2024-07-28

SC 1.2: Global Reference Frames

Chair: Mathis Bloßfeld (Germany)

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 as well as for most regional and national TRFs. In addition, especially for the precise determination of near-Earth satellite orbits, a TRF plays a fundamental role. 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. It is of outmost importance to establish a quality control for TRF realizations since precise as well as long-term stable station coordinates serve as a backbone of numerous geosciences. A first step is to compare the different global TRF solutions currently provided on a non-frequent basis by the three IERS ITRS combination centres. Thanks to technological achievements and the development in the analysis of space-geodetic observations (such as GNSS, SLR, VLBI, and DORIS observations), more than one space technique provides sufficient sensitivity to geodetic datum parameters of TRF realizations. Of special interest is the potential contribution of GNSS to the global scale realization which is usually based on SLR and VLBI only. Besides SLR, also GNSS and DORIS might reliably contribute to the origin realization in the near future. 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 TRF should be able to explicitly model them or should be constructed in such a way that those displacements are accurately modeled. Thereby, not only non-tidal loading models of different Earth system sub-components shall be used but also other geophysical models which are capable to picture global and regional geophysical phenomena such as, e.g., glacial isostatic adjustment (GIA). Nevertheless, technique systematic errors still exist in space geodesy products, which impact the TRF, especially its scale parameter. Dedicated satellite missions with onboard multi-technique sensors could improve further our understanding of technique systematic errors thanks to solving parameters common to multiple 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.

A further 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’s 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 the 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);
  • Comparison of existing global TRF solutions;
  • Methods to determine local tie vectors and to relate instrument reference points to surveyed ground markers;
  • Investigation of 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;
  • Pursuing 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 the IAG technique Services: International GNSS 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. In addition, this Sub-Commission will work closely together with the IHRF (International Height Reference Frame) Coordination Center of the International Gravity Field Service  (IGFS).

Corresponding Member

Georgios Vergos (Greece); Representative of IFGS

Working Groups of Sub-Commission 1.2

JWG 1.2.1: GNSS scale information for global reference frames (joint with IGS)

Chair: Paul Rebishung (France)
Vice-Chair: Tom Herring (USA)

Terms of Reference

Since 2007, the International Terrestrial Reference System (ITRS) has been recommended by the IUGG as the preferred geodetic reference system for positioning on and around the Earth, satellite navigation and Earth Science applications. The ITRS is formally defined as a three-dimensional cartesian coordinate system co-rotating with the Earth, with its origin at the center of mass of the whole Earth system, its unit of length (scale) given by the SI meter and its orientation conventionally defined. The ITRS is realized by successive releases of the International Terrestrial Reference Frame (ITRF) which consists of precise coordinates of a global network of geodetic stations, expressed as accurately as possible in the ITRS. To satisfy particularly demanding applications such as the monitoring of sea level rise from satellite altimetry or the establishment of mass budgets from satellite gravimetry, the ITRF coordinates need to be expressed in the ITRS, in terms of origin and scale, with a millimetric accuracy over decades. However, in view of the discrepancies between the scale of the coordinates determined by the individual space geodetic techniques contributing to the ITRF, this requirement does not appear to be met yet.

The ITRF coordinates are derived from the combination of the observations of four space geodetic techniques: DORIS, GNSS, SLR, and VLBI. Only two of those techniques, SLR and VLBI, have so far contributed to the definition of the scale of the successive ITRF releases. Until the advent of the Galileo system, GNSS were indeed considered unable to provide reliable scale information, due to the combination of two factors:

  • the existence, in global GNSS analyses, of a quasi-linear dependency between the average of station heights (i.e., the terrestrial scale), tropospheric zenith wet delays, satellite and station clock offsets, and the radial component of the antenna phase center offsets (z-PCOs) of GNSS satellites;
  • the absence of reliable, publicly available pre-flight calibrations of GNSS satellite antennas.

This situation evolved with the public release of pre-flight antenna calibrations for the Galileo satellites, which put the IGS in a position where it could provide, for the first time, a GNSS-based estimate of the terrestrial scale. This was put into practice in the third IGS reprocessing campaign (repro3), which contributed to the latest release of the ITRF, ITRF2020.

