Der Internationale himmelsfeste Referenzrahmen (ICRF-3)
Zusammenfassung der Projektergebnisse
In project ‘TP10 International Celestial Reference Frame (ICRF-3)’ we investigated how to best combine various celestial reference frames, not only determined by different analysis centers, but also observed at different radio frequencies. We promote that a combination using normal equation systems is suited best for the process including a transfer of the full covariance information. The final multi-frequency catalog contains positions of 4617 compact extra-galactic radio sources. New is that the full variance-covariance matrix is produced in the rigorous combination process as well. The resulting catalog and the respective full covariance matrix are available at http://www.referenzsysteme.de. We developed the theoretical background and a practical software realization for the combination of multiple catalogs with full variance-covariance propagation, first for combining the normal equation systems on a single session basis, and second for combining accumulated normal equation systems of the individual catalogs. Our software has successfully demonstrated to be able to combine various catalogs, observed at different frequencies and submitted in different formats. Special validation routines were used to characterize the random and systematic errors between the input reference frames and the combined ones. As a result, we presented a Celestial Reference Frame (CRF) based on the combination of independent, multi-frequency radio source position catalogs using nearly 40 years of Very Long Baseline Interferometry observations at the standard geodetic frequencies at X/S-band (8.4/2.3 GHz, S band for ionospheric calibration only) and about 15 years of observations at the higher frequencies of K band (24 GHz) and of Ka/X band (32/8.4 GHz). The resulting CRF contains precise positions of 4617 compact radio astronomical objects, 4536 measured at X band, 824 sources being observed also at K band and 674 at Ka band. We assessed the quality of the combinations with various means, and without exception, all show that the CRF profits from the rigorous combination. The number of sources increases, which is of benefit mainly to the south, and the accuracy of some of the source positions improves. Deformations present in the individual catalogs are mitigated (i.e., declination bias or network deficiencies). The primary advantage of the combination process developed in this project, however, is that the full covariance information across all position components of all radio sources is carried over and available for the combined catalog. This is important for a proper interpretation of the results and their statistics. Because our developments were delayed by changing personnel, ICRF-3 was adopted by the International Astronomical Union (IAU) without a rigorous combination process. Although ICRF-3 also contains source positions at three different observing frequencies (i.e., X, K and Ka band), the final catalog consists of three stand-alone products, which are only aligned to the International Celestial Reference System (ICRS) via identical sources in the individual catalogs pertaining to the so-called ICRF-3 defining sources. Despite the availability of other solutions able to stabilize the X band observations, they could not contribute to ICRF-3 for a lack of readiness of data handling and a combination algorithm for this purpose. Only comparisons were made between individual solutions for an assessment of the level of agreement. In contrast to this, the output of this project is a rigorous combination routine for radio source position catalogs carrying over the full variance-covariance information. For the first time, we have produced a multifrequency catalog of radio source positions with a complete variance covariance matrix. This also serves to enhance and geometrically stabilize the final product, finally filling the gap of all ICRF computations so far. We are confident that our work can improve significantly any future multi-frequency realizations of the ICRS.
Projektbezogene Publikationen (Auswahl)
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Development of a Combination Procedure for Celestial Reference Frame Determination. In P. Willis, Editor, Proceedings of IAG Scientific Assembly 2013. International Association of Geodesy Symposia, pages 63–68. Springer, Berlin, Heidelberg, 2015
Iddink A., Artz T., Nothnagel A.
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First results of S/X and X/Ka-band catalogue combinations with full covariance information, Proceedings of the Journées 2014 "Systèmes de référence spatio-temporels" at Pulkovo Observatory, St. Petersburg, 2015
Iddink A., Jacobs C., Nothnagel A.
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Estimating the Celestial Reference Frame via Intra-Technique Combination, In IVS 2016 General Meeting Proceedings, “New Horizons with VGOS”, Johannesburg, South Africa, March 13-19 2016, Eds: D. Behrend, K. D. Baver, and K. L. Armstrong, NASA/CP–2016–219016, 2016
Iddink A., Artz T., Nothnagel A.
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ivg::ASCOT: The Development of a new VLBI Software Package, In IVS 2016 General Meeting Proceedings, “New Horizons with VGOS”, Johannesburg, South Africa, March 13-19 2016, Eds: D. Behrend, K. D. Baver, and K. L. Armstrong, NASA/CP–2016–219016, 2016
Artz T, Halsig S., Iddink A., Nothnagel N.
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Current Development Progress in ivg::ASCOT. A new VLBI Analysis Software; Proceedings of the 23rd Meeting of the European VLBI Group for Geodesy and Astrometry Working Meeting, May 2017, Gothenburg, Sweden, ed. by R. Haas and G. Elgered, 167-171, 2017
Halsig S, Corbin A, Iddink A, Jaron F, Schubert T, Nothnagel A
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Impact of the Terrestrial Reference Frame on the determination of the Celestial Reference Frame, Geodesy and Geodynamics, 10(1), 58-71, 2018
Karbon M., Belda S., Nilsson T.
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Combining Datum-free Normal Equation Systems for the Determination of Celestial Reference Frames, In: D Behrend. KD Baver, K Armstrong (eds.), International VLBI Service for Geodesy and Astrometry 2018 General Meeting Proceedings, NASA/CP–dbd, Greenbelt MD, tbd, 2019
Karbon M., Nothnagel A.
