Effects of source structure in legacy and broadband geodetic VLBI

Very Long Baseline Interferometry (VLBI) is critical for the realisation of global geodetic reference frames. The new VLBI system (VLBI Geodetic Observing System; VGOS) is designed with the primary goal of filling the gap between the current accuracy of 5 mm for VLBI position determination and the new sub-1 mm target. To achieve the goals of VGOS, new features such as broadband frequency observations (2-14 GHz) and correlation are being implemented. To fully leverage the VGOS system and obtain the most accurate geodetic parameters, it is also necessary to evaluate the effect of source structure on broadband observations, which is the main research objective of this thesis. To address this issue, the VieVS source structure simulator is used in conjunction with simulated schedules representative of actual observed sessions, and real radio source images from the Astrogeo database.

The impact of VLBI image quality on expected source structure effects is assessed by systematically investigating the role of noise level and resolution on geodetic group delay. Using Monte Carlo simulations, the structure index (SI) uncertainties caused by limitations in image quality are calculated and found to be between 0.01 and 0.46 SI units.

At GHz frequencies, extragalactic quasars making up the bulk of celestial reference frame sources vary in their structure and ux density on timescales of months to years. In this study, the impact of quasar variability and observing mode on geodetic observables in the X-band are investigated. The variability of 83 most frequently observed ICRF3 de?ning sources (corresponding to 38% of all geodetic VLBI observations) is analysed and a median standard deviation in the structure index of σ_SI = 0:5 is found; in several cases, the mean SI is significantly different from the reported ICRF2 value. The role of observing mode and VLBI baselines for fixed source structure is furthermore investigated, and variations of up to one SI unit are found.

Simulations are used to determine the effects of source structure on broadband (2{14 GHz) VGOS observations. The simultaneous fitting to the ionosphere and source structure can introduce biases into derived broadband delays at up to hundreds of picoseconds levels. In the most extreme cases, it may not be possible to connect phases across VGOS bands.

In summary, this thesis quantifies the effects of source structure and variability in both legacy and next-generation VLBI observations, and provides recommendations related to observing strategy, with a view of minimising source structure effects.