Report of International Association of Geodesy Section I - Positioning
President: Fritz K. Brunner
Technische Universität Graz
Engineering, Surveying and Metrology
The Handbook of Geodesy (1996) describes the tasks of Section I as:
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.
Structure of Section I
In order to achieve the above described tasks, Section I consists of the Commission X "Global and Regional Geodetic Networks" (President: C. Boucher, France) and of the Special Commission 4 "Application of Geodesy to Engineering" (President: H. Kahmen, Austria). Both commissions have established several working groups and address the longer term interests of positioning on an international basis. For the period 1995 - 1999, Section I set up several Special Study Groups (SSG) to investigate rather limited but highly relevant research topics with the goal to solve the underlying problems. One of the main activity of a SSG is the international coordination of the ongoing research in its field.
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.
Meetings and Highlights
There were many international meetings held on topics of positioning for which IAG functioned as a sponsor. However, in my opinion three meetings had a special mission:
- 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.
The Proceedings "Advances in Positioning and Reference Frames`"(Brunner, 1998) reflect the exciting and steadily growing developments of fundamental GPS work as well as novel applications of static and kinematic GPS surveying techniques. The maintenance and the densification of reference frames are treated for the purpose of establishing global and regional GPS networks. The scientific achievements of the South American Geocentric Reference System project (SIRGAS) are discussed. Several contributions review the state of the art of GPS analysis techniques, ambiguity resolution methods, as well as GPS antenna and site problems. New applications of kinematic GPS positioning and the quality control issues of real-time GPS positioning are presented.
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
- quality control issues of GPS.
The development of continuously operating GPS networks especially as regional active GPS networks is changing the face of positioning. Instantaneous positioning of high precision at distances up to 100 km is just rising above the horizon.
Achievements, trends and research opportunities in positioning
It has been a tradition of Section I Presidents to give a brief outline of their thoughts on the future of Section I. The following notes are my attempt to keep this tradition alive.
It is interesting to realise that the origins of IAG have been in the early arc measurements. At that time small parts of the Earth were measured and estimates about the whole Earth were made. Satellite geodesy reversed the situation and its outstanding success story is well known by such acronyms as IGS, IERS and ITRF. We now have available station coordinates, station velocities and precise orbits and other parameters in order to compute very accurate geodetic networks on a deforming Earth. The traditional backbone of Section I has been national networks and their combinations, like EUREF, taken care of by Commission X. Currently Commission X is involved in the maintenance of geodetic networks and the provision of the necessary terrestrial reference systems. Using IGS and ITRF we can achieve the densification of the global network for national survey purposes.
In this field we notice several recent success stories, like EUREF. My personal favourite is SIRGAS, where a whole continent started to participate in all aspects of modern geodesy, see the papers in Brunner (1998). This project had the right people, the right spirit and the outstanding and sensitive partnership with DGFI and its head, Hermann Drewes.
However, there are still opportunities for other SIRG-- stories. I think that Commission X could provide leadership though workshops etc. by actively approaching the potential users rather then waiting for them to tell what they have achieved and sometimes even what they have not achieved. I am confident that Claude Boucher will address this opportunity to provide a further service by Commission X.
The other development which is still continuing and will show more exciting results in future is the field of continuous monitoring networks at all scales. The use of these networks are twofold:
- deformation studies, and
- remote sensing of atmospheric parameters.
Let me start with the global network. A recent example of the determination of plate tectonic motions using GPS as a subset of IGS was presented by Reigber and Gendt (1996). A phantastic agreement was shown between the displacement vectors derived by GPS and from the NUVEL model. The NUVEL model results are based on geological and geophysical data covering time scales of millions of years. The excellent agreement of these model results with the short period GPS results is quite amazing. It means that the large scale convection processes in the Earth´s mantle are driving the tectonic plates of the Earth´s crust at a steady rate. GPS results show that these rates agree with very high accuracy even at annual periods if the individual plates are considered as rigid bodies.
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
- ambiguity resolution
- site effects
- orbit predictions
- GPS/GLONASS combination
- active GPS networks.
However, the potential precision of GPS is still masked by propagation effects, mainly multipathing and diffraction effects as is easily seen by a comparison of zero baseline versus very short baseline GPS results. This problem needs to be solved before we can achieve another improvement in precision of short site occupation results.
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:
"Not even the future is any more like it used to be."
I would like to thank all geodesists especially, of course, the IAG members for their significant contributions to the discipline of positioning during the past four years. Many thanks!
Brunner F.K. (Ed.) Advances in Positioning and Reference Frames, IAG Scientific Meeting, Rio de Janeiro, Brazil, Springer Verlag (1998), Vol. 118.
Kahmen H., Brückl E. and Wunderlich T. (Eds.) Geodesy for Geotechnical and Structural Engineering, Int. Assoc. of Geodesy, Special Commission 4 (1998).
Reigber C. and Gendt G. Geodätische Messungen der Plattentektonik, Spektrum der Wissenschaften (1996): 115-117.
Bock Y. and Williams S. Integrated Satellite Interferometry in Southern California, EOS, AGU 78 (1997): 293, 299-300.
Beutler G., Rothacher M., Springer T., Kouba J. and Neilan R.E. The International GPS Service (IGS): An interdisciplinary service in support of earth sciences, 32nd COSPAR Scientific Assembly, Nagoya, Japan (1998).
Naito I., Hatanaka Y., Mannoji N., Ichikawa R., Shimada S, Yabuki T., Tsuji H. and Tanaka T. Global Positioning System Project to Improve Japanese Weather, Earthquake Predictions, EOS, AGU 79 (1998): 301, 308, 311.