Baseline geometry effects due to source structure in geodetic VLBI

Abstract

The technique of very long baseline interferometry (VLBI) uses knowledge about the positions of radio telescopes to measure celestial radio source positions with highest precision down to tens of microarcseconds for ground-based experiments. At the same time, VLBI measurements of stable, point-like celestial radio sources can be used to determine precise station positions down to the millimeter level. However, the majority of radio sources exhibit extended intrinsic structure at sub-milliarcsecond (or larger) scales that is time and frequency dependent and leads to systematic effects on the group delay observable. The classical approach to correct for structure if the radio sources are to be used for geodesy is to perform VLBI imaging, where the visibility function measured by the interferometer is the mapping of the specific intensity distribution of the source on the sky as a function of the interferometer baseline (u,v) coordinates through a Fourier transform. Radio source images provide information about the sources’ appearance and compactness. Ultimately, sources are classified according to their compactness, and for geodesy, the best sources are those of which core structure is compact and astrometrically stable: “defining sources”. The sources exhibiting complex structure, “special handling sources”, are always adjusted on the session level in the analysis, as they could otherwise corrupt the station position estimates. In this study we investigate the effects of source structure on the geodetic/astrometric results, i.e., radio source positions, by considering the different baseline-source geometries. For this purpose, we compare results obtained from astrometrically unstable sources observed with many long East‐West baselines (to provide the highest angular resolution) with those from astrometrically stable sources.