On the estimation of a celestial reference frame in the presence of source structure

The spatial structure of sources making up the celestial reference frame (CRF) at radio frequencies is a systematic error source in very long baseline interferometry (VLBI) measurements. Using simulations, we investigate the effects of source structure on the CRF, determined by the actual observational programme of the International VLBI Service for Geodesy and Astrometry. This is done using the source structure simulator of the Vienna VLBI Software. Applying various mock two-component source models, systematic displacements of 10–80 μas in median source position offsets are found. These offsets are predominantly aligned with the direction of the jet. The simulations further show that slight changes in the source model can significantly change the estimated positions. We finally present a new parametrization of source positions in the analysis, namely along the jet direction and perpendicular to it, allowing us to significantly mitigate the effects of source structure on an estimated CRF.