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

TL;DR

This paper investigates how frequency-dependent source structure affects VLBI VGOS source position estimates and emphasizes the importance of image alignment over frequency to improve positional accuracy.

Contribution

It introduces a simulation-based analysis of frequency-related source position shifts in VGOS and highlights the necessity of image alignment to mitigate these effects.

Findings

Source position shifts can be more than three times the offset in radio emission locations.

VGOS source positions are highly variable and imprecise without modeling source structure effects.

Image alignment over frequency is crucial for accurate source position estimation.

Abstract

The VLBI Global Observing System, which is the next generation of geodetic VLBI and is called VGOS, observes simultaneously at 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. Based on an ideal point source model, simulations are made to determine the relation between the source positions as determined by VGOS observations and the locations of the radio emission at the four bands. We obtained the source positions as determined by VGOS observations as a function of the source positions at the four frequency bands for both group and phase delays. The results reveal that if the location of the radio emission at one band is offset with respect to that at the other bands, the position estimate can be shifted to the opposite direction and even by more than three times that offset. The VGOS source positions will be very variable with time and very imprecise in the sense of relating to the locations of the radio emission at the four bands, if the effects of source structure are not modeled. The image alignment over frequency is essential in order to model the effects of source structure in VGOS observations, which is the only way to mitigate these strong frequency-dependent impacts on VGOS source positions.