Towards an Astronomical Use of New-Generation Geodetic Observations
This thesis explores the astronomical use of observations from the VLBI Global Observing System (VGOS), a next-generation geodetic network designed to improve the precision of terrestrial and celestial reference frames and Earth orientation parameters. Although primarily conceived for geodesy, its wide frequency coverage and regular monitoring of compact active galactic nuclei (AGN) provide valuable opportunities for astrophysics. We develop a calibration pipeline that transforms VGOS correlator outputs into full-polarization, calibrated visibilities suitable for astronomical imaging. The procedure combines linear-to-circular polarization conversion, instrumental phase and amplitude calibration, and a new wideband global fringe-fitting algorithm that accounts for dispersive ionospheric effects. Regularized maximum likelihood imaging is then used to reconstruct multi-frequency intensity and polarimetric images. The pipeline is applied to the blazar 1803+784 and five additional AGN: 3C418, 0059+581, 0613+570, 1849+670, and 2229+695. We measure frequency-dependent core shifts, spectral indices, and Faraday rotation properties. For 1803+784, the fitted core-shift index is $k_r = 0.73^{+0.12}_{-0.19}$, suggesting departures from a simple conical jet in energy equipartition. Finally, we apply the reconstructed source models to correct structure-dependent effects in geodetic VLBI observables. The correction reduces closure-delay scatter and post-fit residuals for strongly structured sources, particularly 3C418, while its effectiveness depends on source variability and the temporal proximity of the imaging observations. These results demonstrate the dual scientific value of VGOS as a powerful multi-frequency astronomical VLBI instrument and as a means of improving geodetic measurements.