2020-09-25

SC 1.2: Global Reference Frames

Chair: Xavier Collilieux (France)

Terms of Reference

Sub-commission 1.2 focuses its activity on the definition and realization of the terrestrial reference system (TRS). The TRS realization, named Terrestrial Reference Frame (TRF), is fundamental to study and locate global phenomena or objects at the Earth's surface, in the ocean or in space. It is used as the basis of several operational observation system processing chains such as sea level determination from space and Earth’s rotation monitoring but is also used for most regional and national TRFs. Thus, TRF specifications in terms of origin, scale and orientation have to be optimally realized to satisfy user needs. That’s why sub-commission 1.2 shall study either fundamental questions or more practical aspects that could improve current TRF determinations. Thanks to the accumulation of space geodesy observations and progress in modeling and analysis, non-stationary Earth surface displacements are nowadays clearly evidenced. The next generation of TRF should be able to explicitly model them or should be constructed in such a way that those displacements are accurately modelled. There are currently two different approaches to represent the TRF: Long-term linear and nonlinear TRFs. Time series of quasi-instantaneous frames are proposed but practical implementations still need to be investigated so that the implicit reference frame definition reach the required accuracy. Augmented parametric TRF, coupled with enhanced forward displacement models is an alternative to TRF time series. This approach is in agreement with past modeling of the International Terrestrial Reference Frame (ITRF) but still require progress in forward models (e.g. loading and post-seismic deformations). The dominant non-steady displacement signal is the geocenter motion which is related to the origin definition of the frame. While its main contribution is included in non-tidal loading forward models and while it can be observed by space geodesy, there are still open questions regarding its annual variation. Technique systematic errors still exist in space geodesy products, which impact the TRF definition, especially the scale parameter. Dedicated satellite missions with onboard multi-technique sensors could improve further our understanding of technique systematic errors thanks to solving parameters common tomultiple techniques. However, a set of accurate tie vectors that relate position of various technique instruments at co-location sites will still be of outmost importance to validate those new space-ties and monitor their long-term variations. In parallel, due to the high cost of local tie surveys, it is worth investigating supplementary ways to monitor reference point variations with time. Here, the potential of PSInSAR technique to investigate ground/monument deformation is proposed. A step forward could be established by investigating relativistic reference frames based on a network of -clocks in space linked with time transfer technologies. Such realized frame would be entirely decoupled from ground fixed stations and could be used to reference any point on the Earth's surface. The relativistic frequency shift between clocks in space and on the ground would be a direct measurement of the Earth gravity potential. This technology can be used to realize a world height system based on a network of ground clocks. While this ultimate goal still requires intensive research works, TRF and future World Height Systems need to be studied in closer partnership in order to connect reference benchmarks, gravimeters or clocks to the TRF but also to provide consistent coordinate and height time-variations. The work of this sub-commission will be done in partnership with the International Earth Rotation and Reference Systems Service (IERS) as well as IAG Global Geodetic Observing System (GGOS).

Objectives

The main objectives of sub-commission 1.2 are the following:

  • Definition of the global terrestrial reference frame (origin, scale and orientation, time evolution,standards, conventions, models);
  • Methods to determine local tie vectors and to relate instrument reference points to surveyed groundmarkers;
  • Investigate new methods to determine relative motions at co-location sites;
  • Evaluation of technique systematic errors by focusing on errors at co-location sites;
  • Enhanced forward modeling of the Earth’s surface deformation;
  • Modeling of the reference frame in general relativity;
  • Linking global height reference frames with the terrestrial reference frame;
  • Pursue studies and investigation related to multi-technique satellites (space ties) and concepts of novel dedicated missions with onboard multi-technique sensors.

Link to Services

Sub-Commission 1.2 will establish close links to relevant services for geodetic reference frames, namely the IERS, GGOS and IAG technique services: International GPS Service (IGS), International Laser Ranging Service (ILRS), International VLBI Service for Geodesy and Astrometry (IVS), and International DORIS Service (IDS). A close link with the IERS Conventions Center will be also maintained, especially for chapter 4 (“Terrestrial Reference Systems and Frames”) updates.

Working Groups of Sub-Commission 1.2

WG 1.2.1: Assessing impacts of loading on Reference Frame realizations
Chair: Anthony Mémin (France)

JWG 1.2.2: Methodology for surveying geodetic instrument reference points
(joint with IERS)
Chair: Ryan Hippenstiel (USA)
Vice-Chair: Sten Bergstrand (France)

JWG 1.2.3: Toward reconciling Geocenter Motion estimates
(joint with IERS)
Chair: Kristel Chanard (France)
Vice-Chair: Alexandre Couhert (France)


Study Groups of Sub-Commission 1.2

SG 1.2.1: Relevance of PSInSAR analyses at ITRF co-location sites
Chair: Xavier Collilieux (France)
Vice-Chair: Thomas Fuhrmann (Australia)


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