President: S. Zerbini (Italy)

Vice-Presidents: B. Richter (Germany)

H.G. Kahle (Switzerland)

Secretary: T. Van Dam (USA)

Space geodetic measurements provide nowadays the mean to observe deformation and movements of the Earth’s crust at global, regional and local scale. This is a considerable contribution to global geodynamics by supplying primary constraints for modeling the planet as a whole on the one hand, but also for understanding geophysical phenomena occurring at smaller scales. There are many geodetic signals which can be observed and are representative of the deformation mechanisms of the Earth’s crust at different spatial and temporal scales. The time scales range from seconds to million of years in the case of plate tectonics and from millimeters to continental dimension for the spatial scales.

Gravimetry, both absolute and relative, is a powerful tool providing information to the global terrestrial gravity field and its temporal variations. Superconducting gravimeters allow a continuous acquisition of the gravity signal at a given site with precision of 10^-10. This is important in order to be able to detect and model environmental perturbing effects as well as the weak gravity signals associated with vertical crustal movements of the order of mm/yr.

These geodetic observations together with other geophysical and geological sources of information provide the mean to understanding the structure, dynamics and evolution off the Earth system.

The Commission on Crustal Deformation comprises six sub-Commissions, namely:

1. WEGENER (Working group of European Geoscientists for the Establishment of Networks for Earth science Research);
2. APSG (Asia-Pacific Space Geodynamics program);
3. Central and South America;
4. Antarctica;
5. Africa;
6. Geodetic and Geodynamics programs of the Central European initiative;

encompassing most of the tectonically active areas of the Planet. The sub-Commissions will be dealing with main scientific objectives having common general aspects and, in parallel to these objectives, will follow the development of technology and measurement techniques capable to best fulfil the scientific objectives. This will allow a close interaction between the various sub-Commissions which shall organize, on a regular basis, conferences/assemblies of the sub-Commissions themselves or for selected scientific/technological subjects of common interest to most of them.

Primary, general objectives of the Commission will be:

1. to study tectonic motions, including plate deformation;
2. to study postglacial rebound;
3. to study sea-level fluctuations and changes in relation to vertical tectonics along many parts of the coastlines and in relation to environmental fluctuations/changes affecting the geodetic observations;
4. to promote, develop and coordinate international programs related to observations, analysis and data interpretation for the three fields of investigation mentioned above;
5. to promote the development of appropriate models.

Bureau

President: S. Zerbini (Italy)

Vice-Presidents: B. Richter (Germany)

H.G. Kahle (Switzerland)

Secretary: T. Van Dam (USA)

Members of Bureau:

25. Y. Bock (USA)

Chairperson: A. Tealeb (Egypt)

Chairperson: t.b.d

Geodetic And Geodynamic Programmes Of The Central European Initiative Sub-Commission (GGPCEI)

Chairperson: H.J. Sledzinski (Poland)

Chairperson: A. Capra (Italy)

Chairperson: S. Zerbini (Italy) to be changed

B. Ambrosius (The Netherlands)

T. Baker (UK)

L. Bastos (Portugal)

G. Bianco (Italy)

G. Blewitt (USA)

T. Clark (USA)

J. Degnan (USA)

B. Richter (Germany-Chair)

I. Tomasi (Italy)

Chairpersons will change and a few changes will take place in the Board Membership as well.

Chairperson: Ye Shuhua (China)

The primary objectives of study for the APSG are to:

Management Board:

Chair: E. Groten

Chair: J. Rueger

President: Michael Bevis (USA)

Vice President: Reinhard Dietrich (Germany)

IAG Special Commission 8 (Sea Level and Ice Sheets) will work so that IAG, IGS, IERS, GLOSS, IOC, etc., work in a coordinated manner.