However, the subsequent analysis of the contributions of the four techniques to ITRF2020 conducted at the Institut National de l’Information Géographique et Forestière (IGN) concluded that there was a +4.3 mm scale offset at epoch 2015.0 and a +0.11 mm/yr drift between the Galileo-based scale of the IGS repro3 solutions and the average scale of the SLR and VLBI solutions selected to define the ITRF2020 scale. A comparable offset of +4.9 mm at epoch 2010.0 (but no clear drift) was also found between the repro3 scale and that of the SLR contribution to ITRF2020 in a similar analysis conducted at the Jet Propulsion Laboratory (JPL). A third analysis conducted at the Deutsches Geodätisches Forschungsinstitut at the Technische Universität München (DGFI-TUM) concluded that there was good agreement (0.25 mm at epoch 2010.0; 0.025 mm/yr) between the repro3 scale and that of the VLBI contribution to ITRF2020, but confirmed the existence of an offset and a drift between the repro3 scale and that of the SLR contribution to ITRF2020 (+2.2 mm at epoch 2010.0 and +0.1 mm/yr between the GNSS/VLBI average and SLR).

In this context, it appears worth investigating possible systematic errors in GNSS-based estimates of the terrestrial scale and attempting to draw a budget for these errors. This implies evaluating the accuracy of the pre-flight antenna calibrations available not only for the Galileo satellites, but now also for the GPS Block III, IIF, and IIR satellites, by, e.g., inter-comparisons across satellites, constellations, and signal frequencies. This also implies evaluating the stability of GNSS satellite z-PCOs over their lifetimes. This may also comprise comparisons between scale estimates relying on pre-flight GNSS satellite antenna calibrations with other estimates that rather rely on the dynamical constraint brought by Low Earth Orbiters (LEOs) in the joint processing of GNSS tracking data from ground stations and LEOs. This finally implies evaluating to what extent, besides errors in the GNSS satellite antenna calibrations, other types of errors in GNSS data or their analysis (e.g., inaccuracies of the ground antenna calibrations, multipath, tropospheric delay mismodeling) might bias GNSS-based scale estimates. Some of these other possible contributors may have  implications for scale estimates from other geodetic systems e.g., tropospheric modeling.


Objectives


  • Establish an inventory of the efforts by different groups to determine the terrestrial scale with GNSS;
  • Review the consistency of GNSS-based scale and scale rate estimates across research groups, constellations and frequency combinations, as well as with estimates from other space geodetic techniques;
  • Investigate potential systematic errors in GNSS-based scale and scale rate determination;
  • Investigate methods for in-situ calibrations of the phase response of GNSS antenna;
  • Provide recommendations and strategies to improve the scale of the IGS contribution to the next ITRF releases.

Members

  • Paul Rebischung (France); Chair
  • Tom Herring (USA); Vice-Chair
  • Claudio Abbondanza (USA)
  • Zuheir Altamimi (France)
  • Andria Bilich (USA)
  • Mathis Blossfeld (Germany)
  • Susanne Glaser (Germany)
  • Jing Guo (China)
  • Bruce Haines (USA)
  • Guorong Hu (Australia)
  • Benjamin Männel (Germany)
  • Oliver Montenbruck (Germany)
  • Manuela Seitz (Germany)
  • Peter Steigenberger (Germany)
  • Daniela Thaller (Germany)
  • Arturo Villiger (Switzerland)


JWG 1.2.2: Metrology of space geodetic infrastructure (joint with IERS, GGOS)

Chair: Ryan Hippenstiel (USA)
Vice-Chair: Cornelia Eschelbach (Germany)

Terms of Reference

The ITRS is built upon multiple geodetic techniques: SLR, VLBI, DORIS, and GNSS. At locations where these techniques are co-located, it is vital to determine and understand the vectors between the reference points of each technique. These vectors are determined by local tie surveys conducted terrestrially with various procedures and geodetic instruments. The reference points should be collected and properly aligned to a global reference frame to produce relative and absolute coordinates. As the science of local tie surveys has developed, so has technology and the expectation of higher precision and improved protocols. It is the desire of this JWG to investigate the current and expected best practices available, along with documenting past efforts, both in the field and researched. This JWG will share methodology of existing tie surveys, continued to develop and document recommended procedures, and archive surveys completed by all agencies represented. In addition, efforts will be made to isolate systematic errors of the space geodetic techniques using surveying methods and investigate field procedures that could be completed during a tie survey to provide the operator valuable feedback on potential physical errors found onsite. One critical example of this is quantifying thermal and gravitational deformation in VLBI sensors.

It is the overall goal of this JWG to encourage consistent field practice, terminology, and documentation throughout the community, with a continued eye on the future of tie surveys.

Objectives

Enhance and improve knowledge of space-geodetic infrastructure through precise local tie techniques, research on practices and error sources, and engagement with new technologies.

