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2003-08-14

Section III - Determination of the Gravity Field

posted by
Sz. Rózsa
President Michael G. Sideris (Canada)
Secretary Gerd Boedecker (Germany)
Comm XIII Martin Vermeer (Finland)


Terms of Reference

Section III, determination of the gravity field, is engaged in the determination and modelling of the earths gravity field variations. Knowledge of the gravity field variations is of prime importance for geodesy, geophysics and navigation, and efficient and accurate modelling of such variations is a key geodetic research activity. Within the modelling especially the precise determination of the geoid is of great contemporary significance due to requirements from satellite geodesy and oceanography.

The gravity field may be determined by a multitude of measurements: satellite tracking, absolute and relative gravimetry, gravity gradiometry, GPS-levelling, satellite altimetry, astronomical deflections of the vertical, etc. Currently the global determination of the gravity field is significantly improved due to advances in satellite techniques, the release of terrestrial gravity data from formerly classified sources, and the development of efficient airborne gravity survey techniques. Compared to just a few years ago, the global data coverage is improved to such a degree that the only major regions in the world void of data now are some tropical jungle regions and Antarctica.

The development of new global reference models, incorporating the advance in terrestrial data coverage and new satellite data, will be a major benefit to all geodesists in utilization of all these global gravity field data.

In the field of gravimetry, building up national and international networks of absolute and relative gravimetry continues to be a key element within the framework of Section III. With the increasing accuracy of these nets, now approcaching 10-9 g, the study of non-tidal gravity changes becomes increasingly important for geodynamic studies.

The developments in the the gravity field determination expressed in the formal IAG By-Laws, in which Section III is responsible for:
  • absolute and relative terrestrial gravity measurements,

  • gravity networks and control stations,

  • non-tidal gravity variations,

  • determination of the external gravity field and geoid the different gravity field data types, and

  • reduction and estimation of gravity field quantities.



Structure


Commissions:
  • Comm XIII: International Gravity and Geoid Commission
      President: Martin Vermeer (Finland)


Special Study Groups:
  • SSG 3.167: Regional Land and Marine Geoid Modelling
      Chair: I.N. Tziavos (Greece)

  • SSG 3.177: Synthetic Modelling of the Earth’s Gravity Field
      Chair: Will Featherstone (Australia)

  • SSG 3.184: Use of Remote Sensing Techniques for Validating Heights and Depths
      Chair: Philippa Berry (UK)

  • SSG 3.185: Merging data from dedicated satellite missions with other gravimetric data
      Chair: Nico Sneeuw (Germany)

  • SSG 3.186: Altimetry data processing for gravity, geoid and sea surface topography determination
      Chair: C. Hwang (Taiwan)


International Services:
  • BGI International Gravimetric Bureau
      Director: J-P. Barriot (France)

  • IGeS International Geoid Service
      Director: F. Sansò (Italy)



Commission XIII - International Gravity and Geoid Commission

    President: Martin Vermeer (Finland)

Program of Activities

The International Gravity and Geoid Commission (IGGC) promotes scientific investigations of the Earth's gravity field, its relationship with the form and processes of the Earth's interior and exterior, and its variations with time. It does so through the concerted actions of its members to acquire homogeneous, geolocated gravity value coverage of the Earth's surface and to derive useful representations of the gravity field. Hence it constitutes the primary activity centre of the IAG for information related to the gravity field not only for geodesists, but for all geophysicists.
For this purpose, the IGGC regulates the operations of two services.

The Bureau Gravimétrique International (BGI) maintained by France at Toulouse, was established for the acquisition, processing, and distribution of gravity data in order to promote the free exchange of gravity data among the member countries.

The International Geoid Service (IGeS) at the Politecnico di Milano, was established and is further to be developed as a focal point and service centre for representation of the gravity potential such as the geoid, through data collection and dissemination and research and education in the field of geopotential determination and exploitation for geodetic, geophysical and oceanographic purposes.

The Commission shall seek to collaborate with all international and national organizations concerned with the work of the IGGC, particularly for the benefit of the developing countries.

The Commission will encourage and promote special campaigns to develop and compare instruments, techniques and procedures for the adjustment of the results of gravity surveys and for the precise determination of the geoid.

The Commission will review its functions from time to time to ensure its activities are commensurate with the needs of the geodetic community.

The Commission will organize at least one meeting of a global nature in the four-year period between the IUGG/IAG Assemblies. In addition, it will encourage regional meetings or workshops dedicated to specific problems, where appropriate.

The Commission will issue a compilation of national reports of its member countries on gravity and geoid related activities during the past four-year period, to be submitted at the IUGG/IAG Assemblies.

