[IGSMAIL-0961] Summary of Potsdam Workshop

[G. Beutler]

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IGS Electronic Mail      Fri May 26  0:56:26 PDT 1995      Message Number 0961
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Author: G. Beutler
Subject: Summary of Potsdam Workshop

Subject: The IGS Workshop 1995 "Special Topics and New Directions" in Potsdam

The workshop took place in the beautiful "Jagdschloss Glienicke" between
Potsdam and Berlin. It was hosted by the GeoForschungsZentrum (GFZ),
Potsdam, Germany. The workshop was opened by Prof. Christoph Reigber around
1 p.m. on Monday, May 15; the chairman of the IGS Governing Board briefly
reviewed the IGS workshops in the past, greetings and best wishes from
the IAG for a fruitful workshop were forwarded by Prof. Ivan I. Mueller,
IAG representative in the IGS Governing Board.

Session 1 gave an overview of the development of the IGS in 1994 and the
state of affairs in 1995. It became clear that the first year of official
IGS operations was successful. The analysis center coordinator could
report that the official IGS orbits were always available in time, i.e.
not later than 14 days after the observations. The consistency of the
individual series produced by the seven IGS analysis centers approached the
10 cm level, the accuracy of the official (combined) IGS orbit is of the
same order of magnitude. The essential advantage of the IGS orbit is its
reliability. The coordinator suggested to produce a combined IGS pole in
addition to the combined IGS orbit in future. It was generally
acknowledged that this is the best possible procedure for the rapid
products. The new procedure will be set up soon. The Bulletin B products
still will be used for the IGS final orbits. The Central Bureau Report
revealed that the IGS network was considerably growing (from thirteen to
well over fifty permanent tracking sites) since the GIG campaign in 1991.
Today we may speak of a truly global network, although gaps still exist in
Africa and in Asia. Thanks to the involvement of big research institutions
like the JPL and the GFZ but also thanks to National Survey Agencies like
AUSLIG many gaps (in particular in South America, in the Pacific, in and
around Australia) could be considerably reduced or even removed. The
growth of the IGS network is a convincing proof for the IGS idea as it was
initiated by Ivan Mueller, Ruth Neilan, William Melbourne, and
Gerry Mader in 1989 in Edinburgh. Dennis McCarthy's report (IERS Rapid
Service Subbureau) showed that today the IERS earth rotation series heavily
rely on the more and more consistent and accurate IGS contributions. The
reports of the seven analysis centers were of greatest interest for the
insider. Rapid products, troposphere estimates, ionosphere models
undoubtedly will be important issues in future. It will be worthwile to
carefully study the contributions of this session in the proceedings.

The second half of Monday afternoon was reserved for a "closer look into
the more distant future" (!). It was pointed out in the presentation
prepared by W. Melbourne and R. Neilan that the IGS, its network and
orbits, are also a corner-stone for spaceborne applications of the GPS.
Also, a closer connection with geophysics (ionosphere models, troposphere
models) was predicted. The troposphere aspect was discussed in detail in
the presentation by M. Bevis. Obviously, with only a comparatively small
additional effort (essentially the deployment of high precision barometers
in the IGS network and the use of this information by the analysis centers)
the IGS will be capable of making remarkable contributions to climatology.
Should rapid IGS products (within hours rather than weeks) eventually
become available the contribution to weather prediction would become of
vital interest. In the discussion a two-step approach (deployment of
barometers and treatment of climatologic aspects by IGS analysis centers as
a first step, development of precise rapid products in a second step)
seemed to find general approval. N. Jakowski's presentation brought
insight into the value of the IGS network from the point of view of
atmosphere physics. High spatial resolution and the global station
distribution make the IGS network very interesting for ionosphere physics.
The production of ionosphere maps and the study of special phenomena may be
supported by the IGS network. G. Johnston from the RACAL Company
confirmed that the IGS products (orbits in particular) are of great
importance to the commercial community. More rapid products, ideally
predicted orbits of sub-meter quality for real-time applications would be
of even greater value. R. Ware underlined the fact that the
contribution of the GPS to geophysics will grow considerably in future. He
also pointed out that some of the instrumentation needed at the ground
stations for the estimation of the precipitable water content is already
available.

The first day was concluded with an informal but instructive tour of the
GFZ. An informal grill-party in the garden of the Einstein Science Park
was very much appreciated by the attendants of the workshop.

