Overall Properties Of The ICRF And Gaia-CRF

The celestial reference frame?CRF?is a cornerstone of several scientific domains connected to astronomy,geosciences,and deep space navigation.An accurate CRF opens the doors to millimeter-level geodetic measurements?e.g.,Earth rotation?,in-sights into the Earth's internal structure,and testing of General Relativity and alter-native theories with VLBI.Improving the precision and accuracy of the CRF,as well as building an internally consistent multi-wavelength CRF is an essential task for the astrometry community,which reuqires knowledge of the overall properties of current materializations of the CRF.The thesis first reviews the history of the astrometry and the concept of the In-ternational Celestial Reference System?ICRS?,followed by an introduction of the In-ternational Celestial Reference Frame?ICRF?and Gaia-CRFwith a highlight on the high-frequency ICRFs.In the Sect.3,I present the state-of-art tool for evincing the global systematics in the catalog,the vector spherical harmonics?VSH?,which is used frequently in this work.Section 4 is the most substantial part of this thesis,where I investigate the overall properties of ICRFs.I study the stability of individual source position along the ob-serving span and propose two ensembles of defining sources for the sake of improving the axis stability of the VLBI celestial frame.Then I assess the internal consistency of the VLBI catalog at S/X-band to serve as an estimate of the noise floor level?best positional uncertainty?of the ICRF3.This analysis provides an independent validation of the documented value of 30?as given by the ICRF3 Working Group as well as justifies the improvements of the VLBI astrometry after 2009.Then I study the exter-nal accuracy of the ICRF3 with referred to the Gaia-CRF2 and reveal the severe zonal errors of about 0.2 mas in the X/Ka frame.I study the overall properties of the Gaia-CRF in the Sect.5.I assess the inertial of the stellar frame of the Gaia-CRF1 in the light of the global spin of Gaia DR1 proper motions with two versions of Hipparcos frames and Galactic kinematics.Our results report possible residual rotation of around 0.3 mas yr^-1in the Gaia-CRF1frame.We then estimate the influence of the traditional magnitude-dependent error on the overall systematics of the Gaia-CRF2 and found little influence.It underlines a new aspect of the radio-to-optical frame link,which was thought to be taken place only among bright quasars.The radio-to-optical offset is one of the main systematical error sources for radio-to-optical frame link.Different from previous studies which focus on the VLBI S/X-band position solely,I adopt the source positions at four bands to study the position-frequency relation.The radio-to-optical offset is found to have no stastistically signifi-cant correlation on the observing frequency of the ICRF and the radio source structure,differing from assertion given in the literature.We also find that a fraction?10%?of the sample shows all four positions aligned in the direction of jet,which could be a good sample for studying the mas-scale structure of Active Galactic Nucleus?AGNs?.After clarifying the possible systematics in both the ICRF and Gaia-CRF,I propose a new method for the link between ICRF and Gaia-CRF,which is to use the Gaia-CRF as the reference catalog for estimating source positions from VLBI observations.I test this method by running several VLBI global solutions with different choice of the a priori catalog?ICRF3 S/X-band or the Gaia-CRF2?and compare the VLBI products.The results suggest that the ICRF3 S/X frame and Gaia-CRF agrees well so that there is no significant improvement when using the Gaia-CRF position as the a priori position.But this method could be re-tested based on the future Gaia data releases where the accuracy of Gaia-CRF will be improved.Finally,I summarize all the results and propose several topics that may be done in my next step of career.