SECTION II

ADVANCED SPACE TECHNOLOGY

TECHNOLOGIE SPATIALE AVANCEE

President: R. Rummel (Germany)

Secretaries: P. Willis (France)

G. Beutler (Switzerlands)

I- Objectives

Section II, Advanced Space Technology, is engaged in new space techniques for geodesy and geodynamics. Its objectives are to anticipate and promote their implementation into geodetic/geodynamic work and, in general, support and coordinate the optimal use of modern space technology for the benefit of geodesy.

For the coming term the activities of the section include:

1. Promotion of the realization of space experiments for the improvement of our knowledge of the Earth's gravity field in the medium spatial wavelengths.

2. Improvement of the coordination and combined use of the full range of geodetic space techniques, such us SLR, VLBI, GPS, DORIS, PRARE.

3. Continuation of the successful work of WEGENER with a broadened, new scope.

4. Stimulation of the geodetic use of interferometric SAR and of spaceborne sounding with global satellite navigation systems.

5. Development of concepts on how to come to an optimal worldwide distribution of geodetic/geophysical fundamental stations.

6. Continuation and further improvement of the work of I.G.S.

II-Structure

Commissions :

Commission VIII : International Coordination of Space Techniques for Geodesy and Geodynamics (CSTG)

President: G. Beutler (Switzerland)

Secretary: H. Drewes (Germany)

Special Commissions :

SC 6 : Wegener Project

President : S. Zerbini (Italy)

SC 7 : Gravity Field Determination by Satellite Gravity Gradiometry

President : K.-H. Ilk (Germany)

Special Study Groups :

SSG 2.160 : SAR Interferometry Technology

Chairman : R. Klees (The Netherlands)

SSG 2.161 : Spaceborne Atmospheric GNS Soundings

Chairman : C. Rocken (USA)

SSG 2.162 : Precise Orbits using Multiple Space Techniques

Chairman : A. Marshall (USA), from 1998.07.10 R.Scharroo (remko.scharroo@lr.tudelft.nl), the Netherlands.

International Service

International GPS Service for Geodynamics (IGS)

Chairman of the Governing Board:

G. Beutler (Switzerland)

Director of the Central Bureau :

R.E. Neilan (USA)

Commission VIII

INTERNATIONAL COORDINATION OF SPACE TECHNIQUES FOR GEODESY AND GEODYNAMICS

(C S T G)

President : G. Beutler (Switzerland)

Secretary : H. Drewes (Germany)

I- General Objectives

The Commission on International Coordination of Space Techniques for Geodesy and Geodynamics (CSTG) was established during the XVII-th General Assembly of the IUGG in Canberra in 1979. It is Commission VIII in Section II of the International Association of Geodesy (IAG) and Subcommission B.2 in COSPAR. The charter of the commission is the following :

"Develop links between various groups engaged in the field of space geodesy and geodynamics by various techniques, coordinate work of these groups, elaborate and propose projects implying international cooperation, follow their progress, and report on their advancement and results."

The role of the CSTG is to facilitate the activities of national and international groups through the collection and dissemination of information within the groups and the member countries mainly through correspondence and also through international or regional meetings and to help in setting up coordinated multi-national observing projects and scientific investigation programs.

CSTG operates through an Executive Committee through the National Representatives in carrying out the Commission objectives stated in the charter. In addition a number of Projects and Subcommissions have been created to coordinate activities in the field of space geodesy and geodynamics which could benefit from the coordinating role of the Commission. Each Project and Subcommission has its own organizational structure and must issue its own informational bulletins or newsletters. Projects generally consist of highly focused activities over a limited period of time, Subcommissions cover long-term programs, often with programmatic implications.

It is the Commission plan to promote space geodesy in areas of the world in which there have been few space geodetic measurements and to encourage the continued conduct of comparisons between space geodetic techniques. The commission will also encourage the development of new techniques and application areas.

The CSTG will encourage dissemination of information between the various groups engaged in space geodesy and geodynamics through the publication of the CSTG Bulletin.

