Global Reference Frames
Chair: Xavier Collilieux (France)
Terms of Reference
Global reference frames are fundamental to study and locate global phenomena or objects at the Earth's surface, in the ocean or in space, and to determine Earth's rotation in space. Sub-commission 1.2 focuses its activity on the definition and realization of the terrestrial reference system (TRS) and its link to world height system (WHS). It shall study fundamental questions but also more practical aspects that could improve current terrestrial reference frame (TRF) determinations.
More than 35 years of space geodetic observations are now available. Thanks to this extraordinary datasets, non-stationary Earth surface displacements are now 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 do not affect its accuracy. Time series of frames have been suggested in the past as a potential solution but practical issues still need to be coped with so that the implicit reference frame reach the required accuracy. Multi-technique satellite that should tie all kind of space geodetic could potentially solve most of these practical issues. However, a set of accurate tie vectors that relates position of various technique instruments at co-location sites will still be of outmost importance to validate those new ties and monitor their variations along time. Work on enhanced parametric modeling, coupled with enhanced forward displacement model 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 (loading, seismic). In parallel of this work on the TRF modeling, study of systematic errors in the coordinates provided by various techniques is still mandatory to improve the homogeneity and frame definition of the TRF whatever the solution that will be adopted.
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. Difference of frequencies of clocks would inform on Earth gravity potential differences, this technology being used in the end to determine a world height system based on a network of ground clocks. In such a framework, distinction between geometric and gravity based height system disappears.
While this ultimate goal still requires intensive research works, TRF and future WHS need to be studied in closer partnership in order to locate reference benchmarks, gravimeters or clocks in the TRF but also to provide consistent coordinate and altitude time-variations.
The work of this sub-commission will be done in partnership with the International Earth Rotation and Reference Systems Service (IERS), IAG commission 2 and Inter-Commission Committee on Theory (ICCT) as well as IAG Global Geodetic Observing System (GGOS). Cooperation with International Astronomical Union (IAU) and International Organization for standardization (ISO) will be also continued.
The main objectives of sub-commission 1.2 are the following:
- Standardization activities: ISO, IERS conventions, GGOS bureau for standards;
- Definition of the global terrestrial reference frame (origin, scale and orientation, time evolution, standards, conventions, models);
- Enhanced forward modeling of the Earth’s deformation;
- Modeling of the reference frame in general relativity;
- Linking global height system with the terrestrial reference frame;
- Evaluation of systematic errors by focusing on errors at co-location sites and offset detection methods;
- Methods to determine local tie vectors;
- Impact of multi-technique satellites (space ties).
Links to Services
Sub-Commission 1.2 will establish close links to the relevant services for 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).