supports scientific research based on highly precise and accurate Earth observations using the technologies of Global Navigation Satellite Systems (GNSS), primarily the US Global Positioning System (GPS). The mission of the IGS is “to provide the highest-quality GNSS data and products in support of the terrestrial reference frame, Earth rotation, Earth observation and research, positioning, navigation and timing and other applications that benefit society”. The IGS will continue to support the IAG’s initiative to coordinate cross-technique global geodesy for the next decade, via the development of the Global Geodetic Observing System (GGOS), which focuses on the needs of global geodesy at the mm-level. IGS activities are fundamental to scientific disciplines related to climate, weather, sea level change, and space weather. The IGS also supports many other applications, including precise navigation, machine automation, and surveying and mapping. This article discusses the IGS Strategic Plan and future directions of the globally-coordinated ~400 station IGS network, tracking data and information products, and outlines the scope of a few of its numerous working groups and pilot projects as the world anticipates a truly multi-system GNSS in the coming decade.

Realization of the Terrestrial Reference System by a reprocessed global GPS network

We present a GPS‐only reference frame named PDR05 (Potsdam‐Dresden‐Reprocessing TRF 2005) based on the results of a homogeneous reprocessing of a global GPS network over the time span 1994.0–2006.0. The reference frame is realized in the center of mass (CM) system. This implies that a self‐consistent model considering the reference frame and loading dynamics has been applied. The determined set of coordinates at epoch 2000.0 and their linear changes with time are evaluated in terms of self‐consistency and are compared to other realizations of the Terrestrial Reference System (TRS), such as ITRF2000, ITRF2005, and the GPS‐only realizations IGb00 and IGS05. We demonstrate that GPS is able to realize a TRS with the origin in the center of mass and a scale which are both on an equivalent level of self‐consistency, stability, and precision as other space techniques. The results show the high potential of homogeneously reprocessed GPS networks for future International Terrestrial Reference System (ITRS) realizations and for the realization of the Global Geodetic Observing System (GGOS). Hence, PDR05 represents a stable metrological basis for global geodynamic investigations.

Geodetic Observations Help Scientists Understand Geohazards and Mitigate Disasters: The GGOS Contribution to GEOSS and an Observing System for Geohazards and Disaster Prevention; Frascati, Italy, 5–6 October 2007

Modern space‐geodetic observations have revolutionized our understanding of geohazards, and these observations have a great potential for further scientific discovery and applications, including early warning.To discuss this, about 50 scientists from the geodetic and geohazards communities took part in a workshop organized by the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) as a contribution to the intergovernmental Group on Earth Observations (GEO). Highest‐level representatives of space agencies (European Space Agency, NASA, and Agenzia Spaziale Italiana) emphasized the fundamental importance of the geodetic reference frames for satellite missions and Earth observation. Although high, the present accuracy of these reference frames still is a key limitation in quantifying global change processes such as changes in ice sheets and sea level. Improvements of the reference frame are a pivotal step toward a better understanding of these processes and their impact on society. The representative of GEO identified GGOS as a core element in the Global Earth Observation System of Systems (GEOSS), which aims to integrate Earth observations in order to better serve users in a number of societal benefit areas, including disasters.

The International DORIS Service: genesis and early achievements

All space-geodetic techniques are now organized as separate services of the International Association of Geodesy (IAG), supporting the first pilot project “Global Geodetic Observing System (GGOS)”. The International DORIS (Determination d’Orbite et Radiopositionnement Integres par Satellite) Service (IDS) was created in mid-2003 to organize a DORIS contribution to this project and to foster a larger international cooperation on this topic. The goal of this paper is to summarize the key steps that were taken to create this structure and to present its current organization and recent results. At present, more than 50 groups from 35 different countries participate in the IDS at various levels, including 43 groups hosting DORIS stations in 32 countries all around the globe. Four Analysis Centres (ACs) provide results, such as estimates of weekly or monthly station coordinates, geocentre variations or Earth polar motion, that will soon be used to generate IDS-combined products for geodesy and geodynamics. As a first test, a preliminary combination was performed for all the 2004 data from these four ACs. Three of them show RMS of weighted station residuals with respect to this combination solution between 1 and 2 cm. The main topic under investigation is a discrepancy in the scale factor of the terrestrial reference frame (TRF) to map the individual solutions into the combination solution, which reaches 6 cm (multiplying the unit-less scale factor by the Earth radius to get convert scale to millimetre in vertical at the Earth’s surface). Finally, foreseen improvements of the DORIS technology are discussed as well as future improvements concerning the service organization itself and the accuracy and reliability of its scientific products.

Systematic errors in the Z-geocenter derived using satellite tracking data: a case study from SPOT-4 DORIS data in 1998

Within the scope of the Global Geodetic Observing System, Doppler Orbit Determination and Radiopositioning Integrated by Satellite – as a geodetic technique – can provide precise and continuous monitoring of the geocenter motion related to mass redistribution in the Earth, ocean and atmosphere system. We have reanalyzed 1998 DORIS/SPOT-4 (Satellite pour l’ Observation de la Terre) data that were previously generating inconsistent geocenter positions (−65 cm offset). We show here that this error is due to an incorrect phase center correction provided with the DORIS preprocessed data resulting from a +12 cm offset in the cross-track direction that has been confirmed since. We also conclude that a 1 mm error in the cross-track offset of non-yawing sun-synchronous SPOT satellites will generate a −6.5 mm error in the derived Z-geocenter. Other non-yawing satellites would also be affected by a similar effect whose amplitude could be easily estimated from the orbit inclination

The International GPS Service tracking network: Enabling diverse studies and projects through international cooperation

Abstract The International GPS Service (IGS), formulated beginning in 1989 and formalized in 1994, was founded on the collaborative operation of approximately 30 permanent GPS stations to benefit global geodynamics. The same cooperative principles, today applied to a network of over 300 stations, still serve to maximize global benefit without unnecessary duplication of investment in global infrastructure. The scope of applications of the dataset has grown to include atmospheric, oceanographic, subdaily, and low-earth orbiter activities through working groups and pilot projects fostered within the IGS in the now traditional IGS spirit of collaboration. These activities and the IGS infrastructure are viewed as critical elements to the Global Geodetic Observing System. This presentation will review the present nature of the IGS tracking network and its ability to support new applications.

