SC 1.4: Interaction of Celestial and Terrestrial Reference Frames

Chair: Maria Karbon (Spain)

Terms of Reference

Sub-commission 1.4 focuses its activity on the understanding of the interplay of the reference frames and their auxiliary components; be it at their inception, their dependencies and differences w.r.t. previous iterations or their evolution over time. The final aim is to enable the achievement of the requirements for future releases. The International Terrestrial Reference Frame (ITRF) and the International Celestial Reference Frame (ICRF) and the link between them, expressed by the Earth Orientation Parameters (EOP), are key products of geodesy and astrometry. The demands on all components of this triad are increasing, as the mm/μas level of accuracy is the present goal of the astronomical and geodetic community.

The current method for the ITRS realization is based on a multi-stage processing of observations made with different spatial geodetic techniques, i.e. VLBI, SLR, GNSS and DORIS. The final ITRF is derived from the combination of all four techniques, but not all contribute equally. Most realizations are dominated by GNSS, due to its unrivaled station network and data volume. VLBI and SLR are predominately used for the determination of the scale, and the latter also defines the origin. The current ITRF2020 is based on a combination of the full time series of the station positions and EOP of the four techniques, and the station positions are augmented by PSD (post-seismic deformation) parametric models and seasonal (annual and semi-annual) signals. However, the increased demand on accuracy forces improved station motion modeling and continuous updates.

The ICRS realization on the other hand, is based solely on VLBI. The current ICRF3 features source positions only, predominantly in the X band frequency (8 GHz). In addition, two independent catalogs observed at K (22 GHz) and Ka band (32 GHz) are included as well. The three individual monolithic solutions are aligned to the X band catalog through a match of common sources for the transfer of the datum. The ESA satellite mission Gaia measures the positions of star images relative within its field of view. Hence, the resulting coordinate system of positions and proper motions has to be aligned with the ICRS via quasars that are also visible in the optical frequency range and have an ICRF3 counterpart. Thus, all four frames are determined independently and then aligned to each other through individual sets of datum sources.

The tie between ITRF and ICRF is established through an arbitrary set of reference VLBI stations. At the same time VLBI relies on the ITRF origin as established by SLR, and simultaneously contributes to its scale together with SLR. All techniques contribute to the positions and velocities of the ITRF stations. At the same time, all reference frames inherit the orientation from their preceding realization by applying NNR constraint based on various subsets of network stations or radio sources. Thus, each new frame inherits the orientation uncertainty of all its predecessors and adds its own to it.

The definition of the EOP is based on the transformation between the ICRS and ITRS. However, while a reference frame may remain unaltered for years until a new version is released, EOP are subject to constant change and depend on continuous updates. Further, many applications need accurate predictions. Nonetheless, the EOP must remain consistent with the most recent reference frames. Similar to the ITRF, the individual space geodetic techniques deliver different EOP components with different accuracy, which are then combined in a multi-step process. This combination is linked to the ITRF and ICRF mainly only by the determination process of the EOP within the individual techniques.

Another reference frame that needs to be considered, is the emerging International Terrestial Gravity Reference System and Frame (ITGRF). It is, like the International Height Reference Frame (IHRF), linked to the ITRF via absolute  gravity observationsat co-located sites. The consistency between these systems needs to be established and ensured over variouse temporal and spacial domains. As it stands, the realization of a global vertical reference system supporting geometric and physical heights is still in its infancy.

Last but not least, the nacent Lunar Reference Frame has to be taken into consideration as well, as updated localization standards for both surface and orbital activities at the Moon are needed for future exlporation missions.

Objectives

The entanglements between ITRF, ICRF and EOP mentioned above lead to convoluted dependencies and complex interactions between them, which need to be carefully investigated and understood to reach the GGOS goals.

While some improvements may be reached by

• reviewing the theory,
• augmenting the models involved,
• refining and unifying the processing strategies,

other limitations are imposed by

• the technological limitations of the observing systems,
• the systematic errors inherent to them.

All these points will be addressed within the respective Working Groups, and due the interdependency of these topics a close collaboration and information exchange is encouraged. This incitement is extended to other Commissions in the IAG and beyond.

WG 1.4.1: Improving and homogenisation of geophysical modeling for a better consistency of the reference frames
Chair: Tobias Nilsson (Sweden)

Terms of Reference

In the data analysis done for a new reference frame realisation (e.g., the ITRF or the ICRF), a multitude of models are used to correct for geophysical effects, for example atmospheric refraction, tides, and geophysical loading. Since the models used in the data analysis for the ITRF and the ICRF are not necessarily identical, this will lead to inconsistencies between the frames. These inconsistencies will also affect the EOP.

The aim of this WG is to investigate the geophysical models used in the data analyses to find out their quality, as well how they impact the reference frames and the EOP.

The work will mainly focus on the TRF and CRF, but emerging reference frames like the ITGRF, IHRF and the reference frame for the moon will also be considered.

Objectives

• Investigate the quality of different geophysical models.
• Develop methods for comparing TRF, CRF and EOP realisations and assessing the consistency between them.
• Perform different studies (theoretical, simulations, and analysis of real data) aimed at determining the impact of different geophysical models on the TRF and CRF and their consistency.
• Assess the consistency between the geophysical models used for the ITRF and the ICRF with those used for the ITGRF, the IHRF, and the lunar refence frame.
• Give recommendations to the IAG technique Services on what models to use in the data analysis and on the importance of using consistent models.