Functions

The functions of the IGGC shall be:
  • To find solutions to problems requiring international cooperation in gravity and geoid research and review the results of such investigations;

  • To recommend and coordinate international programmes for scientific investigation in gravity, geoid and related matters which call for concerted action by its members and interested organizations;

  • To make recommendations to international organizations in matters which relate to the Commission's programme;

  • To promote and make recommendations for the exchange of gravity data and the publication of results of scientific investigations;

  • To make recommendations to strengthen education and training programmes in gravity, its technology, and in precise geoid determination;

  • To assist developing countries in geoid- and gravity-related research and technology development;

  • To promote scientific investigation of gravity and geoid on behalf of the international community, taking into account special interests and rights of countries concerning scientific research in the zones under their jurisdiction;

  • To assist in operational aspects, such as: data collection, geoid determinations through computer centres financed nationally or by international organizations, execution of computational tasks of regional character, organization of vertical positioning campaigns for geoid control, and organization of contacts to oceanographers in need of gravity and geoid data;

  • To evaluate, exchange and make available tested software and data sets; and

  • To organize expert meetings and international Schools.


In carrying out all its functions, the IGGC shall bear in mind the special needs and interests of developing countries.

Cooperation

The IGGC shall give due attention to supporting the objectives of the international organizations with which it collaborates and which may request IGGC to act, as appropriate, as the medium for discharging certain of their responsibilities in matters relating to gravity and geoid investigations. The IGGC may also request these organizations to take its requirements into account in planning and executing their own programmes.

Organization

The Assembly of the IGGC shall be its principal organ and, without prejudice to the provisions of paragraph of this Article, shall make all decisions necessary to fulfill the functions of the IGGC.

The Assembly consists of national representatives for countries that are members of IAG. Each country will have one vote, but may accompany its representative to sessions of the Assembly with alternates and advisers as deemed necessary.

In accordance with the By-Laws of the IAG, the president of the Commission is appointed by the Council of the IAG. Every four years, the Assembly shall elect two Vice-Presidents and a Secretary at its ordinary session. They, along with the President, shall constitute the Executive Board. The President may appoint a second Secretary to assist with the operation of the Commission.

The Executive Board shall exercise the responsibilities delegated to it by the Assembly and act on its behalf in the implementation of decisions of the Assembly.

The Executive Board shall review the effectiveness of the operating structure of the Commission and make recommendations to the Assembly on structural or other necessary changes.

Services

IGGC supports the operation of and liaison with the Services:
  • [BGI]- Bureau Gravimétrique International: The Commission supports the continuing operation of the BGI, with emphasis on expanding its gravity data base to include areas for which no gravity data have been released, and to improve coverage of gravity on land and on the oceans.

  • [IGeS]- International Geoid Service: IGeS's mandate is to collect and compute geoids of the world, to organize of geoid schools, to distribute geoid software and to perform studies on the use of satellite altimetry and dedicated satellite missions for geoid research.


Working Groups, Subcommissions

The Commission can establish topical or regional working groups (the latter called subcommissions) to solve specific scientific problems or regionally coordinate activities. The Working Groups shall communicate to its members mainly by circular letters, with copies to the President of the IGGC and the President of Section III, and other relevant interested parties.

Working groups proposed for 1999-2003 include:
  • WG on Intercomparison of Absolute Gravimeters (Leonid Vitushkin)

  • WG on the Arctic Gravity Project (René Forsberg)

  • WG on Antarctica (Alessandro Capra)

  • SC for Europe* (Ambrus Kenyeres)

  • SC for South-America* (Denizar Blitzkow)

  • SC for South-East Asia* (Bill Kearsley)

  • SC for North America (Marc Veronneau)


The subcommissions marked with a * can be seen as continuations of already existing regional subcommissions for geoid studies.

National Representatives.
    Spain Miguel J. Sevilla
    China Jiancheng Li
    Sweden Lars Sjöberg
    New Zealand Merrin Pearse,
    Czech Republic Petr Holota
    Luxembourg Bernard Reisch
    Denmark Rene Forsberg
    Egypt S. Riad
    Germany T. Gruber
    Poland Marcin Barlik
    Australia Bill Kearsley
    Japan Shuhei Okubo,
    Hungary Geza Csapo
    Portugal João Catalão
    Norway Dag Solheim
    South Africa Prof CL Merry
    Canada Anthony Lambert
    Croatia Kresimir Colic
    Belgium B.Ducarme
    Estonia Harli Juergenson
    Russia P.Medvedev
    Israel Dan Sharni
    Italy Riccardo Barzaghi
    USA Steve Kenyon
    Greece Dimitris Arabelos
    UK Roger Hipkin
    Austria N. Kühtreiber



Special Study Group 3.167 - Regional Land and Marine Geoid Modelling

    Chair: I.N. Tziavos (Greece)


Objectives and Tasks

The objectives of SSG 3.167 reflect the research work already done and the results already achieved and focus on open problems and new questions related to geoid modelling. In order to consolidate the current state of knowledge in regional geoid modelling and seek open issues, the following objectives and tasks present themselves:
  • Theoretical work on the modelling of non-linear effects, terrain effects, etc., in the frame of studying the Boundary Value Problems (BVPs) is of main importance. The study of different models and numerical techniques is necessary.