Program of Activities

  • Investigate thermal and gravitational deformation;
  • Consider importance and inclusion of DoV observations;
  • Discuss overall error budget and precision (achieved/necessary) considering the above;
  • Continue to enhance guidelines on procedures (and subsequent feedback for improvement);
  • Archive reports, tie vectors and raw data of all agencies conducting tie surveys;
  • Gather, distribute, and maintain publications on related matters;
  • Coordinate with and solicit feedback from all geodetic techniques on developments;
  • Participate in local tie surveys and/or testing of new methodologies;
  • Attend meetings or participate in virtual/online discussions;
  • Report survey results and/or research efforts towards the Objectives of the JWG;
  • Develop and maintain a library of past and future reference materials.

Members (to be updated as JWG develops)
  • Ryan Hippenstiel (USA); Chair
  • Cornelia Eschelbach (Germany); Vice-Chair
  • Zuheir Altamimi (France)
  • Sten Bergstrand (France)
  • Steven Breidenbach (USA)
  • Benjamin Erickson (USA)
  • Charles Geoghegan (USA)
  • Dionne Hansen (New Zealand)
  • Craig Harrison (Australia)
  • Christopher Holst (Germany)
  • Ulla Kallio (Finland)
  • Michael Lösler (Germany)
  • Kevin Jordan (USA)
  • Saho Matsumoto (Japan)
  • Jack McCubbine (Australia)
  • Damien Pesce (France)
  • Anna Riddell (Australia)
  • Owen Smallfield (New Zealand)
  • Jerome Saunier (France)
  • Elena Martínez Sánchez, (Spain)
  • Daniela Thaller, (Germany)
  • Bart Thomas (Australia)
Corresponding Members
  • Xavier Collilieux (France)
  • Robert Heinkelmann (Germany)


JWG 1.2.3: Impact of geophysical models on reference frames(Joint with Commission 3)

Chair: Jeff Freymueller (USA)

Terms of Reference

This working group, joint between IAG SC 1.2 and IAG SC 3.4 has the goal of assessing and improving geophysical models, to help to develop models that are sufficiently accurate that they could be used to describe non-secular motion in reference frame definition. Two broad categories of models could be considered, with the priority being mass transport models and associated surface loading deformation. Work in the JWG is carried out on a best-effort basis and is described below in the Objectives. Meetings are primarily expected to be virtual/online. However, the goal is to have an in-person meeting at the IAG Scientific Assembly in Fall 2025. A meeting at the Fall 2024 AGU meeting is considered.

The JWG will communicate with other groups under the IAG umbrella that are working on adjacent topics: for example, recommendations about what we need from GIA models.

Objectives for Geophysical Loading Models

  • Make recommendations about quantities that we want/need modelers to compute:
    • Model computations in CM frame, with degree 1 coefficients (with documented units/normalization) to allow models to be expressed in CF or other frames;
    • Geocenter time series or trend (CF minus CM);
    • Intermediate results, like the gravitationally consistent redistribution of mass to the ocean in the case of GIA and present-day mass transport models.

  • Assess existing models to determine the degree of double-counting and missing mass:
    • Continental hydrology;
    • Cryosphere (the largest secular trend in hydrology, but largely missing from most continental hydrology models – but not completely missing either);
    • Non-tidal ocean mass vs gravitational redistribution of water;
    • Groundwater;
    • How do assimilation-based models (such as Gerdener et al.) compare to traditional models?

  • How can we make progress toward a fully integrated global mass change model?

Objectives for Earthquake and Post-seismic Models

  • Assess the state of current models.
  • How do current models (i.e., ITRF PSD models) predict the future?
  • Make recommendations for how to improve such models.

Program of Activities

  • Propose a session for the 2025 AGU or 2026 EGU meeting.
  • Convene a special session at the 2027 IUGG Assembly in Korea, focused on improving global mass transport models.
  • Propose a special session for the 2026 REFAG meeting.
  • Interact with JSG 3.1 (Chair: Lambert Caron):
    • JWG 1.2.3 would like to make recommendations to JSG 3.1 about what kinds of “separate” model computational values should be saved and reported because they are needed and useful for reference frame issues. One example is CF-CM, the trend (and variations if dealing with areas of low viscosity) in the geocenter due to GIA. Also, having the ocean load redistribution and its induced deformation saved and reported separately would be highly useful; and of course, 3D displacements/velocities are needed.
    • JWG 1.2.3 would like to hear from JWG 3.1 about progress towards new models, and assessments of models and uncertainties.
Members (to be updated as JWG develops)

