Abstract
The next-generation, broadband geodetic very-long-baseline
interferometry system, named VGOS, is being developed globally with an
aim to achieve 1~mm accuracy for station positions.
Currently, the systematic errors in VGOS observations are still about
one order-of-magnitude larger than this aim. In this study, we
demonstrate that it is feasible to make images directly from VGOS
observations without the need of complicated calibrations and determine
the source structure effects in VGOS broadband delays through the
process of model fitting to the structure phases from our imaging
results. Source structure effects are investigated in detail, and it is
shown that the systematic errors in VGOS observations are well explained
by these effects. For instance, the root-mean-square (RMS) closure
delays of the observations of sources 0016$+$731 and 1030$+$415 are
24.9~ps and 50.2~ps in session VO0034,
respectively; by correcting source structure effects based on the
images, the RMS values of the residual closure delays are
5.5~ps and 10.1~ps. The jumps in delay
observables with magnitudes of several hundreds of picoseconds are found
to be caused by 2$\pi$ phase shifts among the four
bands due to strong source structure effects. The impact of the
alignment of the images at the four frequency bands in VGOS is
discussed. Our study provides a methodology of deriving images of radio
sources at the four bands of VGOS observations and discusses the
alignment of the four-band images, which is fundamental to mitigate
systematic effects.