  • Study of the differences between the modelling procedures for land and marine geoids with special emphasis on the peculiarities in working across the land/sea boundary. Numerical tests should be carried out in different test areas in order to draw conclusions on the optimal combination of terrestrial, marine, airborne and satellite data.

  • Comparative study between methods in order to identify the key issues that remain to be tackled in the optimal way of working with heterogeneous data. Investigation on the importance of satellite altimetry in combination solution in coastal areas. Optimization of regional scale geoid solutions.

  • Development of new approximation and numerical techniques that hold the promise of a closer representation and/or more effcient regional geoid computation. Special work is recommended in input/output system theory algorithms, improvement of error propagation methodologies and kernel modification techniques in the space and frequency domain.

  • The contribution of GPS to the validation geoid solutions. Improvement of existing and development of new techniques for a common adjustment of GPS and geoid heights. The impact of GPS in studying the compatibility of neighbouring datums through geoid determination. Numerical tests between different datums of neigbouring countries.

  • The importance of geophysical data in regional-scale geoid studies. Modelling of the upper crust density in terrain effect computations. Correlation studies between geophysical and geodynamic parameters and geoid heights.

  • The impact of oceanographic information for an accurate regional-scale marine geoid solution. Sea surface topography studies using oceanographic and geodetic methodologies. Numerical tests and comparisons. The sea surface topography as a product of combination solutions.

  • Availability of regional geoids. Different forms of publishing regional geoids (maps, gridded heights, function coefficients, data compression techniques).

  • The impact of new dedicated satellite missions and airborne gravimetry on precise regional geoid determination.


Proposed program of activities

The first year, it is recommended to the members to work with real or simulation data, in sub-groups or individually, on the topics mentioned before. The first results from all these activities will be discussed, in the frame of the IAG International Symposium on Gravity, Geoid and Geodynamics 2000, which will be held in Banff, Canada, July 31 - Aug. 4, 2000, and where we will have the first official meeting of our SSG.

For the year 2002 (one year before the XXIII IUGG/IAG General Assembly in Saporo) I plan to organize a Symposium in collaboration with chairmen of several or all other study groups of IAG Section III (Determination of the gravity field). This Symposium will be held in Thessaloniki or in the Halkidiki penninsula, a picturesque touristic place close to Thessaloniki. In this Symposium papers will be presented from the members of our study group and other SSGs related to their scientific activities and the results obtained, and the new scientific directions for further investigation will be proposed.

More information about the meetings of the study group, the joint Symposium, and any other activity will be announced in the web home page of the SSG: http://olimpia.topo.auth.gr/ssg3167

Members
    Tziavos, I.N. (Greece)
    Abd-Elmotaal, H. (Egypt)
    Barzaghi, R. (Italy)
    Catalao, J. (Portugal)
    Chen, J.Y. (China)
    Dahl, O.C. (Norway)
    Denker, H. (Germany)
    Featherstone, W. (Australia)
    Haagmans, R. (Netherlands)
    Hipkin, R. (United Kingdom)
    Jiang, Z. (France)
    Kaminskis, J. (Latvia)
    Kuehtreiber, N. (Austria)
    Kuroishi, Y. (Japan)
    Marti, U. (Switzerland)
    Merry, C. (S. Africa)
    Smith, D. (USA)
    Toth, Gy. (Hungary)
    Tsuei, G. (Taiwan)
    Veronneau, M. (Canada)

Associate members
    Duquenne, H. (France)
    Fernandes, J. (Portugal)
    Fukuda, Y. (Japan)
    Hwang, C. (Taiwan)
    Jekeli, Chr. (USA)
    Kearsley, A.H.W. (Australia)
    Knudsen, P. (Denmark)
    Krynski, J. (Poland)
    Kuhn, M. (Germany)
    Milbert, D. (USA)
    Papp, G. (Hungary)
    Prijatna,K. (Indonesia)
    Sanchez, L. (Colombia)
    Sevilla, M. (Spain)
    Zhang, K. (Australia)



