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Register a new userTowards a robust estimation of orientation parameters between ICRF and $Gaia$ celestial reference frames
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Zinovy Malkin
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An analysis of the source position differences between VLBI-based ICRF and $Gaia$-CRF catalogues is a key step in assessing their systematic errors and determining their mutual orientation. One of the main factors that limits the accuracy of determination of the orientation parameters between two frames is the impact of outliers. To mitigate this effect, a new method is proposed based on pixelization data over the equal-area cells, followed by median filtering of the data in each cell. After this, a new data set is formed, consisting of data points near-uniformly distributed over the sphere. The vector spherical harmonics (VSH) decomposition is then applied to this data to finally compute the orientation parameters between ICRF and $Gaia$ frames. To validate the proposed approach, a comparison was made of the ICRF3-SX and $Gaia$~DR2 catalogues using several methods for outliers removal. The results of this work showed that the proposed method is practically insensitive to outliers and thus provides much more robust results of catalogues comparison than the methods used so far. This conclusion was confirmed by analogous test comparison of the $Gaia$~DR2 and OCARS catalogues.
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The current state of the link problem between radio and optical celestial reference frames is considered. The main objectives of the investigations in this direction during the next few years are the preparation of a comparison and the mutual orientation and rotation between the optical {it Gaia} Celestial Reference Frame (GCRF) and the 3rd generation radio International Celestial Reference Frame (ICRF3), obtained from VLBI observations. Both systems, ideally, should be a realization of the ICRS (International Celestial Reference System) at micro-arcsecond level accuracy. Therefore, the link accuracy between the ICRF and GCRF should be obtained with similar error level, which is not a trivial task due to relatively large systematic and random errors in source positions at different frequency bands. In this paper, a brief overview of recent work on the GCRF--ICRF link is presented. Additional possibilities to improve the GCRF--ICRF link accuracy are discussed. The suggestion is made to use astrometric radio sources with optical magnitude to 20$^m$ rather than to 18$^m$ as currently planned for the GCRF--ICRF link. In addition, the use of radio stars is also a prospective method to obtain independent and accurate orientation between the Gaia frame and the ICRF.
In this paper we outline several problems related to the realization of the international celestial and terrestrial reference frames ICRF and ITRF at the millimeter level of accuracy, with emphasis on ICRF issues. The main topics considered are: analysis of the current status of the ICRF, mutual impact of ICRF and ITRF, and some considerations for future ICRF realizations.
The link problem between radio (VLBI/ICRF) and optical (Gaia/GCRF) celestial reference frames is analyzed. Both systems should be a realization of the ICRS (International Celestial Reference System) at microarcsecond level of accuracy. Therefore, the link between the ICRF and GCRF should be obtained with similar accuracy, which is not a trivial task due to relatively large systematic and random errors in source positions at different frequency bands. In this presentation, additional possibilities to improve the GCRF-ICRF link accuracy are discussed. In particular, a possibility to increase the number of ICRF and GCRF common objects is considered using advanced scheduling of the regular IVS sessions such as R1 and R4. It is shown that inclusion of supplement prospective southern sources in these sessions allows enriching southern ICRF zone without noticeable loss of accuracy of geodetic results. Another topic discussed in this presentation is using the correlations between radio source coordinates, which can impact the orientation angles between two frames at a level of a few tens of microarcseconds.
The goal of this presentation is to report the latest progress in creation of the next generation of VLBI-based International Celestial Reference Frame, ICRF3. Two main directions of ICRF3 development are improvement of the S/X-band frame and extension of the ICRF to higher frequencies. Another important task of this work is the preparation for comparison of ICRF3 with the new generation optical frame GCRF expected by the end of the decade as a result of the Gaia mission.
The International Celestial Reference Frame (ICRF, Ma et al. 1998) is currently the best realization of a quasi-inertial reference system. It is based on more than 10 years of cumulated geodetic and astrometric VLBI observations of compact extragalactic objects at centimetric wavelengths. In the perspective of the realization of an accurate optical counterpart of the ICRF using future space astrometry missions like GAIA or SIM, this paper investigates the consistency of celestial reference frames realized through the same subset of compact extragalactic radio sources at optical wavelengths. Celestial reference frames realized in radio wavelengths with the VLBA Calibrator Survey (VCS) data and in optical wavelengths with the Sloan Digital Sky Survey (SDSS) data (DR3 quasar catalogue and DR5) are compared in terms of radio-optical distances between the common sources, global rotation of the axes and offset of the equator.
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