Terms of Reference
According to the by-laws of the IAG, the Section V "Geodynamics" deals with the following topics :
|monitoring and study of time-dependent phenomena, such as polar motion, Earth rotation, Earth tides, recent crustal movements, variations of gravity, sea surface topography including mean sea level,|
|geodetic aspects of international geodynamic projects, such as the Lithosphere project,|
|geophysical interpretation of gravity and related data.|
The objectives of Section V are :
Comm V: Earth Tides
Comm XIV Crustal Deformation
SC3: Fundamental Parameters
SC8: Sea Level and Ice Sheets
Joint Working Groups:
IAG/IAPSO Joint Working Group
IERS International Earth Rotation Service
PSMSL The Permanent Service For Mean Sea Level
BIPM Bureau International des Poids et Mesures - Time Section
President: Shuzo Takemoto(Japan)
Vice-President: Jacques Hinderer(France)
Secretary: Oliver Francis(Belgium)
Program of Activities
The objective of the Commission is to promote international cooperation and coordination of investigations related to the observation, preprocessing, analysis and interpretation of earth tides. By earth tides, we understand all phenomena related to the variation of the Earth's gravity field and to the deformation of the Earth's body induced by the tide generating forces, i.e. the forces acting on the Earth due to differential gravitation of the celestial bodies as the Moon, the Sun and the nearby planets. The Commission will: Collaborate with all international and national organizations concerned with the observation, preprocessing, analysis and interpretation of earth tides; Encourage and promote campaigns to develop, compare and calibrate instrumentation for earth tide observations, techniques of operation, procedures for data preprocessing and data analysis; Organize the 14th International Symposium on Earth Tides at Mizusawa Japan in 2000.
Trevor Baker (UK)
President: S. Zerbini (Italy)
Vice-Presidents: B. Richter (Germany)
H.G. Kahle (Switzerland)
Secretary: T. Van Dam (USA)
Terms of Reference
Space geodetic measurements provide nowadays the mean to observe deformation and movements of the Earths 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 Earths 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:
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:
President: S. Zerbini (Italy)
Vice-Presidents: B. Richter (Germany)
H.G. Kahle (Switzerland)
Secretary: T. Van Dam (USA)
Members of Bureau:
Chairperson: A. Tealeb (Egypt)
Central-South America Sub-Commission
Geodetic And Geodynamic Programmes Of The Central European Initiative Sub-Commission (GGPCEI)
Chairperson: H.J. Sledzinski (Poland)
|to integrate the geodynamic research in the region of Central and Southern Europe based on high accuracy space geodetic surveys and to provide a precise geodetic reference frame for studies on geodynamics of Central Europe, in particular on areas Pannonian Basin, Bohemian Massif, Teisseyre-Tornquist Zone, Carpathian Orogenic Belt, Subalpine Region and Balkanides;|
|to provide a reliable three-dimensional tectonic velocity field covering the Central Europe region and integrate it into hierarchically higher level (i.e. global) tectonic models as well as to prepare and publish geotectonic monographs highlighting and summarizing the latest research and studies on regions under study;|
|to support local area geodynamic research, environmental studies, seismic hazard assessment, meteorology etc. in Central Europe region based upon the high accuracy space geodetic measurements carried out on an integrated geodynamic network of permanent GPS stations in CEI countries;|
Chairperson: A. Capra (Italy)
Working group of European Geoscientists for the Establishment of Networks for Earth-science Research Sub-Commission
Chairperson: S. Zerbini (Italy) to be changed
|to study the three-dimensional deformations and gravity along the African-Eurasian plate boundaries and in the adjacent deformation zones in order to contribute to a better understanding of the associated geodynamical processes;|
|to monitor the three-dimensional deformations in a large region centered around Fennoscandia in order to determine the magnitude and extent of the present-day postglacial rebound in that area thereby extending our knowledge about the viscoelastic properties of the Earth;|
|to investigate height and sea-level variations in order to identify and separate the processes contributing to these variations.|
Science advisory Committee
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.
APSG (Asia Pacific Space Geodynamics) Sub-Commission
Chairperson: Ye Shuhua (China)
The primary objectives of study for the APSG are to:
|measure and monitor, using space techniques, the relative motion between the Eurasian, Pacific, Philippine, and Indo-Australian plates including the plate tectonic motion along the boundaries, as well as local crustal deformation;|
|study the evolution and dynamics of the crustal motion of the island-arc system in the Western Pacific boundary zone and the mountain-building zones of the Tibetan Plateau and Southeast Asia;|
|measure and monitor sea-level change in the Asia-Pacific region using space techniques including altimetry and tide gauge data to study the characteristics and causes of the fluctuations in global sea surface;|
|investigate the dynamics of the Earth as a whole (Earth rotation, gravity changes, etc.) and the mass motions within each layer (including the atmosphere, oceans, lithosphere, mantle, and the core) and their dynamic relations; and|
|investigate natural hazards (earthquakes, volcanic eruptions, sea immersion, etc.) in the region as well as their relation with various Earth motions, and provide basic information for the prediction of natural disasters.|
Special Commission 3
Chair: E. Groten
|the evaluation and discussion of numerical values of fundamental constants and para-meters together with their variations with time in terms of "current best estimates",|
|the proposal of revision of official values and considerations of related observations of existing "fundamental constants" as well as the derivation and elaboration of consistant reference frames and systems in connection with fundamental parameters.|
Refractive Indices of Light, Infrared and Radio Waves in the Atmosphere
Chair: J. Rueger
Special Commission 8
Sea Level and Ice Sheets
President: Michael Bevis (USA)
Vice President: Reinhard Dietrich (Germany)
Terms of Reference
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:
|Develop a meta-data center for Tide Gauges associated with colocated CGPS stations, or where CGPS stations just happen to be located within 25 km or so. This would be web-based. We would like to host this at the University of Hawaii (UH), and perhaps mirror it elsewhere. This metadata center would contain pointers to CGPS data (when it is public) in IGS data centers, and pointers to the TG data in the oceanographic data centers. It would also contain information about leveling ties between the CGPS stations, the TG, and the TGBMs.|
|Encourage the international community to make public all CGPS data collected at or near TGs. Accurate vertical positioning in a global frame is very difficult, and many of us believe it will be best to handle it at the global scale. So getting a truly global data set is the best way to go. The problem that this presents is that a fairly complete dataset will not appear until months after the data are actually collected.|
|The SC wants to encourage the development of several GPS analysis groups that will specialize in processing CGPS data, much of which is NOT available shortly after collection, for the purposes of building a vertical reference system that includes most of the CGPS stations at TGs, as well as many other high quality CGPS stations associated with the IGS tracking network (and the realization of ITRF). At present only one group (Boucher et al.) has committed itself to seeking funds for this purpose. The SC will write letters of support for inclusion in the proposals that specific groups submit to start up processing groups of this kind. We think it important that we have at least three groups processing the data redundantly, and using more than one software package. It is necessary that they accept RINEX as input, not SINEX.|
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:
IAG/IAPSO Joint Working Group
Geodetic Effects of Non-Tidal Oceanic Processes
Chair: R. S. Gross (USA)
To investigate the effects of non-tidal oceanic processes on the Earths rotation, deformation, gravitational field, and geocenter. To foster interactions between the geodetic and oceanographic communities in order to promote greater understanding of these effects.