Special Study Group 3.177 - Synthetic Modelling of the Earth’s Gravity Field

    Chair: Will Featherstone (Australia)

Aims and objectives

The aims and objectives of the SSG include, but are not limited to:
  • address the theoretical and practical basis of producing a synthetic model of the Earth's gravity field;

  • study the effects of the various approximations currently made in physical geodesy;

  • reach a consensus on the correct methods to test or compare the theories, approximations, approaches and

  • software being developed, or in use, for physical geodesy;

  • produce a gravity field model with realistic noise spectra from various sources (both correlated and un-correlated)

  • to allow the study of the propagation of errors in current gravity field modelling methods;

  • produce a model that generates data in various formats, so as to ensure its wide-spread application;

  • supply the results (papers and software) to researchers in physical geodesy and geophysics world-wide



Suggested Methodology

One suggested approach to constructing the SEGM is to model the broad/deep structure of the Earth's gravity field using spherical harmonics, point mass or digital density models (eg. REM). The gravitational effects of the Earth's crust and topography can be modelled using recent global digital density and terrain models and superimposed on the broad deep structure component of the SEGM.

The detailed structure of the SEGM in specific areas can be refined using regional datasets, which avoids problems of data confidentiality and large computation time. These can be superimposed on the global field. In areas where detailed local data are not available, it will be necessary to simulate the gravity field completely artificially (eg. by recursing the global data using fractal techniques).

Members
    Will Featherstone (Australia)
    Yoichi Fukuda (Japan)
    Erik Grafarend (Germany)
    Roger Haagmans (Netherlands) Roger Hipkin (UK)
    Simon Holmes (Australia)
    Chris Jekeli (USA)
    Ruediger Lehman (Germany)
    Zdenek Martinec (Czech Republic)
    Yuri Neyman (Russia)
    Roland Pail (Austria)
    Gabor Papp (Hungary)
    Doug Robertson (USA)
    Walter Schuh (Austria)
    Gabriel Strykowski (Denmark)
    Gyula Toth (Hungary)
    Ilias Tziavos (Greece)
    Petr Vanicek (Canada)
    Peter Vajda (Slovak Republic)
    Martin Vermeer (IGGS, Finland)


Corresponding Members
    Fabio Boschetti (Australia)
    Hans Engels (Germany)
    Jonathan Evans (UK)
    Rene Forsberg (Denmark)
    Jonathan Kirby (Australia)
    Michael Kuhn (Germany)
    Gabi Laske (USA)
    Adam Dziewonski (USA)
    Marcel Mojzes (Slovak Republic)
    Edward Osada (Poland)
    Spiros Pagiatakis (Canada)
    Michael Sideris (Canada)
    Dru Smith (USA)
    Hans Suenkel (Austria)
    Christian Tscherning (Denmark)
    Jack Weightman (UK)



Special Study Group 3.184 - Use of Remote Sensing Techniques for Validating Heights and Depths

    Chair: Philippa Berry (UK)


Terms of Reference

Global and regional scale Digital Terrain Models (DTMs) have a wide range of applications within geodesy. Crucially, topographic and bathymetric data form a key part of the processing and interpretation of gravity data at all spatial scales.

The work done in the frame of SSG3.163 showed that very significant deficiencies exist in current DTMs, and many problems are still open for investigation after four years of work of this SSG. However, the experience gained within SSG3.163 showed that remote sensing techniques are promising tools for the evaluation of heights and depths on regional and global scales, with major contributions expected from current and planned space missions such as the Shuttle Topographic Mission, the ICESAT mission and ENVISAT. Together with the rapidly increasing availability of stereo SAR and interferometric SAR DEM data for topographic work, this means that the next four years will encompass a period where many new datasets are being created.

Objectives

With the growing need for precise elevations and depths on both regional and global scales, it is an appropriate task for geodesists to provide high quality models to the growing market of users. A key objective of this study group is therefore the production of improved topographic and bathymetric data for geodetic requirements. The definition of a strategy for inclusion of new data from current and forthcoming space missions for the requirements of geodesy will also form a key part of the work of this study group.

Activities

Comparisons of actual (measured) depths and predicted depths; bathymetric prediction methods including inversion of gravity data and use of satellite altimetry; also including comparison of bathymetry models with oceanographic model predictions and observations.

Use of space and airborne radar and laser altimetry for the evaluation of errors in regional and global scale DTMs; identification of regions of poor topographic representation and incorporation of improved heights where possible.

Exploitation of new techniques (SAR, InSAR etc) for improvement of regional DTMs, and evaluation of effect of errors in regional DTMs on derived gravity models.

