Commission VIII: International Coordination of Space Techniques for Geodesy and Geodynamics (CSTG)
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.
CSTG Executive Committee 1995-1999
CSTG operates through an Executive Committee and a broad-based Steering Committee, as well as through the National Representatives in carrying out the Commission objectives stated in the charter. At the beginning of the period 1995-1999 the CSTG Executive Committee was composed as follows:
- G. Beutler (Switzerland) President
- H. Drewes (Gemany) Secretary
- B.E. Schutz (USA) Past President
- T. Clark (USA) Chair, VLBI Subcommission
- J. Degnan (USA) Chair, SLR and LLR Subcommission
- P. Willis (France) Chair, Subcommission on Precise Satellite Microwave Systems
- J. Bosworth (USA) Chair, Geodetic and Geodynamics Sites Subcommission
- T. Herring (USA) Chair, Project on Coordination and Combination of Space Geodetic Analysis
Later on, Claude Boucher (France) became member of the EC as the IERS representative, John Dow (Germany) joined the EC as the COSPAR-liaison, and James Campbell (Germany) replaced Tom Clark as Chair of the VLBI Subcommission. In March 1999, with the creation of the IVS (International VLBI Service), Wolfgang Schlüter (Germany) became Chairman of the IVS Directing Board, thus succeeding James Campbell as VLBI Subcommission Chairman.
3. CSTG Bulletins 1995-1999 and other Publications
The CSTG establishes dissemination of information between the various groups engaged in space geodesy and geodynamics through the publication of the CSTG Bulletin. The following four CSTG-Bulletins were issued in the time period 1995-1999:
- Bulletin 12: Status and Programme 1995-1999 (1996)
- Bulletin 13: Progress Report 1997 (1997)
- Bulletin 14: Advanced Space Technology in Geodesy, Achievements and Outlook (1997)
- Bulletin 15: Progress Report 1998 (1999)
Moreover the proceedings of the Munich Section II Symposium are published by CSTG.
4. Major Achievements in the 1995-1999 Time Period
The ILRS and the IVS
In geodesy we have seen the evolution of two extremely successful services, namely the IERS (International Earth Rotation Service) and the IGS (International GPS Service), providing information to the scientific community and serving as a basis for scientific research. It thus seemed important that the old-established space geodetic techniques, namely Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and Lunar Laser Ranging (LLR) get organized in a similar way as the GPS community in the International GPS Service (IGS). We consider it as a remarkable achievement that the
- SLR/LLR Subcommission of CSTG was restructured to become the International Laser Ranging Service (ILRS),
- The VLBI Subcommission of CSTG was restructured to become the International VLBI Service for Geodesy and Astrometry (IVS),
in the time period 1995-1999. The ILRS started operations in fall 1998, the IVS in spring 1999. Both services applied to become official scientific services of the IAG. The proposal will be addressed by the IAG Executive Committee at its meetings in Birmingham. Currently the ILRS and the IVS are considered as services within CSTG. Section II considers the ILRS and the IVS as the successors of the mentioned subcommissions.
The ILRS and the IVS are working in close cooperation with the IERS. The establishment of the services was coordinated between CSTG and IERS.
The Russian Global Navigation Satellite System (GLONASS) was in the focus of interest of the CSTG Subcommission on precise Satellite Microwave Systems. Together with the IGS, the ION (Institute of Navigation), and the IERS the International GLONASS Experiment 1998 (IGEX-98) was organized.
IGEX-98 was the first global GLONASS tracking and analysis experiment. The experiment started in October 1998 and was terminated in April 1999. Since that time GLONASS trracking and data analysis continue on a best effort basis. It is planned to continue activities in the sense of a pilot service. This pilot service shall be organized in close cooperation with the IGS. An international IGEX Workshop will be attached to the 1999 ION conference in Nashville (USA).
In the context of this experiment precise GLONASS orbits are produced and reference frame issues (geometry and time) could be resolved.
FRCN and ISGN
Combination of space techniques is a central issue within CSTG and, e.g., within the IERS for quite some time. In the 1995-1999 time frame several attempts were made to create stable multi-technique networks.
The attempt to form the IUGG Fundamental Reference and Calibration Network eventually failed and led to the process of creating the ISGN, the International Space Geodetic Network. As opposed to the FRCN, which was planned to become a small but global network of about 20 fundamental stations where all (space) geodetic techniques would be available, the ISGN in essence is a selected number of stations of the existing technique-specific networks of the ILRS, the IVS, the IGS, the DORIS, and the GLONASS networks.
The ISGN consists of multi-technique stations with a long-term commitment. The creation of the FRCN Working Group is documented in CSTG Bulletin No. 12, its final report as it was presented to the IAG Executive Committee is included in CSTG Bulletin No. 13. Information concerning the creation of the ISGN is contained in CSTG Bulletin No. 15.
5. Problem Areas
Combining Space Geodetic Analysis
Combination of space geodetic techniques in the analysis is an important issue, but also a difficult one to achieve in practice. The issue was in the center of the project on the coordination and combination of space geodetic analysis (Chair Tom Herring). We believe that important progress was made in the time interval 1995-1999 through he creation of the services ILRS and IVS which are now in a position to build up contacts between the services concerning this topic.