II- Objectives 1995-1999

In geodesy we have seen the evolution of VLBI, Satellite- and Lunar Laser- Ranging, and GPS as very powerful space techniques. We have also seen the evolution of two services, namely the IERS (International Earth Rotation Service) and the IGS (International GPS Service for Geodynamics), providing information to the scientific community. New techniques (Glonass, Doris, Prare) are coming up now, spaceborne applications of the GPS are becoming more and more routine. In this environment we envisage the following main objectives for the next four years :

1. So far, each technique was producing its own results without taking into account (at least not to the extent possible) the achievements of the other techniques. Results (coordinates and earth rotation parameters) are compared by the IERS, a combination is done on a very high level by simple averaging processes. It is clear, however, that more rigorous analysis techniques (ideally common processing of different techniques or equivalent procedures) must lead to better results. Also, it is possible today to observe the same satellites with different techniques (e.g. using SLR, GPS, DORIS, PRARE). The coordination of activities and efforts in these domains is the main objective of the newly established CSTG project "Coordination and Combination of the Analysis in Space Geodesy" and the subcommission "Precise Satellite Microwave Systems".

2. It is clear from the scientific point of view that VLBI, Laser Ranging, and satellite microwave techniques are indispensable as contributors to space geodesy. It is also clear, on the other hand, that operational costs play an essential role. Making optimum use of the existing tracking networks, developing low cost equipment, optimizing observation schedules and processing strategies will be the key issues within the techniques subcommissions.

3. Network optimization will have to be considered by all subcommissions. The problem of collocating the networks and/or of co-locating different techniques in one and the same observatory will be studied.

4. Dissemination of information is considered an essential tool in CSTG. We plan to issue at least one CSTG Bulletin per year.

III-Structure

Executive Committee :

G. Beutler (Switzerland) President

H. Drewes (Germany) Secretary

B.E. Schutz (USA) Past President

T. Clark (USA) Chairman, VLBI Sub- commission

J. Degnan (USA) Chairman, SLR and LLR Subcommission

P. Willis (France) Chair, Subcommission on Precise Satellite Microwave Systems

J. Bosworth (USA) Chairman, Earth Science Sites (ESS) Sub- commission

T. Herring (USA) Chairman, Project on Coordination and Combination of Space Geodetic Analysis

Subcommissions :

Subcommission

VLBI (Very Long Baseline Interferometry)

Chairman : T.A. Clark (USA)

Established as subcommission on "International Radio Interferometric Surveying (IRIS)" in 1983. Renamed as VLBI Subcommission in 1995 at XXI-th IUGG General Assembly in Boulder.

Subcommissiopn

Satellite Laser Ranging (SLR) and

Lunar Laser Ranging (LLR)

Chairman : J. Degnan (USA)

Established in 1986 as SLR subcommission. Scope broadened to include LLR in 1995 at XXI-th IUGG General Assembly.

Subcommission

Precise Satellite Microwave Systems

Chairman : P. Willis (France)

This subcommission was established in 1995 at the XXI-th IUGG General Assembly in Boulder. It replaces the former Subcommission on GPS. This seems to be justified because the IGS (International GPS Service for Geodynamics), formerly a CSTG project, now (in 1995) established as an International Service under Section II, is taking care of the International coordination and promotion of the GPS technique. Coordination between the established GPS techniques and the upcoming GLONASS, DORIS, PRARE systems is the main objective of this subcommission.

Subcommission

Geodetic and Geophysical Sites

Chairman : J. Bosworth (USA)

Established in 1989 as the Site Issues Sub-commission to identify, discuss and disseminate information on the types of monuments, monument stability and local site surveys necessary for various space geodetic measurement systems. Officially renamed the Geodetic and Geophysical Sites Subcommission at the twenty-first IUGG General Assembly in Boulder.

Project

Coordination and Combination of the Analysis in Space Geodesy

Chair : T. Herring (USA)

Established at the XXI-th IUGG General Assembly in Boulder. Coordinating the analysis of different space techniques, encouraging common processing strategies and common observations is the the key objective of this subcommission.

