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SC 1.4 Interaction of Celestial and Terrestrial Reference Frames

Chair: Johannes Böhm (Austria)


In recent years, significant progress has been made in astronomical and geophysical modeling for the analysis of space geodetic observations. Thus, there is the need to investigate the impact of those models on the terrestrial and celestial reference frames (TRF and CRF), and on the consistency between the TRF, CRF, and the Earth orientation (EOP) parameters. Special attention needs to be paid to Very Long Baseline Interferometry (VLBI) observations, since it is the only technique to provide consistent sets of TRF/EOP/CRF. However, the present realization of the ITRF (ITRF2008) is based on a combination of VLBI, GNSS, SLR, and DORIS observations, whereas the present realization of the ICRF (ICRF2) is determined from a single VLBI solution. Consequently, research has to be carried out to integrate the ITRF and ICRF solutions, and also new options like VLBI observations to satellites should be considered for future improvement of the consistency. The GAIA mission scheduled for launch in 2013 is expected to achieve an optical realization of the CRF with precision similar to or better than the ICRF2 and with at least an order of magnitude more objects. As the set of extragalactic objects suitable for both optical and radio observation is limited, such objects will have to be identified, and investigations have to be carried out to permit the best possible connection between the radio and optical CRF realizations.


Sub-Commission Meeting, Vienna, 25 April 2012: » Report and Presentations


WG 1.4.1: Geophysical and Astronomical Effects and the Consistent Determination of Celestial and Terrestrial Reference Frames

Chair: Zinovy Malkin (Russia)


WG 1.4.1 is created to promote and coordinate investigations of the impact of geophysical and astronomical modeling on the terrestrial and celestial reference frames (TRF and CRF) and the consistency between CRF, TRF, and Earth orientation parameters (EOP), the latter serving as the transformation parameters between TRF and CRF. The primary attention will be given to Very Long Baseline Interferometry (VLBI) as the only technique nowadays that can provide highly consistent global solutions for TRF, CRF, and EOP.

Objectives:

  • Encourage and develop cooperation and collaboration in theoretical studies, simulations, and processing of real data aimed at a better understanding of the impact of geophysical and astronomical modeling on TRF, CRF, and EOP derived from VLBI observations.
  • Advance means of comparing models as well as TRF, CRF, and EOP realizations.
  • Compare different theoretical models and their realizations used by VLBI analysis centers. Study the propagation of differences in those models to differences in geodetic and astrometric products.
  • Develop practical recommendations for VLBI analysis centers and the IERS Conventions Center on the optimal models to be used during processing of VLBI observations.

Members: Z. Malkin, J. Böhm, S. Lambert, C. Ma, R. Haas, H. Spicakova, R. Heinkelmann, D. MacMillan.



WG 1.4.2: Co-location on Earth and in Space for the Determination of the Celestial Reference Frame

Chair: Sebastien Lambert (France)


Very Long Baseline Interferometry (VLBI) is the only technique sensitive to the "quasi inertial" celestial reference frame, and the most recent realization of the International Celestial Reference System, the ICRF2, was determined in a VLBI-only solution. The other space geodetic techniques (GNSS, DORIS, SLR) define a dynamical celestial reference frame, but all techniques are combined to determine the International Terrestrial Reference Frame (ITRF) without adding the estimation of sources to that combination, thus causing inconsistencies between the ICRF and the ITRF.

The goal of WG 1.4.2 is to investigate the impact on the ICRF when combining VLBI observations with those from satellite techniques. Historically, this combination is based on local tie information at the co-location sites, but in future also troposphere ties and even space ties could be used, i.e., observing the GNSS constellation or a dedicated micro satellite like GRASP with VLBI, so that a fully consistent system is created.

Objectives:

  • Support the realization of a full combination of the VLBI/GNSS/DORIS/SLR at the normal equation level including estimates of radio source coordinates. This should be done in close cooperation with the IERS Working Group on the Combination at the Observation Level.
  • Investigate various analysis options (local and troposphere ties, twin telescopes) on the results (TRF, EOP, CRF).
  • Analyze VLBI observations to GNSS satellites.
  • Simulate the effect of VLBI observations to GNSS and SLR satellites and assess the impact on the CRF.
  • Simulate future micro satellite missions like GRASP in VLBI analysis software packages.
  • Simulate the benefits of the upcoming VLBI2010 equipment to propose recommendations for the near future.

Members: S. Lambert, Z. Malkin, C. Ma, J. Böhm, R. Haas, R. Heinkelmann, Y. Kwak, Le Poncin-Lafitte, L. Plank, M. Seitz, V. Tornatore.



WG 1.4.3: Maintenance of Celestial Reference Frames and the link to the new GAIA frame

Chair: Chopo Ma (U.S.A.)


To achieve further progress regarding the realization of celestial reference frames it is essential to review the current status, to identify deficiencies and to make proposals for improvements. This task is closely related to various components of the IERS and the techniques analysis coordinators (in particular of the IVS), and requires a close cooperation between the different groups. The activities include the survey of the current status of CRF realization, a review regarding the implementation of IERS Conventions and IAG Fundamental Parameters and different space techniques for CRF realization.

The International Celestial Reference Frame (ICRF2) realized by VLBI is currently defined by the radio positions of 295 extragalactic objects (IERS TN 35, Fey, Gordon and Jacobs, eds., 2009). The ICRF2 was endorsed by the IAU in 2009 and by the IUGG in 2011. The noise floor is ~40 μas and the uncertainty of the axes is ~10 μas. Precise positions of 3414 extragalactic radio sources are included in the ICRF2 catalogue.

The GAIA (Global Astrometry Interferometer for Astrophysics) mission scheduled for launch in 2013 is expected to achieve an optical realization of the CRF with precision similar to or better than the ICRF2 and with at least an order of magnitude more objects. However, as the set of extragalactic objects suitable for both optical and radio observation is limited, one goal of the Working Group is to identify such objects, oversee the relevant observations, and analyze the data to permit the best possible connection between the radio and optical CRF realizations.

For geodetic use the CRF realization must be accessible from the ground. For the foreseeable future this connection will be through VLBI observations. In cooperation with the IVS and IERS, this Working Group will oversee the maintenance and improvement of the ICRF2, in particular the set of sources used for geodetic observations and the ICRF2 defining sources.