Investigation of new mission datasets as these become available; assessment of the impact of these data on existing topographic and bathymetric models. Definition of a strategy for inclusion of new data, primarily from space based techniques, to enhance model representation in priority areas.

Estimation of ice sheet thickness using radar and optical space mission datasets, together with in-situ data.

Initial inputs to this work are anticipated from the directly related session at EGS (April 2000). In ‘Quantifying the earth system: geodesy’s fundamental contribution.’ Session: ‘Topography and Surface Changes’. Convenors: S. Ekholm & P.A.M. Berry. Meetings will also be held during the following conferences on an ad-hoc basis:
  • July 2000: Int. Symposium on Gravity, Geoid & Geodynamics

  • 2001: IAG General Assembly

  • 2002: EGS meeting

  • 2002; AGU fall meeting

  • 2003: EGS meeting

  • 2003: IUGG 23 General Assembly


Members
    Philippa Berry (UK, chair)
    Jerome Benveniste (Italy)
    Denizar Blitzkow (Brazil)
    Anita Brenner (USA)
    Peter Challenor (UK)
    Simon Ekholm (Denmark)
    Joanna Fernandes (Portugal)
    Yoichi Fukuda (Japan)
    Alan Hittelman (USA, NGDC)
    Bill Kearsley (Australia)
    Steve Kenyon (USA, NIMA)
    Roland Klees (Netherlands)
    Nikos Pavlis (USA)
    Dave Sandwell (USA)
    Dru Smith (USA)
    Gabriel Strykowski (Denmark)
    G.-C. Tsuei (China)
    Hilary Wilson (Germany)


Corresponding members
    W. Featherstone (Australia)
    R. Hipkin (UK)
    M. Sideris (Canada)
    C.C. Tscherning (Denmark)



Special Study Group 3.185 - Merging data from dedicated satellite missions with other gravimetric data

    Chair: Nico Sneeuw (Germany)

Terms of Reference

The next years will be exciting times for global gravity field modelling. Starting early 2000, a series of dedicated gravity field missions will fly, that will map the gravity field of the earth with unprecedented accuracy and resolution. These are the hi-lo SST mission CHAMP (spaceborne GPS + accelerometry), the lo-lo SST mission GRACE (range and range-rate + spaceborne GPS + accelerometry) and the SGG mission GOCE (gradiometry + spaceborne GPS).

The data distribution from these missions will be spatially homogeneous, except for polar gaps in case of non-polar orbits. Also spectrally, i.e. in terms of spherical harmonic error coefficients, the mission results are expected to be homogeneous. However, each mission has its own spectral window, that only partially overlaps with others.

At the same time, improvements in terrestrial gravity field information have continued and still continue. The amount of surface gravimetry data keeps increasing. Also developments in airborne gravimetry, regarding data distribution (arctic, marine,...) and quality, greatly enhance the terrestrial gravity database.

Objectives

The analysis of data from these dedicated gravity field missions and the possible combination with existing gravity field information pose interesting and challenging questions, both from theoretical and numerical perspectives. Data from these missions have to be merged among themselves and with the terrestrial gravity information. This merging process will be the central research issue of this Special Study Group. In particular the following objectives are defined.
  • To investigate issues related to merging, where merging is to be understood in the spatial domain, in the spectral domain and at the level of combining normal matrices.

  • To come up with results and proposals that may support CHAMP/GRACE/GOCE communities in their data processing and merging strategies.

These objectives are to be realized through research into the following fields:
  • Satellite-only solutions. Merging data from dedicated gravity field missions into satellite-only models.

  • Combination solutions. Merging data from dedicated gravity field missions with existing gravity field information.

  • Orbital effects. Each mission has its own spatial characteristic, in terms of ground-track pattern, inclination, and height profile. Research items are: polar gaps, regularization and downward continuation.

  • Terrestrial data distribution. The irregular data distribution of terrestrial gravity field information and of altimetry derived gravity anomalies (land-sea problem) requires research into gridding strategy and into alternative representations.

  • Weighting schemes for merging normal matrices. One of the lessons from the EGM96 development is, that weighting of data-subsets and calibration of normal matrices is a topic for extensive further research. This will be a central research issue.

  • (Block-)diagonal approach. In view of the high degree spherical harmonic development, the trade-off between mathematical rigour and computational viability of gravity field solutions is an important research item.

Although CHAMP and GRACE are, due to their long mission life-time, destined to measure time-variations of the Earth's gravity field as well, this

Special Study Group will focus on the stationary geopotential only.

Activities

  • Evaluate existing methods and approaches to "merging" (spatial, spectral and at normal matrix level).

  • Develop strategy and theory for optimal weighting of data-subsets and calibration of normal matrices.

  • Research into necessity of terrestrial data at the spectral domains, covered by the dedicated satellite missions.