IV-National Representatives

The IAG member countries had the opportunity to nominate new National Representatives at the XXIst IUGG General Meeting in Boulder. The National Representatives also considered as the steering committee of the CSTG. Not all IAG country nominated representatives. Nominations are still possible and would undoubtedly facilitate the flow of information.

J. Manning (Australia)

H. Sünkel (Austria)

R. Warnant (Belgium)

J. Kouba (Canada)

Hu Jianguo (China)

M. Bursa (Czech Rep.)

B. Madsen (Denmark)

A. Shaker (Egypt)

J. Kakkuri (Finland)

R. Biancale (France)

R. Schluter (Germany)

G. Veis (Greece)

P. Denys (New Zealand)

B. Ambrosius (Netherlands)

B. Engen (Norway)

I. Fejes (Hungary)

B. Rajal (India)

G. Bianco (Italy)

T. Kato (Japan)

B. Okumu (Kenya)

Kim Cha Un (Korea)

J.B. Zielinski (Poland)

M. Prilepin (Russia)

M.C. Perez-Urquiola (Spain)

G. Nicholson (South Africa)

J. Hefty (Slovakia)

B. Ronnang (Sweden)

W. Gurtner (Switzerland)

R.M. Rezgui (Tunisia)

S.M. Nakiboglou (Turkey)

M. Watkins (USA)

M. Yatskiv (Ukraine)

A. Sinclair (United Kingdom)

Special Commission SC 6

WEGENER PROJECT

Geodetic Investigations Related to the Kinematics and Dynamics of the African,

Arabian and Eurasian Plates

President: S. Zerbini (Italy)

I. Main Objectives

Three main objectives have been defined. They are:

1. The investigation of deformations along the African-Arabian/Eurasian plate boundary which includes:

- determination of the relative plate motions in the framework of 3-D global plate motions,

- estimation of the extent of the deformation zones directly associated with the plate boundaries,

- assessment of the relative magnitudes of horizontal and vertical change and the variation of the deformations occurring across the boundary zones,

- interpretation and use of geodetic results as constraints on geodynamic models of the African-Arabian/Eurasian collision zone;

2. The investigation of post-glacial rebound in Fennoscandia, which includes:

- estimation of the extent of the rebound phenomena in the Fennoscandia region,

- determination of the rate of vertical deformation and the variation of rate as a function of distance from the centres of rebound,

- analysis of the geodetic results to constrain the viscosity of the mantle and the models describing deformation of the lithosphere in response to loading;

3. The investigation of height variation and changes of sea level, which includes:

- contributions to the establishment of a common global height datum,

- determination of improved geoidal information for the plate boundary and Fennoscandian regions,

- estimation of the relative magnitudes of the different factors contributing to height and sea-level variations and the relationship of changes in sea level to global change.

Each of these objectives will draw upon the data collection and analysis of GPS, SLR, VLBI, DORIS, PRARE, absolute and relative gravimetry, satellite altimetry and additional data sources.

II. Program of Activities

The currently scheduled activities comprise both field measurements and analysis. These include:

1. Research focused on the completion and maintenance of a reference frame adequate for the monitoring of horizontal and vertical crustal motion on the spatial and time scales addressed by the foregoing objectives.

2. Extensive space geodetic observations in the reference frame along the southern boundary of the Eurasian plate from the Azores to the eastern limit of its collision with the Arabian plate.

3. Space geodetic and terrestrial measurements to investigate the extent and magnitudes of Fennoscandian post glacial rebound and its associated effects, in this reference frame.

4. Space geodetic and terrestrial measurements to determine the origins and magnitudes of height variation and their relevance for understanding the interaction of crustal motion and sea level fluctuations.

5. Development and exploitation of models and analytical techniques to facilitate the estimation of time-dependant three-dimensional positional change, the separation of exogenic and endogenic effects, and the time-dependant components of the gravity field.

