Quality Assessment of VTEC Products Derived from Observations with the Next-Generation VLBI System

Abstract

The ionosphere hosts all charged particles in the Earth’s atmosphere. It is a dispersive medium for radio signals allowing to investigate the variability and changes of this medium with the use of ground-based geodetic instruments operating in the microwave regime. One of the parameters of interest in connection to ionosphere is the vertical total electron content (VTEC) that can be derived from multi-frequency measurements carried out with space-geodetic techniques such as global satellite navigation system (GNSS) and geodetic very long baseline interferometry (VLBI). The latter technique relies on measurements of the time difference in the arrival of microwave signals from natural and very distant radio sources (quasars) at two or more radio telescopes located on the Earth surface. The focus of this thesis are the VTEC products derived from the next-generation VLBI system, known as the VLBI global observing system (VGOS). The latter has already reached an operationally stable international network. However, VGOS continuously evolves into a truly global infrastructure. Since late 2017, VGOS observations, organized as 24-hour sessions, were already carried out. Currently, there are over forty sessions that are available. In this master’s thesis, a Python code was developed to process these sessions and derive VTEC time series above VGOS stations. In modelling the ionosphere with VGOS, piece-wise linear (PWL) function with a temporal resolution of one hour is used to consider the time variability of the ionosphere above VGOS sites. The modified dip (modip) latitude is used to consider the influence of the Earth’s magnetic field on the ionosphere, and Sun-fixed coordinate system is used to account for the strong correlation between the ionosphere and the position of the Sun. In addition, latitudinal ionospheric gradients and VGOS instrumental offsets are considered in the adjustment process and assumed to remain constant during the 24-hour session. The results show that VTEC time series from VGOS observations show similar temporal behavior of ionosphere when compared with VTEC time series from global ionosphere maps (GIMs) and Madrigal total electron content maps (MTMs). This is also the case when VTEC time series from VGOS observations are compared with VTEC time series from observations carried out with both geodetic VLBI and GNSS stations that are co-located with VGOS telescopes. Furthermore, a bias between VGOS-derived VTEC time series and the remaining VTEC time series from GIMs, MTMs and GNSS observations was identified. This bias, which often has a positive sign, varies in size across the investigated sessions, reaching few TEC unit (TECU). The smallest and largest RMS differences w.r.t. GIMs-based VTEC time series occur at Onsala Space Observatory (ONSA13NE and ONSA13SW) and at Kokee Park Geophysical Observatory (KOKEE12M), reaching 2.6 ± 0.5 and 5.4 ± 1.1 TECU, respectively. The smallest and largest RMS differences w.r.t. MTMs-based VTEC time series are noticed for ISHIOKA and GGAO12M, reaching 1.6 ± 0.2 and 3.3 ± 1.0 TECU, respectively. Based on the investigated VGOS sessions, the uncertainty of VGOS-derived VTEC time series is on the order of 0.12 ± 0.03 TECU for all VGOS station except KOKEE12M, where the uncertainty is on the order of 0.22 ± 0.05. In addition, the results indicate that the uncertainty of the north ionospheric gradient is often better resolved than the uncertainty of the south gradient due to the current geometric configuration of the operational VGOS network. In general, VGOS-derived VTEC has the potential to complement local and global ionospheric maps in the vicinity of VGOS stations.