Mitigating the effect of source structure in geodetic VLBI by re-weighting observations using closure delays and baseline-to-jet orientation

An ideal target for geodetic very long baseline interferometry (VLBI) is a strong and point-like radio source. In reality, most celestial sources used in geodetic VLBI have spatial structure. This is as a major source of error in VLBI Global Observing System (VGOS) and also affects legacy S/X observations. Source structure causes a systematic delay, which can affect the geodetic estimates if not modelled or otherwise accounted for. In this work, we aim to mitigate its impact by extending the stochastic model used in the least-squares fitting of the VLBI group delays. We have developed a weighting scheme to re-weight the observations by parameterizing the source structure component in terms of closure delays and jet orientation relative to the observing baseline. It was implemented in the Vienna VLBI Software. To assess the performance of the extended stochastic model, we analysed the CONT17 legacy sessions and generated suitable reference solutions for comparison. The effects of re-weighting were evaluated with respect to the session fit statistics, source-wise residuals, and geodetic parameters. We find that this relatively simple noise model consistently improves the session fit by about 5% with moderate variation from session to session. The geodetic estimates are not affected to a significant level by this new weighting method. Source-wise we see improved post-fit residuals for 63 out of a total of 91 sources observed.