6. Inter-disciplinary interpretation of the results.

7. Evaluation and comparison of the observational techniques (in terms of reliability, continuity and accuracy) for meeting these goals.

III. WEGENER Structure

The WEGENER project structure has been organized in the following way:

Science Advisory Committee

H.G. Kahle (Switzerland)

I. Marson (Italy)

M. Pearlman (USA)

H.-P. Plag (Germany) - Chairman

R. Rummel (Germany)

D. Smith (USA)

W. Spakman (The Netherlands)

S. Tatevian (Russia)

P. Wilson (Germany)

S. Zerbini (Italy)

Executive Committe

G. Beutler (Switzerland)

J. Bosworth (USA)

C. Boucher (France)

B. Engen (Norway)

I. Kumkova (Russia)

J. LaBrecque (USA)

C. Reigber (Germany)

H. Seeger (Germany)

S. Zerbini (Italy) - Chairman

Technology Committee

B. Ambrosius (The Netherlands)

T. Baker (United Kingdom)

L. Bastos (Portugal)

G. Bianco (Italy)

G. Blewitt (United Kingdom)

T. Clark (USA)

J. Degnan (USA)

B. Richter (Germany) - Chairman

P. Tomasi (Italy)

Special Commission SC 7

Gravity Field Determination

by Satellite Gravity Gradiometry

President: K.-H. Ilk (Germany)

I. Main Objectives

The necessity of a high-resolution spaceborne gravity field mission was defined already in 1969 in the so--called Williamstown Report by the leading geo--scientists at that time. The idea was to derive the gravity field and positions at the earth's surface and in space consistently at the same level of precision. For various technological as well as political reasons such a mission could not be realized within the last twenty five years despite the intensive work of many individual scientists and scientific groups and the International Association of Geodesy (IAG) as a whole.

The main objective of SC7 is therefore to create a forum that integrates all current international activities related to gravity field determination by satellite gravity gradiometry and to prepare the conditions for a future mission. In detail, the special commission shall

- represent IAG interests in such a mission on a political level,

- support a gravity gradiometry mission by scientific studies,

- investigate scientific and commercial applications of a very precise high resolution gravity field,

- assist in coordination and definition of national and international concepts related to gravity gradiometry,

- act as advisor to national and international bodies responsible for such a mission,

- inform the geodetic community about all these activities.

Steering and Advisory Committee

To make the work of the Special Commission as effective as possible and to integrate all interests to meet these objectives a 'Steering and Advisory Committee' within the Special Commission has been created. The commission consists currently of fifteen scientists actively involved in satellite gravity gradiometry:

G. Balmino (France)

K.H. Ilk (Germany)

W. Keller (Germany)

R. Koop (The Netherlands)

Ph. Moore (UK)

H.J. Paik (USA)

R. Rapp (USA)

R. Rummel (Germany)

F. Sansò (Italy)

P. Schwintzer (Germany)

C.K. Shum (USA)

H. Sünkel (Austria)

B. Tapley (USA)

C.C. Tscherning (Denmark)

M. Vermeer. (Finland)

Task Groups

The following four task groups are bodies of the Special Commission:

Ad hoc Group 'Scientific objectives'

CIGAR-IV Study Group

STEP-Geodesy Working Group

Working Group 'Application of Boundary Value Techniques to Satellite Gradiometry'

Ad hoc Group "Scientific objectives"

As agreed at a joint meeting of NASA'S EOS-SEC and ESA'S Earth Observation and Advisory Committee at ESA headquarter on May, 11 to 12, 1995 a joint group has been formed that will review the science rational for a dedicated gravity field mission and formulate a concise and readable document for future discussion of the science community with space agencies, government organizations, etc.

CIGAR-IV Study Group

In the years since 1987 ESA supported a number of studies to investigate various questions related to the processing of gravity gradient measurements, eventually combined with precise satellite--to--satellite tracking data. Within these so--called CIGAR studies (I to III) all European scientists working in the field of future gravity field determination techniques found a forum for discussion and exchange of ideas. Present CIGAR-IV Scientific manager: H. Sünkel.