|Investigate the effects of non-tidal oceanic processes on the Earths rotation, deformation, gravitational field, and geocenter by analyzing both oceanographic data and the products of Oceanic General Circulation Models (OGCMs).|
|Investigate the response of the oceans to atmospheric pressure changes.|
|Investigate methods of improving the representation of mass signals in OGCMs which depend, among other factors, on the formulation of the continuity equation and on the freshwater flux boundary conditions.|
|Promote improved computations of the geodetic effects of oceanic processes by advocating for the inclusion of atmospheric pressure in addition to surface winds as forcing mechanisms of OGCMs and for the assimilation by OGCMs of oceanographic data such as sea surface height and ocean-bottom pressure measurements.|
International Earth Rotation Service (IERS)
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.
Terms of Reference (short version)
The primary objectives of the IERS are to serve the astronomical, geodetic and geophysical communities by providing the following:
|The International Celestial Reference System (ICRS) and its realization, the International Celestial Reference Frame (ICRF).|
|The International Terrestrial Reference System (ITRS) and ist realization, the International Terrestrial Reference Frame (ITRF).|
|Earth orientation parameters required to study earth orientation variations and to transform between the ICRF and the ITRF.|
|Geophysical data to interpret time/space variations in the ICRF, ITRF or earth orientation parameters, and model such variations.|
|Standards, constants and models (i.e., conventions) encouraging international adherence.|
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:
|International Celestial Reference Frame.|
|International Terrestrial Reference Frame.|
|Monthly Earth orientation data.|
|Daily rapid service estimates of near real-time earth orientation data and their predictions.|
|Announcements of the differences between astronomical and civil time for time distribution by radio stations.|
|Leap second announcements.|
|Products related to global geophysical fluids such as mass and angular momentum distribution.|
|Annual report and technical notes on conventions and other topics.|
|Long term earth orientation information.|
The accuracies of these products are sufficient to support current scientific and technical objectives including the following:
|Fundamental astronomical and geodetic reference systems.|
|Monitoring and modeling earth rotation/orientation.|
|Monitoring and modeling deformations of the solid earth.|
|Monitoring mass variations in the geophysical fluids, including the atmosphere and the hydrosphere.|
|Artificial satellite orbit determination.|
|Geophysical and atmospheric research, studies of dynamical interactions between geophysical fluids and the solid earth.|
The IERS accomplishes its mission through the following components:
|Technique Centers (TC)|
|Product Centers (PC)|
|Combination Research Center(s)|
|Analysis Coordinator (AC)|
|Central Bureau (CB)|
|Directing Board (DB)|
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)
The Permanent Service
For Mean Sea Level
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).
Data and Address
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
PSMSL RLR and Metric Datasets
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).
Geocentric Coordinates Of Tide Gauge Benchmarks
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.
Linkage to the IOC Gloss Programme and to WOCE Sea Level Centres
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:
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: firstname.lastname@example.org (Attn. Lesley Rickards) for the Bidston WSLC and email@example.com (Attn. Mark Merrifield) for the Hawaii WSLC.
Bureau International des
Poids et Mesures (BIPM) - Time Section
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 :
|establishing TAI and UTC (except for the UTC leap second occurence and announcement, in charge of IERS),|
|providing the data making TAI and UTC available in the standard laboratories,|
|participating to the worldwide coordination for time comparisons.|
Activities and services
(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:
|Only strict GPS common views are used to minimize Selective Availability effects.|
|The international network figures local stars on continental distances added to two long-distance links between the NIST (Boulder, Colorado, USA), the CRL (Tokyo, Japan) and the OP (Paris, France),|
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.
Data and Publications
|Circular T (monthly): Corrections to the readings of laboratory clocks to get TAI and UTC. Data on time comparisons. Informations.|
|Annual Report of Time Section of BIPM: Methods of evaluation of TAI. Data on the clocks and time comparisons. Data from the primary frequency standards, BIPM results on time scales.|
|Schedules for GPS and GLONASS satellite tracking (for participating laboratories), issued about twice a year.|
BIPM Data Service
|Internet http://www.bipm.fr or anonymous ftp to 188.8.131.52 (subdirectory TAI) See the READ.ME file for complete description.|