A PSMSL/IGS technical committee is working on standards for CGPS positioning of tide gauges (TGs). This has proven rather controversial, and given that many of these stations have already been constructed, that there are a variety of approaches in effect, and sound arguments in favor of each approach, this document will end up describing the approaches taken, rather than defining a single standard approach (which would not win universal support). So the idea now is to produce a website describing general principles and case studies. The PSMSL/IGS technical committee will be incorporated into the broader IAG activity under SC8. It is clear that CGPS positioning of TGs will be a major (probably the major) focus of SC8. Operationally, the problem of tracking the shifting levels of the earth's land, ice and sea surfaces is best considered jointly. So the Special Commission has to consider the problem of a global vertical reference system. This obviously intersects with IERS and IGS and their activities with the ITRF.

Christian Le-Provost and Philip Woodworth played an important role in Birmingham, in getting the Oceanographic/ Climate Change community to agree to co-sponsor or co-endorse this IAG special commission. The concept is that there should be only one (international or 'global') group involved in the GPS at gauges business which should be regarded as a "IAG (SC8), IAPSO (CMSLT), IGS special project, PSMSL, GLOSS project". Supporting actions from allied organizations, such as the IGS, are expected

The great advantage of getting IGS involved in the Special Commission is that IGS is an operational orgnanization, and it focuses on getting things done. The GPS positioning of tide gauges will require multiple groups working on the problem. The SC/WG can help coordinate this activity.

Reinhard Deitrich serves as a Vice President of the Special Commission, with a special responsibility to coordinate activities associated with ice surface height changes. This is an important area. SC8 plans are under development but are expected to include the following:

Once the processing groups are up and running, we will need to identify a group that will perform systematic comparisons of the various results. Other activities will be identified in the next few months.

Invitations will be extended to representatives from the following countries:

Chair: R. S. Gross (USA)

To investigate the effects of non-tidal oceanic processes on the Earth‚s rotation, deformation, gravitational field, and geocenter. To foster interactions between the geodetic and oceanographic communities in order to promote greater understanding of these effects.

Chair: Christoph Reigber

Director, Central Bureau, Daniel Gambis

The International Earth Rotation Service (IERS) was created by the IAU and the IUGG in 1987 and it started its activities in January 1988. After ten years of activity the service started to initiate a re-organization which will be completed in mid 2000. The new Terms of Reference formulated and adopted by the IERS Directing Board in March 1999 are given hereafter.

The primary objectives of the IERS are to serve the astronomical, geodetic and geophysical communities by providing the following:

IERS is composed of a broad spectrum of activities performed by governmental or selected commercial organizations.

IERS collects, archives and distributes products to satisfy the objectives of a wide range of applications, research and experimentation. These products include the following:

The accuracies of these products are sufficient to support current scientific and technical objectives including the following:

The IERS accomplishes its mission through the following components:

Some of these components (e.g., Technique Centers) may be autonomous operations, structurally independent from IERS, but which cooperate with the IERS. A participating organization may also function as one or several of these components (except as a Directing Board).

Information on IERS global products is provided by FTP, World Wide Web, Technical Notes and Annual Reports. Detailed information about the service and its functions can be accessed via http://hpiers.obspm.fr/

(Membership will be modified mid 2000)

Director: P. L. Woodworth (UK)

Since 1933, the Permanent Service for Mean Sea Level (PSMSL) has been responsible for the collection, publication, analysis and interpretation of sea level data from the global network of tide gauges. It is based at the Proudman Oceanographic Laboratory, Bidston Observatory, United Kingdom and is a member of the Federation of Astronomical and Geophysical Data Analysis Services (FAGS) established by the International Council of Scientific Unions (ICSU). It is supported by FAGS, by the Intergovernmental Oceanographic Commission (IOC) and by the U.K. Natural Environment Research Council. Information on FAGS and its various services, including the PSMSL, can be found in FAGS (1989).

Copies of the PSMSL dataset can be obtained over the Internet by Anonymous FTP: anonymous ftp to ftp.pol.ac.uk, cd pub/psmsl, and 'get' the README and other files. Copies can also be obtained over the web at: http://www.pol.ac.uk/psmsl/psmsl.info.html

Alternatively, data can be obtained on a CDROM. In special circumstances, subsets of the dataset can be provided on floppy disk or as computer printout.