STEP-Geodesy Working Group

At this moment a main candidate for the realization of a gravity field mission is the geodesy experiment on STEP. It consists of a combination of spaceborne GPS and (very likely) single component gradiometry at reasonably low altitude and with an almost polar orbit. This mission has not been selected im spring 1993 as medium mission 2 (M2) of the ESA science program but will compete again in spring 1996 for selection as M3.

Working Group

'Application of Boundary Value Techniques to Satellite Gradiometry'

This working group, under the chairmanship of W. Keller, is intended to continue the successful work of IAG Section IV Special Study Group 'Application of Boundary Value Techniques to Space-- and Airborne Gradiometry'.

The aim is to study satellite gravity gradiometry in the framework of the so--called spacewise approach. In this approach the gradiometer data are considered as sampled boundary values on a surface representing the satellite orbit. The relationship of those boundary data with the unknown gravitational potential is formulated as a boundary value problem and solved with related techniques.

The work of the group will concentrate on some unsolved problems related to existence and uniqueness of the solution, data reduction problems, development of numerical solution strategies, investigation of aliasing effects and to investigation of various geodynamic applications.

Members:

A. Albertella (Italy)

V. Belikov (Russia)

W. Freeden (Germany)

M.v.Gelderen (The Netherlands)

B. Heck (Germany)

M. Hirsch (Australia)

R. Klees (Germany)

M. Schreiner (Germany)

M. Thalhammer (Germany)

M. Vermeer (Finland)

Special Study Group 2.160

Spaceborne INSAR Technology

Chairman: R. Klees (The Netherlands)

I- Terms Of Reference

The special study group shall concentrate on the following topics :

1. Basic Principles and Theory of spaceborne INSAR

2. Modelling and software development for deriving DEMs

3. Modelling and software development for detecting deformations

4. Limitations to INSAR

5. Validation of INSAR results

II- Program Of Activities

The main focus point for present use of spaceborne INSAR (INterferometric Synthetic Aperture Radar) data is the generation of digital elevation models (DEMs) and the detection of deformations of the surface and of artifical objects. Although these kind of INSAR applications are rather new, this field develops very fast since the first results have become known in the mid-eighties.

In Geodesy, however, the technique is rather unknown although the generation of DEMs and especially the detection of surface and object deformations are treated in geodesy for a long time using other techniques.

Therefore, one term of reference of the SSG should be to promote INSAR within the geodetic community. This covers especially a review of synthetic aperture radar technique, its basic principles and theory, and the INSAR data processing. The activities should result in one of more overview articles on the state-of-the-art of the technique, its basic principles, the data processing, the applications, and the limitations.

One main application of INSAR is the generation of DEMs. Although airborne across-track interferometer might be best suited at the moment for that purpose, the SSG should focus on spaceborne single antenna SAR, e.g. ERS-1, ERS-2, RADARSAT. The SSG should contribute to the improvement of the mathematical models and of the processing algorithms to achieve optimal accuracies.

Main focus should be on the use of spaceborne INSAR for detecting deformations. The activities should include both modelling and processing aspects. With respect to modelling the SSG should concentrate on the the role of topography (DEMs) for detecting of deformations and on the problem of decorrelation. Especially the influence of the atmosphere on the results needs to be investigated. With respect to the data processing the SSG should contribute to the development and comparison of different algorithms for phase unwrapping and to the application of filter techniques.

The application of INSAR for detecting deformations have been shown in several field experiments. However, most of them have been run under rather "nice" conditions. Therefore, there is a need to investigate what can be obtained under less optimal conditions. Different conditions of surfaces, such as vegetation, slopes, urbanization, humidity should be assessed. The different error sources should be identified and quantified, e.g. by processing appropriate SAR images taken under different conditions. The mathematical models have to be improved correspondingly and have to be checked by comparison with independent ground truth. The next step then contains the integration of INSAR results with other observations, especially with GPS, levelling, and atmospheric data. Therefore, the SSG should focus on the problem of validation of INSAR results and integration with information provided by other sensors.

