Evaluation of VLBI Observations with Sensitivity and Robustness Analyses

Abstract

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.

On the improvement of the sensitivity levels of VLBI solutions from a combination with GNSS

“…In the multi-session solution, the displacement vector is obtained from the adjusted coordinate differences represented by different sessions. According to deformation analyses, the cofactor matrices will be constant for each session in the multi-session solution if the observation weights, the measurement schedule, and the station distribution are identical (Konak et al, 2019;Küreç Nehbit et al, 2020). However, this condition cannot be realized in reality for any space geodetic technique because the total observation weights at a station cannot be controlled.…”

“…Based on the cofactor matrix of unknown parameters Q xx that represents the full set of unknown parameters of an individual VLBI session, the cofactor matrix of the displacement vector Q d i d i =Q x i x i can be obtained for each VLBI station. The details of determining the cofactor matrix of the displacement vector for each station and each radio source were described in Küreç Nehbit et al (2020).…”

“…Therefore, it is substantial to investigate the effects of the a priori coordinates on the positions of the VLBI radio telescopes and radio sources. One suitable method is the sensitivity analysis (Even-Tzur, 2006;Küreç Nehbit et al, 2020).…”

“…For instance, it was used for the investigation of the datum definition (Even-Tzur, 2006), of monitoring crustal deformation (Hsu and Hsiao, 2002), of a priori sensitivity levels (Küreç and Konak, 2014), and of the determination of the experimental sensitivity levels (Konak et al, 2019). Küreç Nehbit et al (2020) performed quality assessments, such as the robustness and sensitivity analyses, for the first time based on measurements of the VLBI technique. Therein, the sensitivity and robustness levels of the VLBI stations, and of the radio sources have been determined.…”

“…Therein, the sensitivity and robustness levels of the VLBI stations, and of the radio sources have been determined. Although the best sensitivity level is obtained with maximal eigenvalue, the eigenvalues in the z-direction were selected to obtain the sensitivity levels of a VLBI station, and the eigenvalues in declination direction were selected for the sensitivity level of a radio source during the numerical application in that study (Küreç Nehbit et al, 2020). In contrast to that, in the present study the axis with the maximal eigenvalue among the respective coordinate directions was used.…”