The gravity field and GGOS

Abstract The gravity field of the earth is a natural element of the Global Geodetic Observing System (GGOS). Gravity field quantities are like spatial geodetic observations of potential very high accuracy, with measurements, currently at part-per-billion (ppb) accuracy, but gravity field quantities are also unique as they can be globally represented by harmonic functions (long-wavelength geopotential model primarily from satellite gravity field missions), or based on point sampling (airborne and in situ absolute and superconducting gravimetry). From a GGOS global perspective, one of the main challenges is to ensure the consistency of the global and regional geopotential and geoid models, and the temporal changes of the gravity field at large spatial scales. The International Gravity Field Service, an umbrella “level-2” IAG service (incorporating the International Gravity Bureau, International Geoid Service, International Center for Earth Tides, International Center for Global Earth models, and other future new services for, e.g., digital terrain models), would be a natural key element contributing to GGOS. Major parts of the work of the services would, however, remain complementary to the GGOS contributions, which focus on the long-wavelength components of the geopotential and its temporal variations, the consistent procedures for regional data processing in a unified vertical datum and Terrestrial Reference Frame, and the ensuring validations of long-wavelength gravity field data products.

Integrated Global Geodetic Observing System (IGGOS)—science rationale

Abstract The International Association of Geodesy has decided to establish an Integrated Global Geodetic Observing System (IGGOS). The objective of IGGOS is to integrate in a well-defined global terrestrial reference frame the three fundamental pillars of geodesy, which are the determination of all variations of surface geometry of our planet (land, ice and ocean surfaces), of the irregularities in Earth rotation sub-divided in changes of nutation, polar motion and spin rate, and of the spatial and temporal variations of gravity and of the geoid. This integration will have to be done with a relative precision of 1 part-per-billion and be maintained stable in space and time over decades. IGGOS will quantify on a global scale surface changes, mass anomalies, mass transport and mass exchange and exchange in angular momentum in system Earth. It will be a novel and unique contribution to Earth system and Global Change research. It is intended to make IGGOS part of the Integrated Global Observing Strategy (IGOS).

The IGGOS viewed from the Space Geodetic Services

Abstract The Space Geodetic (Technique) Services of the International Association of Geodesy, in particular the International VLBI Service for Geodesy and Astrometry (IVS), the International Laser Ranging Service (ILRS) and the International GPS Service (IGS) will make major contributions to the proposed Integrated Global Geodetic Observing System (IGGOS) through the provision of time series of parameters which describe the fundamental components of global reference frames, in particular the International Celestial Reference Frame (ICRF) and the International Terrestrial Reference Frame (ITRF), and parameters of the transformation between them (Earth Orientation Parameters (EOP's), nutation, origin and scale). The three techniques VLBI, SLR and GPS are represented by services, which in the last years have taken over the coordination of the supporting components including observation sites, data routing and management, and analysis facilities. Moreover, they have the responsibility for consolidated product delivery by timely provision of high quality time series, with temporal latency in some cases approaching near real time. The results of the different services reflect the distinct characteristics of the technique observables, which are partly similar and partly complementary. Due to the independence and different sensitivity of the techniques, systematic errors between the three techniques can be detected by appropriate combination of the results and to some extent at the level of the observables. Such a procedure will increase the reliability and the consistency of the products in satisfying the requirements of IGGOS. This paper reviews the different techniques with regard to their advantages and indicates sources of systematic errors. It will focus on requirements which have to be fulfilled for the combination of the service-dependent time series and might provide a basis for discussion on future plans in order to evolve the services themselves towards fulfilling the objectives of IGGOS.

The GGOS as the backbone for global observing and local monitoring: A user driven perspective

Abstract A key geodetic contribution to both the three Global Observing Systems and initiatives like the European Global Monitoring for Environment and Security is an accurate, long-term stable, and easily accessible reference frame as the backbone. Many emerging scientific as well as non-scientific high-accuracy applications require access to an unique, technique-independent reference frame decontaminated for short-term fluctuations due to global Earth system processes. Such a reference frame can only be maintained and made available through an observing system such as the Global Geodetic Observing System (GGOS), which is currently implemented and expected to provide sufficient information on changes in the Earth figure, its rotation and its gravity field. Based on a number of examples from monitoring of infrastructure, point positioning, maintenance of national references frames to global changes studies, likely future accuracy requirements for a global terrestrial reference frame are set up as function of time scales. Expected accuracy requirements for a large range of high-accuracy applications are less than 5 mm for diurnal and sub-diurnal time scales, 2–3 mm on monthly to seasonal time scales, better than 1 mm/year on decadal to 50 years time scales. Based on these requirements, specifications for a geodetic observing system meeting the accuracy requirements can be derived.