Members preliminary

• Tobias Nilsson (Sweden); Chair
• Sergei Bolotin (USA)
• Susanne Glaser (Germany)
• Robert Heinkelmann (Germany)
• Maria Karbon (Spain)
• Hana Krasna (Austria)
• Lucia McCallum (Australia)
• Daniella Thaller (Germany)
• Minghui Xu (Germany)

WG 1.4.2: Studying and modeling the structure of the AGNs and its evolution over time and frequency for the future CRFs
Chair: Minghui Xu (Germany)

Terms of Reference

The aim of this WG is to characterize, monitor, model, and interpret the structure of the AGNs in order to improve the position accuracy for the next generation CRFs and thus provide a more stable CRF for geodesy. This WG will lead a pilot project to develop a road map to make structure maps, align them over frequency and time in the presence of core shift, and calibrate the visibilities based on them. The final products from this project are group and/or phase delays that are free of the effects of source structure and refer to the positions of the selected feature of the AGNs.

Objectives

• Construct the structure models (aligned images) from VLBI observations, e.g., the VGOS observations and the astrometric VLBI observations from VLBA and compare the different algorithms/methods for deriving images.
• Define the key parameters for the future CRFs and explore different approaches, such as the radio feature defining the reference position, jet direction, and Gaussian parameters of the structure.
• Study the role of the jet directions in VLBI data analysis and the frame ties between multiple radio wavelengths and optical wavelengths for the best frame ties.
• Monitor the structure evolution over time and frequency in geodetic VLBI observations.
• Explore the usage of geodetic VLBI observations for astrophysics by providing aligned images and core-shift measurements on a weekly basis.
• Create a knowledge-transfer result-validation loop in the three disciplines of geodesy, astrometry, and astrophysics on the ground of the new generation of geodetic VLBI system.

Members preliminary

• Minghui Xu (Germnay); Chair
• Robert Heinkelmann (Germany)
• Maria Karbon (Spain)
• Sebastien Lambert (France)
• Zinovy Malkin (Russia)
• Tobias Nilsson (Sweden)

JWG 1.4.3: Consistent realization of TRF, CRF and EOP (joint with IAU Commission A2 and IERS)
Chair: Robert Heinkelmann (Germany)
Vice-Chair: Manuela Seitz (Germany)

Terms of Reference

Many applications, e.g. in geodesy, astronomy, or navigation, rely on the consistency between terrestrial (TRF) and celestial (CRF) reference frames and Earth Orientation Parameters (EOP). The EOP connect the CRF and TRF in terms of their orientation and rotation differences. The EOP can only be considered as physically meaningful when determined consistently with the reference frames. The quality requirements for the applications including societal contributions were  uantified through the GGOS as 1 mm accuracy and 0.1 mm/yr stability, i.e. about 33 μas and 3.3 μas/yr in terms of EOP. For Earth system science based on EOP the consistency is a crucial characteristic.

Today, the quality requirements for reference frames and EOP are not met.

Data and model inconsistency. Currently, TRF and CRF are determined independently of each other. Individual Working Groups (CRF) or Combination Centers (TRF) compute the frames through reprocessing/combination efforts every five to ten years. The releases of the terrestrial and celestial frames do not happen at the same time. In this way, the frames are computed based on different input data and on different analysis models in case of updates of the conventional models. Following independent approaches, the consistency of a new release of one of the frames can only be quantified and thus ensured to the last release of the respective other frame. If the frames are not fully consistent, the EOP based on these frames cannot be consistent.

Multi-technique vs. single technique analysis. DORIS, GNSS, SLR and VLBI observations are combined with local tie vectors at co-location sites for the TRF computation, whereas the CRF is directly connected to the TRF through VLBI alone. This situation does not change when applying alternative data analysis  procedures. Nevertheless,as VLBI networks are sparse in comparison to multi-technique networks, it has been shown that the terrestrial part of the Earth orientation significantly improves through the combination with satellite-based data. The celestial parts of Earth orientation, dUT1 (UT1 ∼ ERA) and CPO, determined by VLBI observations only – and possibly by LLR data –, can in turn improve due to correlations between the EOP within the VLBI data analysis. CRF realizations in  other wavelengths are aligned to the X/S VLBI CRF and thus do not contribute to the CRF orientation for ICRF3.

Nevertheless, they permit an independent validation. Apart from the rotation and spin, catalogues based on Gaia (optical) data releases can provide independent insight into deformations and other technique-dependent systematic errors and thus present another independent validation for the VLBI-based CRF. 

Prediction problem. The reference frames and the EOP are customarily applied in prediction mode, e.g. for geodetic and astrometric data analyses. Accordingly, values have to be given beyond the data time-span considered for the reference frame realization. As long as no significant non-linearity occurs, the global  coordinates can be used very well for predicting the position into the future. For most applications, predicted EOP have to be available as well. These predicted EOP require consistency to the frames and to the reprocessed EOP at the same time. It is impossible to fulfill both requirements when new reference frame releases become available.

Objectives

Addressing the above-mentioned issues, the WG will:

• compute multi-technique CRF-TRF solutions together with EOP in one step, which will serve as a basis to quantify the consistency of the current conventional reference frames and EOP as well as the consistency of reprocessed and predicted EOP;
• investigate the impact of different analysis options, model choices and combination strategies on the consistency between TRF, CRF and EOP;
• study the differences between multi-technique and VLBI-only solutions;
• study the differences between VLBI solutions at different radio wavelengths;
• study the differences between Gaia (optical) and VLBI (radio) reference frames;
• study the effects on the results, when different data time spans are considered;
• compare the practically achievable consistency with the quality requirements theoretically addressed by the GGOS;
• derive conclusions about future observing systems or analysis procedures in case the quality requirements cannot be met with the current infrastructure and approaches.

Members preliminary

• Robert Heinkelmann (Germany); Chair
• Manuela Seiz (Germany); Vice-Chair
• Maria Karbon (Spain)
• Tobias Nilsson (Sweden)
• Minghui Xu (Gernamy)