Another milestone was the development of the SINEX, the software independent exchange format by the IGS. The SINEX was propagated through the CSTG pilot project and will hopefully soon be accepted as the official result exchange format in space geodesy, in particular within the services IGS, ILRS, and IVS.
Nevertheless, the issue is far from being resolved. Much more work and a close cooperation with the IERS will be required in the next four years period of CSTG. Efforts should be combined with the IERS. We believe that the restructuring process of the IERS asks for IERS leadership in this domain in future.
Let us mention that the GEMSTONE Workshop (25-28 January, 1999), organized by our Japanese colleagues from Communication Research Laboratory (CRL), gave an excellent overview of the issues related to collocation and combined analylsis in space geodesy. The workshop was well attended by the international scientific community, in particular by members of CSTG.
Space Geodetic Satellite Missions
There were many satellite missions in the past and there will be more in future in which the satellite is used as an observing platform to study aspects of the Earth relevant to geodesy and geodynamics. Let us mention in particular that altimetry missions significantly improved our knowledge of the sea surface topography, ocean currents, tidal motions of the oceans, etc.
For space geodesy the TOPEX/POSEIDON mission was a kind of rosetta stone mission because its orbit was determined using three independent systems, the French DORIS system, SLR tracking, and the GPS. All three systems proved their capability. The radial component of the orbit (which is of crucial importance for altimetry missions) could be established with an accuracy of a few centimeters. TOPEX/POSEIDON was neither the first, nor will it be the last altimetry mission. We regret that the commission did not play an active role in this important mission.
For geodesy, geodynamics, and atmosphere physics the upcoming missions CHAMP (Challenging Mini-Satellite Payload for Geophysical Research and application, German mission), GRACE (Gravity Recovery and Climate Experiment, U.S./German mission), and GOCE (Gravity field and Ocean Current Explorer, ESA mission) are milestones. It is expected that our knowledge of the gravity field (using spaceborne GPS receivers, accelerometers, or gradiometers) to measure the non-gravitational forces resp. gravity gradients will significantly increase through such missions.
Also, CHAMP, GRACE and GOCE are able to produce atmosphere profiles using the occultation method: the signal (phase and code) of a GPS satellite is monitored by a spaceborne GPS receiver on the Low Earth Orbiter (LEO) during the time period the line of sight LEO-GPS satellite scans through the Earth's atmosphere. These developments underline that interdisciplinary aspects are becoming more and more important in Space Geodesy.
We believe that CSTG should play a significant role in the development and coordination of space geodetic satellite missions in future.
CSTG activities are well incorporated in IAG. The link to COSPAR is more problematic. Attempts are made to get a strong CSTG participation for future COSPAR Scientific Assemblies. The next one will be held in summer 2000 in Warsaw, where a common symposium of the COSPAR pannel of precise orbit determination and CSTG is proposed.
Based on the experiences of the time period 1999-2001 the CSTG Executive Committee should come up with a proposal whether or not to maintain the link of the commission with COSPAR by the time of the Warsaw meeting.
6. Section II Proposals concerning the future structure of IAG
CSTG, as a member of Section II, was heavily involved in the development of the proposals which might, eventually, lead to a reorganization of IAG. At its business meeting of February 12, 1999 in Munich, Section II decided to present a proposal concerning the reorganization of IAG at the IAG Executive Committee Meeting, March 22/23, 1999 in Paris.
The current structure and possible future modifications of the IAG were in the focus of the Section II Symposium in October 1998 in Munich. It was concluded that the IAG should invoke a thorough review process within the next legislation period 1999-2003. This process should include at least one special retreat of the IAG Executive Committee with a well selected list of guests. It must involve all IAG sections and other relevant entities (like IAG services). Section II is convinced that this review is necessary because
- geodesy underwent a dramatic development since the creation of the current structure,
- space geodesy became increasingly important and plays a dominant role in all sections,
- the current section structure does not reflect the present-day situation in an adequate way,
- IAG services (like IERS, IGS, etc.) play an increasingly important role also for research in geodesy and geodynamics. They are not well (if at all) integrated in the current structure.
Section II is convinced that these facts fully justify a reorganization. The process is not yet in an advanced stage. Section II is aware of four more or less concrete proposals, namely
- the structure proposed by the IAG president, K-P. Schwarz at the Munich symposium, where the strucure is centered around a central project (global observing system);
- the structure proposed by Martine Feissel also in Munich, which in essence maintains the section structure but gives much more weight to services, projects, and research;
- merge sections 1,2,3, thus yielding three new sections (measurement methods, modeling, geodynamics),
- form sections according to the key words geometry, rotation, gravity field.
These proposals are not yet formulated in a way that they could be compared in a meaningful way. This is why Section II is convinced that more time is required for the restructuring process. At the IAG Executive Committee Meeting on March 22-23 in Paris, Section II presented the following concrete proposals:
- The IAG by-laws shall be adapted in order to allow a reorganization of IAG in the period 1999-2003 through the mechanism described above. The procedure contains in particular
- one or two retreats of the "enlarged" IAG Executive Committee
- proposal to be adopted at the 2001 mid-term meeting
- a structure that can be put in place for the time period 2003-2007.
- The "old" IAG structure (with minor changes as proposed by the Cassinis Committee) shall remain in place for the period 1999-2003.
These proposals were in essence accepted by the IAG Executive Committee. First actions will take place at the Birmingham General Assembly.