Towards an astronomical use of new-generation geodetic observations. I. From the correlator to full-polarization images

Context. The current algorithms used for the calibration and analysis of very long baseline interferometry (VLBI) networks that only use linear polarizers (as is the case of the VLBI Global Observing System, VGOS) do not properly account for instrumental and source-intrinsic polarimetry, which can cause errors in geodetic and astronomical products. Aims. We aim to develop a calibration pipeline for VLBI interferometers that observe in a basis of linear polarization, as is the case of VGOS. The products from this pipeline can be used to obtain valuable full-polarization astronomical information from the observed sources, and they can be used to potentially improve the geodetic results. Methods. We used the algorithm PolConvert to write the correlation products in a basis of circular polarization that is compatible with the standard VLBI calibration procedures. In addition to this, we implemented a wide-band global fringe-fitting algorithm that accounts for dispersive effects (ionospheric delay) and allows us to perform full-polarization imaging of all the observed sources, covering the whole frequency band of VGOS. Results. We present the outcome of our pipeline applied to a global IVS VGOS epoch of observations and show example imaging results in total intensity and polarization. We also discuss issues encountered during the analysis and suggest points of improvement in the VGOS system for an optimum geodetic and astronomical exploitation of this interferometer.