Supporting activities which also aim at improving and strenghtening the output and the cooperation within the group should be: the distribution of relevant reports, papers etc. at time of submission among all members, the exchange of information on interesting INSAR-meetings, the choice of some basic INSAR scenes for modelling and error studies, the creation of an INSAR bibliography, and the exchange of information concerning interferometric software packages. Moreover, the SSG intents to initiate and organize a field experiment for investigating the application of INSAR for detecting surface deformations under real conditions.

III- Membership

R. Bamler (Germany)

S. Coulson(Italy)

Ph. Hartl (Germany)

R. Klees (The Netherlands) - Chairman

D. Massonnet (France)

M. Murakami (Japan)

C. Pearson (New Zealand)

M. Schmidt (Germany)

P. Vachon (Canada)

E. van Halsema (The Netherlands)

Special Study Group 2.161

Spaceborne Atmospheric

GNS Soundings

Chairman : C Rocken (USA)

I-Introduction :

GPS and GLONASS are Global Navigational Systems (GNS) that were developed for precise positioning and timing on earth and vicinity. GNS signals that travel through the atmosphere to low earth orbiting satellites (LEOs) can also be used to obtain profiles of atmospheric and ionospheric properties.

This is done using radio occultation methods, originally developed at Stanford University and at the Jet Propulsion Laboratory (JPL) for planetary exploration. Since the successful launch of the first GPS receiver for atmospheric soundings in April, 1995, these techniques are now also applied to study the earth's atmosphere.

In light of promising early results from this proof-of-concept launch, we can expect a dramatic increase in activities to use GNS for active atmospheric sounding from space over the next years.

II-Objectives :

Future work and research in the field of active atmospheric GNS soundings will lead to progress in (a) the technology of occultation methods, and in (b) the impact that these data will have on society and on other fields of science. This study group shall function as a focal point in the discussions on progress in the following areas:

(1) The Technique of Using GPS for Active Atmospheric Limb Sounding

(a) Hardware and software developments for sounding the lower troposphere

(b) Use of Y-code and/or additional carrier frequencies

(c) Correcting for multipath effects

(d) Separating water vapor and temperature effects

(e) The ionosphere as noise and signal

(f) Profiling and tomographic techniques

(g) Comparison of GNS Limb Sounding with other techniques (Validation Studies)

(2) The Impact of the Sounding Data:

(a) Data assimilation into meteorological models - what data type shall be used?

(b) Combination of GNS Limb Sounding data with other atmospheric data

(c) Timeliness requirements for weather and climate studies

(d) Future experiments

(e) Operational LEO constellations - (How many satellites, Optimal Orbits, etc.)

(f) Use of the data: Meteorology, SAR, Communications, Airlines, etc. ...

III-Members :

M.Gurbonov Russia

S. Sokolovskiy Russia

P. Schwintzer Germany

X. Zou USA

T.P. Yunck USA

T.K. Meehan USA

B. Herman USA

C. Rocken USA

M. Exner USA

G.A. Hajj USA

P. Hoeg Denmark

A. Jungstand Germany

N. Jakowski Germany

R. Govind Australia

K.R. Hardy USA

D. Anderson USA

J. Eyre UK

W. Smith USA

J. Davies USA

W. Spakman The Netherlands

Special Study Group 2.162

Precise Orbits Using Multiple Space Techniques.

Chairman : A. Marshall (USA)

I. Terms Of Reference

TOPEX/POSEIDON (T/P) carries five independent tracking systems including Satellite Laser Ranging (SLR), Doppler Orbitography and Radio Positioning Integrated by Satellite (DORIS), Global Positioning System (GPS), the Tracking and Data Relay Satellite System (TDRSS), and its own radar altimeter. For the first time, the force model errors, especially gravity, have been reduced to a point where a comparison of the various satellite tracking systems at or near their noise level is possible. Results, as expected, show that each system has its own strengths and weaknesses.