  • Address the question to what level block-diagonal approaches or even diagonal ones (quadrature) might replace full normal matrix inversion in certain spherical harmonic ranges. Or is full-matrix inversion the way to go?


The CHAMP testbed: CHAMP will be launched during the first half of 2000. Data will follow later that year. These data will be analyzed and employed in the framework of this SSG.

Provide the CHAMP/GRACE/GOCE communities with proposals, schemes and advice as to their merging strategies.

Members
    N. Sneeuw (Germany)
    A. Albertella (Italy)
    S.V. Bettadpur (USA)
    J. Bouman (Netherlands)
    A. Bruton (Canada)
    T. Gruber (Germany)
    S. Kenyon (USA)
    J. Kusche (Germany)
    J.M. Lemoine (France)
    N.K. Pavlis (USA)
    W.D. Schuh (Austria)



Special Study Group 3.186 - Altimetry data processing for gravity, geoid and sea surface topography determination

    Chair: C. Hwang (Taiwan)


Terms Of Reference

Since the Seasat mission of 1978, satellite altimetry has found its wide applications in geodesy, geophysics and oceanography. As new satellite missions such as GFO-1, ENVISAT and JASON-1 will contribute more to the existing data sets of Seasat, Geosat, ERS-1/2, and TOPEX/POSEDION, these applications will continue to grow. But there are still many applications to be explored, many problems to be solved, and many data processing techniques to be improved. For example, coastal geoids, gravity anomalies, tide-models and bathymetry models derived from satellite altimetry have important engineering applications, which did not receive much attention in the past. But exploiting satellite altimetry in coastal areas requires much more sophisticated correction models and data processing techniques than in the open oceans. The data and coordinate systems of different satellite missions should be properly weighted/corrected and unified in order to obtain an optimal multi-satellite data set for subsequent analyses. Shipborne gravity data are abundant in many areas of the oceans, and have high quality and good spatial resolution. They should be combined with altimetry data for global gravity and geoid computation and estimation of high-degree geopotential model. Bathymetry model is an important element in, e.g., the general circulation model of the world oceans and the hydrodynamic tide model, and should be optimally derived with altimetry and other data. Eddies in coastal areas are associated with coastal upwellings, which are extremely important for marine production. Can altimetry be used to identify coastal eddies? How accurate should the altimeter measurement and the tide and geoid models be in order to do this?

This SSG point outs selected problems of satellite altimetry as listed below to challenge all altimetric scientists.

Activities

This SSG encourages members to tackle the following problems:
  • improving the quality of coastal altimeter data by improving geophysical corrections, retracking waveforms and "tuning" altimeter measurements.

  • promoting engineering applications of coastal altimetry with high quality coastal geoid, gravity anomaly, bathymetry, ocean tide and sea surface topography models from altimetry.

  • investigating the best method and the best altimeter data type for computing gravity anomalies, mean sea surface heights from multi-satellite altimeter data; a team will be formed to assess the accuracies of various global mssh and gravity anomaly models.

  • developing a best technique to compute bathymetry from altimeter-derived geoids or gravity anomalies, with emphasis on the downward continuation and filtering problems.

  • finding a best strategy and data sources to combine shipborne gravity/airborne gravity and altimeter data for generating global and regional gravity anomalies and geoids.

  • Improving orbit accuracies of altimetric satellites and accuracies of the long wavelength gravity field by crossover and other methods.

  • unifying the coordinate systems between two or more satellite missions for determining long-term time series of oceanographic parameters.

Members
    V. D. Andritsanos (Greece)
    O. Andersen (Denmark)
    D. Chao (China)
    S. A. Chen (Taiwan)
    Y. Fukuda (Japan)
    J. W. Kim (Korea)
    J. Klokocnik (Czech Republic)
    P. Knudsen (Denmark)
    J. Li (China)
    P. Medevedev (Russia)
    P. Moore (UK)
    M. Rentsch (Germany)
    T. Schöne (Germany)
    C. K. Shum (USA)
    G. Venuti (Italy)
    Y. Wang (USA)
    Y. Yi (USA)



International Services

International Gravimetric Bureau (BGI) - http://bgi.cnes.fr:8110
    Director: J-P. Barriot (France)

Objectives and Terms of Reference

The main task of BGI is to collect, on a world-wide basis, all gravity measurements and pertinent information about the gravity field of the Earth, to compile them and store them in a computerized data base in order to redistribute them on request to a large variety of users for scientific purposes. The data consists of: gravimeter observations (mainly location – three co-ordinates, gravity value, corrections, anomalies...), mean free air gravity values, gravity maps, reference station descriptions, publications dealing with the Earth's gravity field. Other data types are sometimes used for data validation and geophysical analysis, such as satellite altimetry derived geoid height and gravity anomalies, digital terrain models, spherical harmonic coefficients of current global geopotential models.