The PSMSL will be pleased to provide data, information and advice to all organisations and individuals interested in measuring and analysing sea level changes. It should be contacted at:

Permanent Service for Mean Sea Level

Bidston Observatory, Birkenhead

Merseyside CH43 7RA, UK

Phone: (44) 151-653-8633

Email psmsl@pol.ac.uk

The database of the Permanent Service for Mean Sea Level (PSMSL) contains monthly and annual mean values of sea level from over 1800 tide gauge stations around the world.

The PSMSL receives monthly and annual mean values of sea level from almost 200 national authorities, distributed around the world, responsible for sea level monitoring in each country or region. Data from each station are entered directly as received from the authority into the PSMSL raw data file for that station (usually called the METRIC file in PSMSL publications). The monthly and annual means so entered for any one year are necessarily required to be measured to a common datum, although, at this stage, datum continuity between years is not essential. While the PSMSL makes every attempt to spot inconsistent or erroneous data, the responsibility for the monthly and annual means entered into the METRIC files in this way is entirely that of the supplying authority. A description of data checks routinely made by the PSMSL is given below and in Woodworth, Spencer and Alcock (1990) and IOC (1992a).

In order to construct time series of sea level measurements at each station, the monthly and annual means have to be reduced to a common datum. This reduction is performed by the PSMSL making use of the tide gauge datum history provided by the supplying authority. To date, approximately two thirds of the stations in the PSMSL database have had their data adjusted in this way, forming the 'REVISED LOCAL REFERENCE' (or 'RLR') dataset. For scientific purposes, the RLR dataset is normally superior to the 'METRIC', although the latter, which contains the total PSMSL data holdings, can also be analysed bearing in mind the above datum continuity considerations. (See below for further comments on METRIC and RLR differences).

The RLR datum at each station is defined to be approximately 7000mm below mean sea level, with this arbitrary choice made many years ago in order to avoid negative numbers in the resulting RLR monthly and annual mean values. The detailed relationships at each site between RLR datum, benchmark heights, tide gauge zero etc. are not normally required by analysts of the dataset, although they can be made available on request. The contents of the PSMSL dataset were described in a report 'Data Holdings of the PSMSL November 1993' (Spencer and Woodworth, 1993). Further information about the PSMSL, together with maps and plots of the spatial and temporal distribution of PSMSL data, can be found in reports by Woodworth (1990, 1991) and Woodworth, Spencer and Alcock (1990).

In 1988 a meeting of tide gauge experts and geodesists was held at the Woods Hole Oceanographic Institution under the auspices of the IAPSO Comm. on Mean Sea Level and Tides. The conclusions of the meeting were published as Carter et al. (1989). A key recommendation of the meeting was that geocentric coordinates of tide gauge benchmarks, derived primarily from differential Global Positioning System (GPS) measurements relative to International Earth Rotation Service (IERS) 'fundamental points' (but not exclusively e.g. DORIS), should be stored at the PSMSL alongside the sea level data.

A follow-up meeting was held in December 1993 at the Institute of Oceanographic Sciences Deacon Laboratory, Wormley, Surrey to review progress, with the conclusions and recommendations published as Carter (1994). Since the Woods Hole meeting, considerable developments have taken place with the GPS technique in particular and with the organistion of centres to analyse such data, in particular with the development of the International GPS Service for Geodynamics (IGS).

In March 1997 a third major meeting on this topic was held at the Jet Propulsion Laboratory, California organised by the IGS and the PSMSL. A report on the meeting is now available (Neilan et al., 1998). In May 1999 a fourth meeting was held in Toulouse, France alongside the Sixth Meeting of the GLOSS Group of Experts. At that meeting, plans were made for the production of an 'IOC Manual' on 'How to Operate GPS at Tide Gauge Sites'. The PSMSL will work closely with the IGS and other organisations in order to provide time series of vertical land movements alongside the sea level time series.

