IAG | 2021-04-20

JWG 1.2.3: Toward reconciling Geocenter Motion estimates (joint with IERS)

(joint with IERS)
Chair: Kristel Chanard (France)

Vice-Chair: Alexandre Couhert (France)

Terms of Reference

The International Terrestrial Reference Frame (ITRF) origin is realized through Satellite Laser Ranging (SLR) orbit dynamics determining the Center of Mass (CM) of the Earth system, e.g. the solid Earth and its fluid envelops. The ITRF origin is considered, over secular time scales, to be the mean Earth CM, averaged over the time span of SLR observations (IERS Conventions 2010). Over shorter time scales, the ITRF origin behaves as an approximated Center of Figure (CF) of the solid Earth surface. The motion of CM with respect to CF is commonly called geocenter motion. For number of operational and scientific applications, such as improving the ITRF accuracy or refining estimates of sea level variations, the ITRF origin should coincide with CM at any time. Thus, accessing true geocentric positions requires, to this day, to adopt a model for geocenter motion. However, due to discrepancies in models derived from various techniques and methods, no conventional model for geocenter motion has not been conventionally accepted yet. It is therefore the focus of this working group to identify scientific and technical obstacles leading to inconsistencies in geocenter motion estimates obtained from various geodetic techniques or forward geophysical models. Consequently, the working group will first gather geocenter motion time series derived from geodetic products, along with detailed information on methods of estimation, compare estimates and closely investigate discrepancies. We seek to identify potential sources of geodetic systematic errors and/or inconsistencies in methodologies used to retrieve geocenter motion (network effect, etc.), at both the annual and interannual time periods. A special attention will then be given to improving and/or developing new methods, less sensitive to errors in geodetic products and provide refined geocenter motion estimates.



·         To review all methods to estimate geocenter motion, both from geodetic data and forward geophysical modelling, and systematically compare results.

·         To focus on discrepancies in geocenter motion estimates and investigate potential biases in methods and/or systematic errors in geodetic products.

·         To study the relative merit of geocenter motion data types (SLR, DORIS, GNSS, GNSS+LEOs). Special emphasis should be placed in evaluating the network-effect biases.

·         To evaluate consistencies in methods used to retrieve geocenter motion (translational and inverse approaches, forward modelling).

·         To assess the impact of errors in geocenter motion through variability in estimates for operational and scientific users.


Program of Activities

·         Organize a group meeting to discuss the above objectives.

·         Gather estimates of geocenter motion from working group members and proceed to systematic comparison highlighting discrepancies.

·         Publishing a report on the current status of geocenter motion and associated error budget (and possibly provide common components to all estimates as a mean geocenter motion model).

·         Contribution to international meetings and conferences (AGU, EGU, IUGG).

·         Managing a website with all geocenter motion models and detailed information on estimates.

·         Common publications by working group members.


List of members


Kristel Chanard (France), Chair

Xavier Collilieux (France)

Alexandre Couhert (France), Vice-Chair

Robert Dill (Germany)

Suzanne Glaser (Germany)

Christopher Kotsakis (Greece)

Flavien Mercier (France)

Laurent Métivier (France)

Paul Rebischung (France)

John Ries (USA)

Ricardo Riva (Nehterlands)

Krystof Sosnica (Poland)

Dariusz Strugarek (Poland)

Xiaoping Wu (USA)

Radoslaw Zajdel (Poland)


(additional members can be added to this list)

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