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.
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.
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:
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deformation studies, and |
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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
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reference frame definitions and their maintenance |
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ambiguity resolution |
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site effects |
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orbit predictions |
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GPS/GLONASS combination |
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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."
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.