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In space geodetic data analysis, improving tropospheric delay modelling is motivated by the correlation between tropospheric zenith total delay (ZTD) and the station height. The gradients are correlated with the horizontal displacements. In the microwave techniques such as GPS, DORIS, and VLBI, the tropospheric delay effects are correlated over the collocation sites. This correlation allows for the estimation of common tropospheric parameters, often referred to as tropospheric ties. These tropospheric ties provide valuable complementary information for the computation of the terrestrial reference frame (TRF) and have been the subject of investigation in many studies. In this study, we investigate the effects of tropospheric ties on the daily TRF combination at the observation level using a batch least-squares estimation. The observations of GPS, DORIS, and VLBI were collected from 06 May 2014 to 20 May 2014 during the CONT14 campaign of VLBI. The tropospheric delay and gradient ties are computed using different numeric weather prediction (NWP) data sets provided by ECMWF and NCEP. We examined different levels of tropospheric ties 0.01, 5, and 10 mm for ZTD and 0.001, 0.5, and 1.0 mm for gradients in the combination of techniques. The results show that the combined solution with tropospheric ties derived from the four NWP data sets does not exhibit significant differences. For VLBI, the repeatability of station coordinates and network scale were found to be improved by around 20% and 30%, respectively. The stronger tropospheric ties show a higher improvement in VLBI baseline repeatability. However, applying tropospheric ties at GPS-DORIS collocation sites does not significantly affect the repeatability of station coordinates and network scale. Both ZTD and gradient ties enhance the repeatability of polar motion components in EOPs, while no observable contribution is observed for dUT1 and celestial pole offsets. [ABSTRACT FROM AUTHOR]

This paper introduces the reader to our Kalman filter developed for geodetic VLBI (very long baseline interferometry) data analysis. The focus lies on the EOP (Earth Orientation Parameter) determination based on the Continuous VLBI Campaign 2014 (CONT14) data, but also earlier CONT campaigns are analyzed. For validation and comparison purposes we use EOP determined with the classical LSM (least squares method) estimated from the same VLBI data set as the Kalman solution with a daily resolution. To gain higher resolved EOP from LSM we run solutions which yield hourly estimates for polar motion and dUT1 = Universal Time (UT1) – Coordinated Universal Time (UTC). As an external validation data set we use a GPS (Global Positioning System) solution providing hourly polar motion results. Further, we describe our approach for determining the noise driving the Kalman filter. It has to be chosen carefully, since it can lead to a significant degradation of the results. We illustrate this issue in context with the de-correlation of polar motion and nutation. Finally, we find that the agreement with respect to GPS can be improved by up to 50% using our filter compared to the LSM approach, reaching a similar precision than the GPS solution. Especially the power of erroneous high-frequency signals can be reduced dramatically, opening up new possibilities for high-frequency EOP studies and investigations of the models involved in VLBI data analysis. We prove that the Kalman filter is more than on par with the classical least squares method and that it is a valuable alternative, especially on the advent of the VLBI2010 Global Observing System and within the GGOS frame work.

Very Long Baseline Interferometry (VLBI) plays an indispensable role in the realization of global terrestrial and celestial reference frames and in the determination of the full set of the Earth Orientation Parameters (EOP). The main goal of this research is to assess the quality of the VLBI observations based on the sensitivity and robustness criteria. Sensitivity is defined as the minimum displacement value that can be detected in coordinate unknowns. Robustness describes the deformation strength induced by the maximum undetectable errors with the internal reliability analysis. The location of a VLBI station and the total weights of the observations at the station are most important for the sensitivity analysis. Furthermore, the total observation number of a radio source and the quality of the observations are important for the sensitivity levels of the radio sources. According to the robustness analysis of station coordinates, the worst robustness values are caused by atmospheric delay effects with high temporal and spatial variability. During CONT14, it is determined that FORTLEZA, WESTFORD, and TSUKUB32 have robustness values changing between 0.8 and 1.3 mm, which are significantly worse in comparison to the other stations. The radio sources 0506-612, NRAO150, and 3C345 have worse sensitivity levels compared to other radio sources. It can be concluded that the sensitivity and robustness analysis are reliable measures to obtain high accuracy VLBI solutions.