Therefore, future precision orbit determination improvements for T/P, as well as other satellites such as GPS, ERS-2, and TDRS, will likely entail a combination of multiple tracking techniques. The focus of this study group will be to further evaluate and characterize the various tracking systems, develop and assess new tracking techniques, and apply the products to improve the state-of-the-art in precision orbit determination.

II. Program Activities

1. Characterize the strengths and weaknesses of all of the current and proposed precise tracking techniques including SLR, DORIS, GPS, TDRSS, GLONASS, Precise Range and Range-Rate Equipment (PRARE), and satellite altimetry.

2. Where possible, assess the impact of multiple tracking techniques on a single spacecraft (i.e. T/P, GPS-35, GPS-36, ERS-2, EUVE).

3. Attempt to resolve discrepancies between the various techniques (i.e. the unexplained "Z-bias" observed between the SLR/DORIS and GPS based T/P orbits).

4. Develop and evaluate alternative tracking techniques to further improve satellite positioning (i.e. use the SLR/DORIS orbits from T/P to refine the TDRS and TDRSS-user satellite ephemerides).

III- Membership

B. Ambrosius (The Netherlands)

P. Andersen (Norway)

W. Bertiger (USA)

R. Coleman (New Zealand)

J. Dow (Germany)

B. Haines (USA)

C. Huang (China)

S. Luthcke (USA)

A. Marshall (USA) - Chairman

F.H. Massmann (Germany)

F. Nouel (France)

E. Pavlis (USA)

J. Ries (USA)

E. Schrama (The Netherlands)

L. Senhal (Czech Republic)

C.K. Shum (USA)

P. Visser (The Netherlands)

M. Watkins (USA)

S. Zhu (Germany)

The International GPS Service for Geodynamics (IGS)

Chairman of the Governing Board :

G. Beutler (Switzerland)

Director of Central Bureau : R.E. Neilan (USA)

1. Development

The International GPS Service for Geodynamics (IGSwas established as an official IAG service on January 1, 1994. First discussions took place in 1989 at the IAG General Assembly in Edinburgh. The IAG Planning Committee

for the IGS was established by the IAG in April 1990 in Paris. This Committee, chaired by Prof. I.I. Mueller, issued the "Call for Participation" in April 1991. Based on the positive responses (about 100 agencies volunteered to take over responsibilities within the IGS) the Planning Committee was reorganized at the XX-th General Assembly in Vienna and renamed as IGS Campaign Oversight Committee. This new Committee conducted the 1992 IGS Test Campaign from 21 June - 23 September. The campaign was successful beyond any expectation. Therefore, the IGS Pilot Service was established on 1 November 1993 to bridge the gap between the 1992 IGS Test Campaign and the start of the official service. The IGS was approved by the IAG as an official service at the IAG General Assembly in Beijing in 1993.

2. The IGS Mission

According to the IGS terms of reference the primary objective of the IGS is to provide a service to support, through GPS data products, geodetic and geophysical research activities. The IGS collects, archives and distributes GPS observation data sets of sufficient accuracy to satisfy the objectives of a wide range of applications and experimentation. These data sets are used by the IGS to generate the following data products :

- high accuracy GPS satellite ephemerides

- earth rotation parameters

-coordinates and velocities of the IGS tracking stations

- GPS satellite and tracking station clock information

- atmospheric information.

3. The Structure

The IGS accomplishes its mission through :

- Networks of tracking stations

- Data centers

- Analysis Centers and Associate Analysis Centers

- Analysis Center Coordinator (at present J. Kouba) - Central Bureau (Director R.E. Neilan)

- Governing Board (Chairman, 1995 G. Beutler)

The network was growing from about 30 stations in 1992 to about 100 stations in 1995. All stations are equipped with high-precision P-code receivers and with powerful data links allowing near-real-time data transmission.

