Impact of the image alignment over frequency for the VLBI Global Observing System

Aims. The VLBI Global Observing System (VGOS), which is the next generation of geodetic very long baseline interferometry (VLBI), observes simultaneously in four frequency bands in the range 3.0-10.7 GHz (expected to be extended to 14 GHz). Because source structure changes with frequency, we aim to study the source position estimates from the observations of this new VLBI system. Methods. Based on an ideal point source model, simulations are run to determine the relation between the source positions as determined by VGOS observations and the locations of the radio emission in the four bands. Results. We obtained the source positions as determined by VGOS observations as a function of the source positions in the four frequency bands for both group and phase delays. The results reveal that if the location of the radio emission in one band is offset with respect to that in the other bands, the VGOS position estimates can be shifted by a larger amount (up to more than three times that offset), possibly in the opposite direction. Conclusions. The VGOS source positions will be highly variable with time and imprecise as to the locations of the radio emission in the four bands if the effects of source structure are not modeled. The image alignment over frequency is essential for modeling these effects, which is the only way to mitigate the impact of such frequency-dependent radio emission on the resulting VGOS source positions.