BGI has been developing various algorithms and software for data validation and analysis, as well as its own data management system. A large number of services are offered to the users (see below).

All kinds of gravity data can be sent to BGI, with or without restrictions of redistribution to be specified by the contributors, sometimes in the form of a protocol of usage.

Structure and membership

BGI is one of the offices of the Federation of Astronomical and Geophysical Data Analysis Services (FAGS). It may also be considered as an executive office of the International Gravity and Geoid Commission (IGGC).

It has a Directing Board composed of the following members:
    Voting members:
    • M. Vermeer (Finland) IGGC President

    • R. Forsberg (Denmark) IGGC Vice-President

    • M. Sideris (Canada) IGGC Vice-President

    • J.P. Barriot (France) BGI Director

    • G. Boedecker (Canada) Section III Secretary

    • J.E. Faller (USA) to be elected

    • E. Groten (Germany) to be elected

    • P.P. Medvedev (Russia) to be elected

    • S. Takemoto (Japan) to be elected

    Non voting members:
    • L. Robertsson (France) Chair of WG6

    • B. Richter (Germany) Chair of WG7

    • M. Becker (Germany) Chair of WG8

    • J. Liard (Canada) Secretary

    • E. Klingele (Switzerland) Secretary

    Ex-officio:
    • F. Sanso (Italy) IGeS Director

    • P. Paquet (Belgium) (FAGS repr)


The central office is located in Toulouse, France, in the premises of the Observatoire Midi-Pyrénées, of which it is one of the services. The other supporting organizations are: the Centre National d'Etudes Spatiales, the Bureau de Recherches Géologiques et Minières, the Institut Géographique National, the Centre National de la Recherche Scientifique (via the Institut National des Sciences de l'Univers), the Ecole Supérieure des Géomètres et Topographes, the Institut de Recherche pour le Développement, the Service Hydrographique et Océanographique de la Marine. There exists a covenant between these agencies to guarantee their support to the BGI.

Address

Bureau Gravimétrique International
18, Avenue Edouard Belin
31401 Toulouse Cedex 4, France
Phone: 33-5 61 33 29 80
Email: Jean-Pierre.Barriot@cnes.fr

BGI Bulletin d'Information

The office issues a Bulletin d'Information twice a year (generally in June and December).

It contains:
  • general information in the field of the Bureau itself, about new available data sets,

  • communications at meetings dealing with gravimetry (e.g. IGGC meeting).


Every four years, an issue (which may be an additional one) contains the National Reports of Activities in Gravimetry.

The full catalogue of the holdings is issued every two years. The Bulletin is sent free of charge to individuals and institutions which currently provide information and/or data to the Bureau. In other cases, information and subscription prices can be obtained on request. There exist 85 issues and about 360 subscribers as of December 1999.

Providing data to BGI

Essential quantities and information for gravity data submission are:

Position of the site:
    latitude, longitude (to the best possible accuracy),
    elevation or depth:
    for land data: elevation of the site (on the physical surface of the Earth)
    for water stations: water depth.

Measured (observed) gravity, corrected to eliminate the periodic gravitational effects of the Sun and Moon, and the instrument drift.

Reference (base) station (s) used. For each reference station (a site occupied in the survey where a previously determined gravity value is available and used to help establish datum and scale for the survey), give name, reference station number (if known), brief description of location of site, and the reference gravity value used for that station. Give the datum of the reference value; example: IGSN 71.

Give supplementary elevation data for measurements made on towers, on upper floor of buildings, inside of mines or tunnels, atop glacial ice. When applicable, specify whether gravity value applied to actual measurement site or it has been reduced to the Earth's physical surface (surface topography or water surface). Also give depth of actual measurement site below the water surface for underwater measurements.

For marine gravity stations, gravity value should be corrected to eliminate effects of ship motion, or this effect should be provided and clearly explained.

Additional information are optional, but welcome.

Services

The most frequent service BGI can provide is data retrieval over a limited area. Data are sent on diskettes or printouts or transferred electronically. Data coverage plots may also be provided, usually over 20° * 20° areas. Cases of massive data retrieval requests may be considered; they are studied and may be processed in a specific way. The simplest way for users is to acquire the open files of the BGI data base which are on two CDs.

Other services include:
  • data screening,

  • provision of gravity base station information,

  • data evaluation and gridding,

  • computation of mean values,

  • contouring,

  • supply of, or information on existing maps


The costs of the services have been established in view of the categories of users-mostly contributors of measurements and scientists, and also considering the large amount of our host organizations. The charging policy is explained in detail in the Bulletin d'Information.