Closure quantities measured by very-long-baseline interferometry (VLBI) observations are independent of instrumental and propagation instabilities and antenna gain factors, but are sensitive to source structure. A new method is proposed to calculate a structure index based on the median values of closure quantities rather than the brightness distribution of a source. The results are comparable to structure indices based on imaging observations at other epochs and demonstrate the flexibility of deriving structure indices from exactly the same observations as used for geodetic analysis and without imaging analysis. A three-component model for the structure of source 3C371 is developed by model-fitting closure phases. It provides a real case of tracing how the structure effect identified by closure phases in the same observations as the delay observables affects the geodetic analysis, and investigating which geodetic parameters are corrupted to what extent by the structure effect. Using the resulting structure correction based on the three-component model of source 3C371, two solutions, with and without correcting the structure effect, are made. With corrections, the overall rms of this source is reduced by 1 ps, and the impacts of the structure effect introduced by this single source are up to 1.4 mm on station positions and up to 4.4 microarcseconds on Earth orientation parameters. This study is considered as a starting point for handling the source structure effect on geodetic VLBI from geodetic sessions themselves. [ABSTRACT FROM AUTHOR]

Very Long Baseline Interferometry (VLBI) plays an indispensable role in the realization of global terrestrial and celestial reference frames and in the determination of the full set of the Earth Orientation Parameters (EOP). The main goal of this research is to assess the quality of the VLBI observations based on the sensitivity and robustness criteria. Sensitivity is defined as the minimum displacement value that can be detected in coordinate unknowns. Robustness describes the deformation strength induced by the maximum undetectable errors with the internal reliability analysis. The location of a VLBI station and the total weights of the observations at the station are most important for the sensitivity analysis. Furthermore, the total observation number of a radio source and the quality of the observations are important for the sensitivity levels of the radio sources. According to the robustness analysis of station coordinates, the worst robustness values are caused by atmospheric delay effects with high temporal and spatial variability. During CONT14, it is determined that FORTLEZA, WESTFORD, and TSUKUB32 have robustness values changing between 0.8 and 1.3 mm, which are significantly worse in comparison to the other stations. The radio sources 0506-612, NRAO150, and 3C345 have worse sensitivity levels compared to other radio sources. It can be concluded that the sensitivity and robustness analysis are reliable measures to obtain high accuracy VLBI solutions. [ABSTRACT FROM AUTHOR]

The celestial sources observed by geodetic very‐long‐baseline interferometry (VLBI), most of which are quasars, are resolved—they have a finite angular size as measured by the interferometer—yet modern geodetic VLBI analysis techniques still treat these sources as if they are point‐like. The errors introduced by source structure, the angular distribution of emission, have traditionally been believed to be a small fraction of the total measurement error budget. We analyzed the VLBI data of the CONT14 continuous observing campaign using standard geodetic and imaging techniques, along with an analysis of closures, combinations of interferometric measurements where the error contributions from station‐based effects such as tropospheric and ionospheric errors, clock and cable length errors, station position errors, and so on, cancel exactly, leaving only nonclosing errors, principally source structure and measurement noise. Our statistical analysis of the closure information, based on actual VLBI observations revealed that nearly half (40%) of the total geodetic VLBI error budget (in terms of variance) in CONT14 came from source‐structure effects, nearly the same variance as all station‐based error sources (43%). The remaining error due to other nonsource‐structure, nonclosing errors is also significant (17%). Previous studies, which were primarily simulation‐based, did not account for the full range of effects of source structure on real measurements. Source structure is a major contributor to errors in geodetic VLBI, and source structure must be taken into account in the entire VLBI operational chain, from scheduling to analysis, in order to mitigate its effects on astrometric and geodetic measurements. Key Points: Source‐structure errors in geodetic VLBI were determined from actual IVS observations, not simulationsSource‐structure and measurement noise were measured using closure relationshipsSource‐structure errors in CONT14 are approximately as large as all station‐based errors combined [ABSTRACT FROM AUTHOR]