Imaging VGOS Observations and Investigating Source Structure Effects

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 broadband delays are still about 20 ps. 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 and 50.2 ps in session VO0034, respectively; after correcting source structure effects based on the images, the RMS values of the residual closure delays are 5.5 and 10.1 ps. The jumps in delay observables with magnitudes of several hundreds of picoseconds are found to be caused by 2π 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 for deriving images of radio sources at the four bands of VGOS observations and discusses the alignment of the images at the four bands, which is fundamental to mitigating these systematic effects.