Section I is concerned with the scientific aspects of the measurement and analysis of regional and global geodetic networks as well as satellite, inertial, kinematic and marine positioning. The practical results of this research work should be made available through recommendations to National Survey Organisations. Applications of geodesy in engineering is a recent new task of Section I.
Tremendous advances of GPS surveying have occurred, especially in precision and applicability. However, there are some remaining issues of accuracy and reliability of GPS surveying (hardware and software) which need to be addressed carefully. Recently, GPS measurements have shown the potential to be used as remote sensing tool of atmospheric parameters.
Two main driving forces of the developments by Section I can be recognised:
GPS technology is changing the methods and practical tasks of positioning,
and the engineering applications of geodesy are a growing field with an
important geodetic kernel.
In 1995 the following special study groups were established:
SSG 1.153: Precise Marine Positioning: Surface and Seafloor (D. Egge, Germany)
SSG 1.154: Quality Issues in Real Time GPS Positioning (C. Rizos, Australia)
SSG 1.155: Active GPS Networks (H. Tsuji, Japan)
SSG 1.156: Advanced GPS Analysis for Precise Positioning (G. Blewitt, UK)
SSG 1.157: GPS Ambiguity Resolution and Validation (P.J. de Jonge, Netherlands)
SSG 1.158: GPS Antenna and Site Effects (J. Johansson, Sweden)
SSG 1.159: Use of GPS Positioning for Atmospheric Monitoring (M. Bevis, USA)
In 1997, the SSG 1.153 was discontinued by the IAG Executive due to lack of communication.
Most SSG worked extremely well which can be seen from their reports.
The Steering Committee of Section I comprises of the Section President (F.K. Brunner, Austria), the Section Secretary (Y. Bock, USA) and the President of Commission X (C. Boucher, France). Meetings of the Steering Committee were held at IAG Executive Committee Meetings and at major international symposia.
The reports by Commission X, SC4 and SSGs groups are given in the following
section. The complete report of SC4 is available from the web-side-version
of the Travaux.
|Section I organised the symposium on "Advances in Positioning and Reference Frames" as a part of the Scientific Meeting of IAG in Rio de Janeiro|
|SC4 organised a Symposium on "Geodesy for Geotechnical and Structural Engineering" in Eisenstadt|
|Section I organised the Symposium G1 "Positioning" at the General Assembly of IUGG in Birmingham.|
At the Eisenstadt meeting (Kahmen, 1998) a first attempt was made to connect geotechnical problems with geodetic measurement and analysis capabilities.
Highlights of "Positioning" of the Birmingham meeting will be contained in the forthcoming IAG Proceedings. The presentations of nearly 110 posters show the tremendous impact GPS currently has on the development of positioning through achievements in:
|global reference frame definition|
|subsequent definitions and maintenance of national networks|
|nearly instantaneous ambiguity resolution|
|much better understanding of site environmental effects|
|and quality control issues of GPS.|
|deformation studies, and|
|remote sensing of atmospheric parameters.|
On a regional scale, we have seen the establishment of several networks to monitor tectonic deformations, such as a GPS array in Japan comprising of about 1000 GPS stations, or the PGGA in California. Results from these networks have already proven to provide vital information on the tectonic deformations around fault and subduction zones.
Let me mention to you an exciting example of the combination of the results of a GPS permanent array at discrete points with INSAR results, which of course have a continuous surface distribution, Bock and Williams (1997). I believe that both techniques but especially their combination, where applicable, will be a growing and exciting research field with many significant results.
On a local scale, we can observe the establishment of continuous GPS networks to monitor deformation of volcanoes, landslide areas, and built structures such as bridges, towers and dams. The first two applications of GPS positioning have already been recognised as important geodetic contributions to IDNDR. Furthermore, there are other geophysical phenomena which so far are lacking objective information by geodetic measurements, such as "mountain spreading". What are the real limitations of the applicability of GPS/Glonass/etc for the study of natural disasters and the possible detection of precursors of these hazards? Definitely an exciting field also for the future.
Now, let us look at the "other face" of GPS continuous arrays, i.e. the remote sensing capability. It did not take very long for the geodetic community to realise the potential of GPS to be used as a remote sensing tool of the Earth´s atmosphere: ionosphere and water vapour. The ionospheric total electron content (TEC) can be measured using the two GPS frequencies due to the dispersive propagation effect. Ionospheric TEC computations are now routine operations using the IGS data, see Beutler et al. (1998).
Since the wet delay of the troposphere is a significant error source in precise positioning it can in-turn be considered a signal in the position results and thus estimated. As a result, the value of the vertical column of water vapour, called precipitable water vapour, can be determined. Water vapour is a highly variable greenhouse gas and a vital parameter driving the weather patterns. An impressive example of remote sensing of the water vapour distribution was shown recently (Naito et al., 1998). The temporal variation of the precipitable water vapour was nearly 6 cm during two days of a weather front moving across Japan.
Propagation effects in occultation situations can be used as a remote sensing tool of ionospheric and tropospheric parameters. This will be a very exciting field for future contributions of geodetic instrumentation and analysis to the research areas of other IUGG Associations.
During the past four years we have experienced tremendous progress or at least an improved understanding in issues such as
|reference frame definitions and their maintenance|
|active GPS networks.|
I believe that we will continue to see many new applications of satellite positioning. This field will expand mainly through the combination of different sensors into new measurement systems. Sensor fusion will lead to new exciting fields of positioning especially in application of geodesy to engineering, for example the guidance of construction vehicles.
Hopefully, I was able to show, is that we are also seeing a change in our research pattern in Section I. Traditionally, we investigated the market driven needs, mainly specified by national survey organisations. The work concentrated on horizontal and vertical control with all the necessary classical geodesy. Now we move to a new situation where our instrumentation and analysis capabilities open up new opportunities not seen by the "almighty market". It is our creativity which finds new areas of novel applications of positioning.
Let me close my remarks as outgoing President of Section I by quoting an Austrian saying: