Integrated VLBI

Very Long Baseline Interferometry (VLBI) is the only technique for the determination of the celestial reference frame (CRF) which is realized by positions of extragalactic radio sources, and it is the primary technique for monitoring the full set of Earth orientation parameters (EOP). In particular it is unique for observing Universal Time and precession/nutation over longer time spans. It also essentially contributes to a stable scale of the terrestrial reference frame (TRF). Moreover, VLBI observations contain information about a large variety of geodynamic, astronomical, and cosmological parameters which are determined within the project Integrated VLBI. We take advantage of the long history of VLBI observations of more than 30 years and use these data to estimate geodynamic and astronomical parameters in an integrated and consistent one-step approach. In this project a new software package called VieVS (Vienna VLBI Software) which has been developed recently for processing single VLBI sessions will be extended to analyse the whole set of existing geodetic VLBI data in a so-called global solution. VLBI global solutions are typically used to determine the TRF in terms of station positions and velocities, and the CRF in terms of radio source coordinates. In this project we additionally estimate global geodynamic and astronomical parameters like the Free Core Nutation (FCN) period, which shows up in frequency-dependent solid Earth tidal displacements as well as in the nutation model. Thus, in our integrated approach we treat the FCN period in both models as a single parameter to gain more reliable and accurate results. Other geodynamic parameters of interest are complex Love and Shida numbers, which describe the reaction of the anelastic Earth and its gravity field to the tidal forces caused by the largest bodies of the solar system, and to loading by the variable atmosphere and ocean masses. These effects give rise to station displacements and also to variations of the Earth orientation parameters, and for both effects amplitudes and phases of tidal harmonics are determined. In the CRF, a (true or apparent) velocity field or a multi-pole pattern in time series of estimated source coordinates would allow to derive information about astronomical effects, such as an un-modelled acceleration of the solar system towards the galactic centre or a rotation of our galaxy against a background of extragalactic sources. Further we estimate the post-Newtonian parameter ? in the model of gravitational deflection of radio waves according to general relativity. As preparatory work in the project Integrated VLBI, careful inspection of all available data has to be done to detect outliers and to identify poor station performance. After that, sophisticated simulations are carried out to assess all possible interactions between observational quantities and geodynamic or astronomical parameters. Real observations are then processed by VLBI global solutions to determine the parameters of interest that allow further interpretations and conclusions concerning many geophysical and astronomical effects.

Geodetic Very Long Baseline Interferometry (VLBI) is the only space geodetic technique for the realization of celestial reference systems with positions of extragalactic radio sources billions of light years away and the observation of all Earth orientation parameters, consisting of precession/nutation, polar motion, and the Earth rotation angle. Furthermore, geodetic VLBI is a primary technique for the determination of the scale of terrestrial reference frames, i.e., for the size of the Earth. In addition to these primary tasks, VLBI observations contain information about a large variety of geodynamical and astronomical parameters which are typically estimated by averaging results from individual 24-hour sessions. In project Integrated VLBI, we added the global solution tool to the Vienna VLBI Software (VieVS) enabling us to use VLBI observations over 30 years and to rigorously derive the parameters of interest by stacking normal equations.The largest part of the project was devoted to the determination of Love and Shida numbers which describe the Earths anelastic response to the tidal potential induced by Sun and Moon, which can be as large as 40cm in radial direction. In particular we estimated so-called frequency-dependent Love and Shida numbers in the diurnal band which are significantly improved compared to earlier studies. Further interest was in the Free Core Nutation (FCN) which is one of several free rotational modes of the rotating Earth and shows up as Nearly Diurnal Free Wobble (NDFW) in the terrestrial frame, thus causing a resonance effect on the solid Earth tidal displacement. In the celestial reference frame it is visible as a retrograde motion of the Earth figure axis with a period of about 431 days. For the first time, we estimated the period of the FCN from both astronomical and geodynamic phenomena at the same time with the global solution in VieVS and found very good results in agreement with conventional values.The new global solution with VieVS was also used for the estimation of other parameters. For example, we investigated the crustal deformation within Europe from the analysis of all European VLBI sessions, we assessed the impact of neglected station motions on Earth rotation parameters and the celestial reference frame, and we made first tests towards the estimation of several relativistic parameters from the long history of VLBI observations. The backbone of all these investigations is the determination of consistent celestial and terrestrial reference frames and Earth orientation parameters estimated with the VieVS global solution.Finally, the in-house VieVS with its global solution has been very attractive for bachelor and master theses. Thus, project Integrated VLBI has been very successful in acquainting new students with geodesy and VLBI in particular.