  • Jeff Freymueller (USA); Chair
  • Sophie Coulson (USA)
  • Maylis de la Serve (France)
  • Jean-Paul Boy (France)
  • Laura Jensen (Germany)
  • Anthony Mémin France)
  • Anna Klos (Poland)
  • Claudio Abbondanza (USA)
  • Manuela Seitz (Germany)
  • Laurent Métivier (France)
  • Karen Simon (Canada)


JWG 1.2.4: Evaluation of the terrestrial reference frames (joint with IERS)

Chair: Guilhem Moreaux (France)
Vice-Chair: Andreja Susnik (UK)

Terms of Reference

Periodically, the International Terrestrial Reference System (ITRS) combination centres (CCs) of the International Earth Rotation and Reference Systems Service (IERS), namely the Institut National de l’Information Géographique et Forestière (IGN, France), the Deutsches Geodätisches Forschungsinstitut at the Technische Universität München (DGFI-TUM, Germany) and the Jet Propulsion Laboratory, (JPL, USA), compute new realizations of the ITRS. The official realization is named International Terrestrial Reference Frame (ITRF) and is published by the IERS ITRS product centre at IGN. DGFI-TUM and JPL compute their own ITRS realizations named DTRF and JTRF, respectively. These global TRF solutions comprise accurate station positions and are obtained by a combination of individual contributions of the four space geodetic techniques DORIS, GNSS, SLR and VLBI. All three solutions are based on identical input data of the IAG technique Services, namely the IDS, IGS, ILRS, and IVS. Since each IERS ITRS CC follows its own combination strategy, each solution comprises a solution-specific array of products (e.g., station positions, velocities, post-seismic deformation models, non-tidal loading correction time series, periodic corrections). In addition to the above mentioned ITRS realizations, global techniquespecific ITRF solutions as well as regional densifications of the ITRF are computed by different institutions.

The ITRF is used daily by a huge number of individuals and organizations in applications such as surveying, aircraft navigation, responding to disaster emergency etc. Furthermore, the ITRF provides the fundamental basis needed for a broad variety of Earth system research applications. Examples are the determination, monitoring, and interpretation of global change phenomena on different temporal and spatial time scales (i.e. sea-level rise and ice sheet melting), of Earth Orientation and Rotation, relativity, lunar science, as well as the calibration and evaluation of ocean and ice altimetry missions. Furthermore, it is the basis for the understanding of dynamics and modeling of satellite orbits and can be used in scientific research such as plate tectonics and crustal deformation monitoring and static and time-varying gravity field modeling. In short, the ITRF serves as the backbone in geosciences.

This JWG aims at complementing the evaluation of the ITRS realizations by the IERS ITRS product center with a special focus on the intercomparison of different global TRF solutions. The assessment aims at investigating conceptual differences of the three ITRS realizations with previous realizations based on user and application requirements important for, e.g., the precise orbit determination (POD) of low-, medium- and high-Earth-orbiting satellites and the estimation of Earth Orientation Parameters (EOPs) as well as the (mean) regional/global sea level rise.

Objectives

Main objectives:

  • Intercomparison of the combination strategies followed by the IERS ITRS CCs.
  • Assessment/quantification of station position time series differences between the ITRS realizations and w.r.t. geophysical models (e.g., loading displacements).
  • Promotion and development of alternative rigorous (and independent) methods for the intercomparison of global TRF solutions (e.g., POD of low-, medium- and high-Earth-orbiting satellites).
  • Explore methods and procedures for the quality control of TRF solutions.

Members


  • Guilhem Moreaux (France); Chair
  • Andreja Susnik (UK); Vice-Chair
  • Dimitrios Ampatzidis (Greece)
  • Peter Clarke (UK)
  • Alexandre Couhert (France)
  • Rolf Dach (Switzerland)
  • Linda Geisser (Switzerland)
  • Frank Lemoine (USA)
  • Anton Reinhold (Germany)
  • Sergei Rudenko (Germany)
  • Erik Schönemann (ESA)
  • Patrick Schreiner (Germany)
Corresponding members

  • Claudio Abbondanza (USA)
  • Zuheir Altamimi (France)
  • Paul Rebischung (France)
  • Manuela Seitz (Germany)

Ex-officio members

  • Mathis Blossfeld (Germany)
  • Daniella Thaller (Germany); IERS CB
  • Robert Heinkelmann (Germany); IERS AC

JSG 3.1: Model representation and geodetic signature of solid-Earth rheology in surface loading problems (joint with IAG Commissions 2 and 3)

Chair: Lambert Caron (USA)
Vice-Chair: Rebekka Steffen (Sweden)

Refer to the the structure of Commission 3 on the website.




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