The Global Sea Level Observing System (GLOSS) is a programme coordinated by the Intergovernmental Oceanographic Commission (IOC) for the establishment of global and regional sea level networks for oceanographic, climate change and coastal research purposes. The main component of GLOSS is the 'Global Core Network' (GCN) of 287 stations around the world for long term climate change and oceanographic sea level monitoring.

The PSMSL took a major lead in the planning of GLOSS, which in the long term will result in a significant improvement in the quantity and quality of data delivered to the PSMSL. Further information on the development of GLOSS from a PSMSL perspective can be found in IOC (1990b, 1991, 1992b) and

Woodworth (1990, 1991, 1998), while the PSMSL can provide details of current status at any time.

Over the past couple of years many GLOSS Contacts at national sea level authorities have contributed detailed information about their tide gauge installations to the PSMSL in order to enable the compilation of a 'GLOSS Handbook'. This publication is the main source reference of information about the GLOSS network. The Handbook, edited by Dr.Lesley Rickards on behalf of the PSMSL and IOC, was first published in early 1991. Later versions have become available also on floppy disk and on CDROM and, most recently, on the web via:

http://www.pol.ac.uk/psmsl/gloss.info.html.

Recent years have seen major efforts to collect higher frequency (typically hourly) sea level data in order to provide an 'in-situ' World Ocean Circulation Experiment (WOCE) dataset, primarily for comparison to and validation of sea level data obtained from satellite radar altimetry (WOCE, 1988a, 1988b). The designated 'WOCE tide gauges' are mostly GLOSS island sites and pairs of gauges across straits and total about 100 stations. There are two WOCE Sea Level Centres (WSLC's): one at the British Oceanographic

Data Centre at Bidston Observatory alongside the PSMSL (the so-called 'delayed mode centre'), and the other at the University of Hawaii Sea Level

Center (the so-called 'fast centre').

Any requests to the PSMSL for WOCE higher frequency data will be automatically passed to the WSLC's for servicing. Alternatively, they may be contacted directly via email at: bodcmail@pol.ac.uk (Attn. Lesley Rickards) for the Bidston WSLC and markm@soest.hawaii.edu (Attn. Mark Merrifield) for the Hawaii WSLC.

Director Time Section : E.F. Arias (BIPM)

Since 1 January 1988 the Bureau International des Poids et Mesures (BIPM) is fully responsible for the maintenance of International Atomic Time (TAI) and of Coordinated Universal Time (UTC).

The BIPM is in charge of :

(a) Time scales

TAI is established on the basis of atomic clock data and atomic frequency standards from some two hundred atomic clocks in nearly 60 laboratories or national centers.

TAI and UTC are made available by the dissemination of corrections to be applied to the readings of the master clocks of the participating laboratories. Since January 1998 TAI has been calculated using one-month blocks of data, instead of two as used previously.

The stability of TAI is about 3x10-15 for averaging times of 1 to 2 months. The TAI scale unit differs from the SI second on the rotating geoid, in values in the range -5x10-15 s to +5x10-15 s during 1999 with an uncertainty of 4x10-15 s.

In addition to TAI, the BIPM establishes a scientific time scale TT(BIPM) for applications requiring ultimate long-term stability. A new version of this time scale, based on data reprocessing, is available every year and covers several past years.

(b) Time comparisons

The activities of the BIPM Time Section are based on accurate time comparisons between remote clocks, which are mostly based on the tracking of GPS satellites. The BIPM organizes these time comparisons by providing international GPS common-view tracking schedules and by checking differential calibration of GPS time receivers. The BIPM treats raw GPS data according to a unified procedure:

Since July 1999 long-distance links are corrected for measured ionospheric delays obtained from IGS ionospheric maps and for precise satellite ephemerides.

The ultimate uncertainty is of a few nanoseconds for a tracking duration of 13 minutes.

Publications

BIPM Data Service