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Tropospheric delay modeling is challenging in high-precision Very Long Baseline Interferometry (VLBI) analysis due to the rapid water vapor variation and imperfect observation geometry, where observations from Global Navigation Satellite Systems (GNSS) co-locations can enhance the VLBI analysis. We investigate the impact of tropospheric ties in the VLBI and GNSS integrated processing during the CONT05–CONT17 campaigns, and present a method that automatically handles the systematic tropospheric tie biases. Applying tropospheric ties at VLBI–GNSS co-locations enhances the observation geometry and improves the solution reliability. The VLBI network is stabilized, with station coordinate repeatability improved by 12% horizontally and by 28% vertically, and the network scale improved by 32%. The Earth Orientation Parameters (EOP) improve by up to 20%. Both zenith delay and gradient ties contribute to the improvement of EOP, whereas the gradient ties contribute mainly to the improvement of length of day and celestial pole offsets.

Terrestrial reference frames (TRF), such as the ITRF2008, are primary products of geodesy. In this paper, we present TRF solutions based on Kalman filtering of very long baseline interferometry (VLBI) data, for which we estimate steady station coordinates over more than 30 years that are updated for every single VLBI session. By applying different levels of process noise, non-linear signals, such as seasonal and seismic effects, are taken into account. The corresponding stochastic model is derived site-dependent from geophysical loading deformation time series and is adapted during periods of post-seismic deformations. Our results demonstrate that the choice of stochastic process has a much smaller impact on the coordinate time series and velocities than the overall noise level. If process noise is applied, tests with and without additionally estimating seasonal signals indicate no difference between the resulting coordinate time series for periods when observational data are available. In a comparison with epoch reference frames, the Kalman filter solutions provide better short-term stability. Furthermore, we find out that the Kalman filter solutions are of similar quality when compared to a consistent least-squares solution, however, with the enhanced attribute of being easier to update as, for instance, in a post-earthquake period.

Within the International Very Long Baseline Interferometry (VLBI) Service for Geodesy and Astrometry (IVS), long time-series of zenith wet and total troposphere delays have been combined at the level of parameter estimates. The data sets were submitted by eight IVS Analysis Centers (ACs) and cover January 1984 to December 2004. In this paper, the combination method is presented and the time-series submitted by the eight IVS ACs are compared with each other. The combined zenith delays are compared with time-series provided by the International Global Navigation Satellite System (GNSS) Service (IGS), and with zenith delays derived from the European Centre for Medium-Range Weather Forecasts (ECMWF). Before the combination, outliers are eliminated from the individual time-series using the robust BIBER (bounded influence by standardized residuals) estimator. For each station and AC, relative weight factors are obtained by variance component estimation. The mean bias of the IVS ACs’ time-series with respect to the IVS combined time-series is 0.89 mm and the mean root mean square is 7.67 mm. Small differences between stations and ACs can be found, which are due to the inhomogeneous analysis options, different parameterizations, and different treatment of missing in-situ pressure records. Compared to the IGS zenith total delays, the combined IVS series show small positive mean biases and different long-term trends. Zenith wet delays from the ECMWF are used to validate the IVS combined series. Inconsistencies, e.g., long-term inhomogeneity of the in-situ pressure data used for the determination of VLBI zenith delays, are identified.

In 2017, the astronomical and geodetic communities will celebrate the 50-year anniversary of the first Very Long Baseline Interferometry (VLBI) experiments. The Journal of Geodesy Special Issue titled “VLBI contribution to reference frames and Earth’s rotation studies” is dedicated to this great jubilee. VLBI plays a key role in astronomy and geodesy. It provides unprecedented spatial resolution and position measurements on the sky with an accuracy superior to other techniques. VLBI is a key space geodetic technique since the 1970s, which fundamentally contributes to the maintenance of terrestrial (TRF) and celestial (CRF) reference frames, including the International Terrestrial (ITRF) and Celestial (ICRF) Reference Frames, and monitoring of the Earth’s rotation with respect to the ITRF and ICRF through determination of highly accurate Earth orientation parameters (EOP). Furthermore, the VLBI technique is unique in determining the Universal Time and the precession–nutation of the Earth’s rotation axis in space. Consequently, it is the only technique capable to provide a consistent TRF–EOP–CRF solution. VLBI also essentially contributes to Solar System dynamics and space and terrestrial navigation, atmospheric studies and refining geophysical models, crustal movements and plate tectonics, time and frequency transfer, and testing physical theories. The most accurate and valuable VLBI results are obtained from global VLBI station networks involving radio telescopes located in different countries on different continents. Therefore, a successful realization of the VLBI observing programs requires both inter-institutional and international cooperation. Since 1999, this cooperation is realized through the International VLBI Service for Geodesy and Astrometry (IVS). Moreover, the IVS provides the combined IVS products, such as EOP and VLBI TRF realization (VTRF). The latter essentially contributes to the multi-technique ITRF solution providing the longest station position time series and defining, together with Satellite Laser Ranging (SLR), the ITRF scale and scale rate. Including the radio source positions in the combination is under investigation. A dedicated paper in this issue describes the current state and perspectives of the IVS operations. The previous Special Issue of Journal of Geodesy devoted to VLBI and published in June 2007 (volume 81, issue 6–8) was highly successful. Since that time, many important changes and impressive improvements in the VLBI technique and analysis happened, such as VLBI2010 technical design, the start of the next-generation global VLBI network VLBI Global Observing System (VGOS), publishing ICRF2 and preparing ICRF3, and implementing new IVS combined EOP–TRF products based on combination of datum-free normal equations. All these researches and developments aim at achieving the 1 mm accuracy in TRF and EOP. To reach this goal, the development of technology should be accompanied by improvement in the geophysical and astronomical modelling as well as in the analysis strategy. This Special Issue of Journal of Geodesy is intended to put together papers describing the latest developments in geodetic and astrometric VLBI. Taking into account the scope of the Journal and the limited size of the volume, only the papers devoted to the geodetic and astrometric data analysis were called for. After the Call for papers was sent to the community, we received many responses from our colleagues expressing their interest to contribute to the volume. We believe that the papers published here comprise a representative collection reflecting the main topics of geodetic and astrometric data analysis, which will provide a good reference source for future research in the field. It is a pleasure to express our gratitude to all the authors presenting their results in this issue. All the papers submitted to this volume have passed a peer-review process following the standard procedure of the Journal of Geodesy. We are much indebted to many reviewers whose valuable comments and suggestions helped to prepare the final papers of high quality included in this volume. Finally, we would like to acknowledge the hard and continuous work of all the IVS components over many years—stations, correlators, technology development, operational and analysis centers that regularly deliver data and products of highest quality for scientific analysis and practical applications.