"The IGS and the Earth's Atmosphere" was the topic of session 3 on Tuesday
morning, May 16. Different approaches to model the ionosphere were
presented before the coffee break. The report by B. Wilson gave
an impression of the progress made at JPL. The code measurements of the
entire network were used to produce global ionosphere maps which could be
compared to independent TEC estimates stemming e.g. from the
TOPEX/POSEIDON mission. An agreement of a few TECs was reported. Regional
and global ionosphere maps were also presented by St. Schaer using
the phase information on the double difference level. The resulting
single-layer models assumes of course that the mapping function is known.
Absolute calibration may be a problem, the operational aspects are easily
dealt with. L. Wanninger pointed out that at present we are again enjoying
years of low solar activity. Geomagnetic storms and short period
fluctuations of the ionosphere will become important issues again in a
few years. Geophysical information (like the kp index) might be extracted
from the IGS network. A. Jungstand pointed out the value of the IGS network
for a real-time monitoring of the earth's atmosphere. J. Dow reported
that local and regional ionosphere models using different base functions
are currently under development at ESA.

After the coffee break the speakers focussed on the troposphere. From the
papers presented by R. Ware and M. Schenewerk one could again conclude
that GPS sensing of atmospheric water vapour may be done in a relatively
easy way using the IGS network. The paper presented by G. Gendt clearly
showed that already today a high degree of consistency is reached in the
troposphere estimates of different IGS processing centers.

"Orbits, Clocks, and other Modeling Issues" was the title of session 4. It
was opened by an indepth review of the attitude model for GPS satellites by
Y. Bar-Sever. Taking into account the pure geometrical effect and
more closely constraining the stochastic part of the force field led to a
significant improvement of JPL's routine orbit computation. Implementation
of this new attitude model will be an issue for other IGS analysis centers,
too. M. Rothacher argued that the Rock4/42 models are no longer adequate,
that they should be replaced in future by empirical models with parameters
to be estimated. It is expected that the long-arc performance will be
improved considerably. Two speakers (T. Martin-Muir resp.  P. Tetreault)
reviewed the clock estimation procedures at ESA resp. EMR. J. Zumberge had
a closer look at formats (always a controversial topic in public). He gave
arguments to define a new format (file type) for satellite and station
clocks (in addition to the satellite clock information which is included in
the SP3 orbit format).

The day ended in peace and harmony on a boat trip including an excellent
dinner -- nobody complained about insufficient formats in this context !

Session 5 on Wednesday morning was devoted to the "Densification of the
ITRF through Regional GPS Networks" and technical aspects related to it.
It became clear that the SINEX format is essentially ready (P. Davis),
that the ITRF section of the IERS Central Bureau will accompany the IGS
pilot project (C. Boucher), and that important technical issues have to be
solved when different antenna types are used (M. Rothacher). The pilot
phase of this project is about to begin; it will be announced in a separate
IGS-Mail. P. Fang underlined that rapid orbits are considered as a serious
issue at SIO in particular for regional geodynamics networks. K. Larson
used selected portions of archived IGS data to deduce Australian,
Antarctic, Pacific, and Indian plate velocities. The session was concluded
with reports about regional networks, which may be considered as potential
contributors for the densification which the IGS community has in mind and
as typical examples for many other permanent regional arrays built up at
present:  J. Kakkuri gave an overview of Finnish permanent array, P.
Pesec presented the activities of the Central European Regional Geodynamics
Project, M. Schenewerk introduced the US CORS network.

The workshop was closed around 1 p.m. on Wednesday, May 17. The attendants
gained deeper insight into atmosphere modeling aspects (troposphere
and ionosphere) and into the state of the densification project. There can
be little doubt that the workshop will have a long-lasting effect, provided
the IGS community will draw the right conclusions and set up the
necessary action items. Such conclusions were drafted at the "wrap-up
meeting" of those Governing Board members still present on Wednesday
afternoon. Decisions have to be taken at the regular Governing Board
meeting in July 1995 in Boulder.

Organizing a workshop with about 100 participants is always a major
challenge. The convenors, the program committee, and the local organizing
committee, Christoph Reigber, Galina Dick, and Gerd Gendt in particular,
may look back to an interesting, intensive and a very successful event. A
long-lasting applause proved that everybody agreed with that particular
conclusion !


                                                   Gerhard Beutler
                                                   on behalf of the
                                                   IGS Governing Board



[Mailed From: Gerhard Beutler <BEUTLER @ aiub.unibe.ch>]