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).

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

to be completed!

Objectives

Main 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. 
• Provide recommendations and strategies to improve the scale of the IGS contribution to the next ITRF releases.
• Investigate methods for in-situ calibrations of the phase response of GNSS antenna.

Members

• Paul Rebischung (France); Chair
• Tom Herring (USA); Vice-Chair

Corresponding Members

• Georgios Vergos (Greece), representative of IFGS

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)
• Rolf Dach (Switzerland)
• Linda Geisser (Switzerland)
• Frank Lemoine (USA)
• Anton Reinhold (Germany)
• Sergei Rudenko (Germany)
• Erik Schönemann (ESA)
• Patrick Schreiner (Germany)
• Daniela Thaller (Germany)