Imaging VGOS observations and investigating source structure effects

Abstract

Geodetic very long baseline interferometry (VLBI) is a space-geodetic technique that has regularly made global astrometric/geodetic observations since 1979, which are the basis for creating the International Celestial Reference Frame (ICRF2; Fey et al., 2015) and obtaining a full set of Earth Orientation Parameters. Together with other three space-geodetic techniques, that is, Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DO-RIS), VLBI plays an important role in establishing the International Terrestrial Reference Frame (ITRF; Altamimi et al., 2016). Requirements for the ITRF have increased dramatically since the 1980s, and the most stringent one from sea level studies is 1 mm position accuracy and 0.1 mm/yr velocity stability on global scales. The International VLBI Service for Geodesy and Astrometry (IVS; Nothnagel et al., 2017; Schuh & Behrend, 2012; please see https://ivscc.gsfc.nasa.gov/index.html) is developing the next-generation geodetic VLBI system, known as the VLBI Global Observing System (VGOS; Niell et al., 2007;Petrachenko et al., 2009) in order to achieve the goal of 1 mm accuracy. This is one order of magnitude beyond the capability of the traditional S/X VLBI system. Although the VGOS observations from the 500-km-long baseline, GGAO12M-WESTFORD, have obtained geodetic results of a weighted root-mean-square deviation of the baseline length residuals about the weighted mean of 1.6 mm (Niell et al., 2018), few studies have analyzed error sources in the VGOS observations, for instance source structure effects, to evaluate the capability of this new VLBI system. Source structure effects hardly have any influence on this short baseline but can cause significant errors in observations of longer baselines, as demonstrated by the residual delays from geodetic solutions of VGOS observations in Bolotin et al. (2019). Celestial radio sources observed by geodetic VLBI, mostly quasars, are treated as point-like sources in routine geodetic solutions, but in fact they are

Observable quality assessment of broadband very long baseline interferometry system

“…Therefore, our findings indicate that the predominant causative factor of the jumps in delay and δTEC is source structure, which can cause large frequency-dependent phase variations across the four bands. This has been demonstrated by our recent imaging results based on closure phases and closure amplitudes (figures 11 and 12 in Xu et al, 2020).…”

“…Evidence for another critical error source in the VGOS system is the presence of discrete jumps in the closure delays and closure TECs, for instance with a delay offset of about 310 ps or integer multiples of that. The likely cause is found to be source-structure-induced phase changes across the four bands (Xu et al, 2020).…”

Deriving Ionospheric Total Electron Content by VLBI Global Observing System Data Analysis During the CONT17 Campaign

“…Some of the radio sources (e.g., 0552 + 398) have large structure effects when observed on the VGOS baselines. They are spatially extended; some of them have two‐point structures and the others have multiple‐point structures (e.g., Xu et al., 2021). As it can be seen in Charlot (1990) the phase curvature will be affected by this error source and according to Equation , dTEC values will be changed as well.…”

“…Small errors in determining and applying this delay, coupled with small errors due to source structure effects and errors in phase calibration may cause a phase curvature across frequency that is mistakenly lumped into a dTEC contribution. Some of the radio sources (e.g., 0552 + 398) have large structure effects when observed on the VGOS baselines. They are spatially extended; some of them have two‐point structures and the others have multiple‐point structures (e.g., Xu et al., 2021). As it can be seen in Charlot (1990) the phase curvature will be affected by this error source and according to Equation , dTEC values will be changed as well. The ionosphere is not always well‐determined in the fringe fitting process.…”