Some of the services may be provided free of charge upon request, to data contributors, individuals working in universities, such as students, and generally to any person who can contribute to the BGI activities on a data or documentation exchange basis.

Activities.

  • continue publication of the Bulletin d'Information,

  • continue data collection, archiving and distribution: emphasis will be on those countries which have not, or seldom, contributed to the BGI data bank. First priority is then given to careful data evaluation; Land data and marine data are validated using different software. Satellite altimetry derived free-air anomalies are to be more and more frequently used to validate sea measurements.

  • assist IGGC in setting up the International Absolute Gravity Data Base Station (IAGBN), and assist in the intercomparisons of instrument.

  • establish simple procedures for the collection and archiving of absolute measurements.

  • link with the commission for the Geoid in data preparation in view of geoid computations and evaluations to be performed by the International Service for the Geoid.

  • assist in promoting satellites techniques to improve our global knowledge of the Earth's gravity field: satellite-to-satellite tracking, satellite gradiometry, etc...



International Geoid Service (IGeS) -http://ipmtf14.topo.polimi.it/~iges/
    Director: F. Sansò (Italy)

Objectives

The main tasks of IGeS are:
  • to collect data referring to the geoid on a worldwide scale,when possible to validate them and to disseminate them upon request among the scientific community; other auxiliary data can also be collected by IGeS, when useful for the geoid determination, and might be made available with the sharp exclusion of gravity anomalies data,

  • to collect, test and, when allowed, to distribute software for the geoid determination,

  • to conduct researches on the best procedures for the geoid determination, possibly from different sources conveniently combined,

  • to provide the international community with technical schools where consolidated techniques of geoid determination, be demonstrated and students trained in the use of the relevant software,

  • to produce, at least once per year, an IGeS Bulletin on geoid related matters.


Data and software given to IGeS remain property of the source, which can dictate the conditions of use and restrict their distribution. IGeS itself can indeed perform geoid computations within different projects, but not in economic competition with Firms or Public Organizations institutionally devoted to that.

Structure

The Service is for the moment provided by a Main Centre, at the Politecnico of Milano, and by individual scientists, called advisors, though in future more Centres could join the organization. IGeS is related to IAG, being the operative arm of the International Commission for the Geoid, operating within IAG - Section III. As such it has a Directing Board which receives a report and defines the long-term program of the Service.

The Directing Board is composed by:
  • President of Section III

  • Secretaries of Section III

  • Director of BGI

  • Director of IGeS;

in this way a strong link is created between the two services of Section III, namely IGeS and BGI.

The Director of IGeS is nominated by the President of the International Geoid Commission, upon recommendation of the past Directing Board.

The IGeS-Main Centre is supported by Italian authorities, which nominate its Director, upon recommendation of the International Geoid Commission. Its structure, tools and activities are illustrated in the IGeS reports to the International Geoid Commission. In the present period Director of IGeS as well as of its main centre is Fernando Sansï (Italy). The IGeS advisors are individual members of IGeS, which have had an outstanding activity in the field of geoid determination and also can represent IGeS in both research and teaching activities.

At present, beyond the members of the Directing Board, the following distinguished scientists are IGeS advisors:
  • C.C. Tscherning (Denmark)

  • M. Sideris (Canada)

  • W. Kearsley (Australia)

  • D. Milbert (USA)

  • H. Denker (Germany)

  • M. Vermeer (Finland)

  • D. Arabelos (Greece)

  • M. Sevilla (Spain)

  • B. Benciolini (Italy)

  • R. Barzaghi (Italy)

The list is open and nominations are welcome by IGeS Director.

Finally within the structure of IGeS, Working Groups can be established for specific purposes, limited in time. At present one W.G. (Global Gravity Field validation) has been set up with the chair of T. Gruber (thomas.gruber@dlr.de). The purpose of the W.G. is to standardise the procedures of validation and combination of global models using data from the forthcoming gravity field spatial missions and terrestrial measurements.

Future Programme

Beyond usual activities of IGeS, the following programs are worth of specific mention:
  • participation to the International ESA Gradiometric Mission (GOCE);

  • organization and support to the International Project for the determination of the Geoid in South America, under the chair of Denizar Blitzkow (dblitzko@usp.br);

  • computation of improved geoids for Italy and the Mediterranean area;

  • study and possibly first computations for the solution of the problem of the unification of height datums;

  • study of improved methodologies for the determination of the geoid at global and local level;

  • organization of 3 International Geoid Schools, the first of which will be in Malaysia in February 2000.