At present 7 Analysis Centers, namely

- CODE (a cooperation of Astronomical Institute, Bern, Federal Office of Topography, Wabern, Institute for Applied Geodesy (Frankfurt)

- Institut Geographique National (France)

- EMR (Natural Resources, Canada)

- ESA (European Space Agency, Germany)

- GFZ (Geoforschungszentrum, Germany)

- JPL (Jet Propulsion Laboratory, USA)

- NGS (NOAA, National Oceanic and Atmospheric Administration)

- SIO (Scripps Institution of Oceanography)

are producing on a daily basis satellite ephemerides for all available satellites, earth rotation parameters (x- and y- components of the pole position in the earth fixed reference frame, length of day estimates).

Moreover the Analysis Centers are producing so-called free network solutions for the station coordinates and their velocities. These solutions are used by the ITRF section of the IERS for the production and maintenance of the International Terrestrial Reference Frame (ITRF) together with the results from the other space techniques.

The principal role of the Analysis Center Coordinator (Jan Kouba, Natural Resources, Canada) is to coordinate the activities of the IGS Analysis Centers and to produce the combined, official IGS products (see below).

The IGS Central Bureau is responsible for the general management of the IGS. The Central Bureau also acts as the GPS Coordinating Center for the IERS.

A key instrument of the Central Bureau is the Central Bureau Information System (CBIS). This CBIS provides public access to the state of the network and the other IGS components, and to the official products of the IGS. It also provides a means of electronic messaging among IGS participants through the IGS Message- and the IGS Report- series. The CBIS is a client of the Internet's World Wide Web.

The IGS Governing Board consist of 15 members. It should be well balanced from the institutional and the geographical point of view.

The current membership in the Governing Board :

G. Beutler (Switzerland), Chairman

Y. Bock (USA)

G. Blewitt (Great Britain)

J. Dow (Germany)

B. Engen (Norway)

C. Boucher (France)

J. Manning (Australia)

J. Kouba (Canada)

G. Mader (USA)

B. Melbourne (USA)

I.I. Mueller (USA)

R. Neilan (USA), Director Central Bureau

C. Noll (USA)

C. Reigber (Germany)

B. Schutz (USA)

Persons representing organizations which participate in any of the IGS components are considered as IGS Associate Members. IGS Associate Members together with the GB vote for the incoming members of the Governing Board. A list of the IGS Associate is available upon request from the IGS Central Bureau. The list is also available in the IGS Directory (last edition: summer 1996).

4. The Products

The daily products of the individual Analysis Centers (satellite ephemerides and earth rotation parameters) are made available to the scientific community (at least) on a weekly basis through the three Global Data Centers

- CDDIS (Crustal Dynamics Data Information System) at NASA Goddard Space Fligth Center, USA),

- IGN (Institut Geographique National, France),

- SIO (Scripps Institution of Oceanography).

They are also available through many regional and National agencies.

The individual products are analysed on a weekly basis by the IGS Analysis Center Coordinator. The latter is responsible to generate the so-called official IGS Orbit through a weighted linear combination of the individual

contributions. Since 1995 a combined IGS pole estimate based on the same principles as the combined orbit is available, too. The quality of the products is of the order of 5-10 cm per satellite coordinate and 0.1 - 0.2 mas for the pole coordinates, which is of the same order of magnitude as that of the best individual series. The official IGS orbits and the earth rotation files associated with them are also made available through the Global Data Centers and through the Central Bureau Information System.

5. Publications

- IGS Messages and IGS Reports, available through the Central Bureau

- Information System (CBIS)

- IGS Colleague Directory, updated regularly, available through the Central Bureau

- IGS Resource Information (short information about the IGS, available through the IGS Central Bureau)

- IGS Annual Report (since start of the official service, report for 1994 available through the IGS Central Bureau)

6. Addresses

For more information please contact :

Ruth E. Neilan

Director IGS Central Bureau

Jet Propulsion Laboratory

M/S 238-540

4800 Oak Grove Drive

Pasadena, CA 91109 USA

Tel : ++1 818 354 8330

Fax : ++1 818 393 6686

Internet : igscb@igscb.jpl.nasa.gov

or : ren@logos.jpl.nasa.gov
Last change 1998.07.09 by cct.