JSG T.35

JSG T.35: Definition of next generation terrestrial reference frames

Chair: Christopher Kotsakis (Greece)
Affiliation:Comm. 1 and GGOS

Terms of Reference

A Terrestrial Reference Frame (TRF) is required for measuring the Earth orientation in space, for positioning objects at the Earth’s surface as well as satellites in orbit around the Earth, and for the analysis of geophysical processes and their spatiotemporal variations. TRFs are currently constructed by sets of tri-dimensional coordinates of ground stations, which implicitly realize the three orthogonal axes of the corresponding frame. To account for Earth’s deformations, these coordinates have been commonly modelled as piece-wise linear functions of time which are estimated from space geodetic data under various processing strategies, resulting to the usual type of geodetic frame solutions in terms of station coordinates (at some reference epoch) and constant velocities. Most recently, post-seismic deformation has been added as well in geodetic frame solutions. The requirements of the Earth science community for the accuracy level of such secular TRFs for present-day applications are in the order of 1 mm and 0.1 mm/year, which is not generally achievable at the present time. Improvements in data analysis models, coordinate variation models, optimal estimation procedures and datum definition choices (e.g. NNR conditions) should still be investigated in order to enhance the present positioning accuracy under the “linear” TRF framework.

Moreover, the consideration of seasonal changes in the station positions due to the effect of geophysical loading signals and other complex tectonic motions has created an additional interest towards the development of “non-linear” TRFs aiming to provide highly accurate coordinates of the quasi-instantaneous positions in a global network. This approach overcomes the limitation of global secular frames which model the average positions over a long time span, yet it creates significant new challenges and open problems that need to be resolved to meet the aforementioned accuracy requirements.

The above considerations provide the motivation for this JSG whose work will be focused to studying and improving the current approaches for the definition and realization of global TRFs from space geodetic data, in support of Earth mapping and monitoring applications. The principal aim is to identify the major issues causing the current internal/external accuracy limitations in global TRF solutions, and to investigate possible ways to overcome them either in the linear or the non-linear modeling framework.

Objectives

• To review and compare from the theoretical point of view the current approaches for the definition and realization of global TRFs, including data reduction strategies and frame estimation methodologies.
• To evaluate the distortion caused by hidden datum information within the unconstrained normal equations (NEQs) to combination solutions by the “minimum constraints” approach, and to develop efficient tools enforcing the appropriate rank deficiency in input NEQs when computing TRF solutions.
• To study the role of the 7/14-parameter Helmert transformation model in handling non-linear (non-secular) global frames, as well as to investigate the frame transformation problem in the presence of modeled seasonal variations in the respective coordinates.
• To study theoretical and numerical aspects of the stacking problem, both at the NEQ level and at the coordinate time-series level, with unknown non-linear seasonal terms when estimating a global frame from space geodetic data.
• To compare the aforementioned methodology with other alternative approaches in non-linear frame modeling, such as the computation of high-rate time series of global TRFs.
• To investigate the modeling choices for the datum definition in global TRFs with particular emphasis on the frame orientation and the different types of no-net-rotation (NNR) conditions.

Program of activities

• Active participation at major geodetic meetings, promotion of related sessions at international scientific symposia and publication of important findings related to the JSG objectives.
• Proposal for a state-of-art review paper in global frame theory, realization methodologies and open problems, co-authored by the JSG members.
• Organize a related session at the forthcoming Hotine-Marussi Symposium.
• Launching a web page with emphasis on exchange of research ideas, recent results, updated bibliographic list of references and relevant publications from other disciplines.

Membership

Christopher Kotsakis (Greece), chair
Zuheir Altamimi (France)
Michael Bevis (USA)
Mathis Bloßfeld (Germany)
David Coulot (France)
Athanasios Dermanis (Greece)
Richard Gross (USA)
Tom Herring (USA)
Michael Schindelegger (Austria)
Manuela Seitz (Germany)
Krzysztof Sośnica (Poland)