We report the results of position ties for short baselines at eight geodetic sites based on phase delays that are extracted from global geodetic very-long-baseline interferometry (VLBI) observations rather than dedicated short-baseline experiments. An analysis of phase delay observables from two antennas at the Geodetic Observatory Wettzell, Germany, extracted from 107 global 24-hour VLBI sessions since 2019 yields weighted root-mean-square scatters about the mean baseline vector of 0.3, 0.3, and 0.8 mm in the east, north, and up directions, respectively. Position ties are also obtained for other short baselines between legacy antennas and nearby, newly built antennas. They are critical for maintaining a consistent continuation of the realization of the terrestrial reference frame, especially when including the new VGOS network. The phase delays of the baseline WETTZ13N–WETTZELL enable an investigation of sources of error at the sub-millimeter level. We found that a systematic variation of larger than 1 mm can be introduced to the up estimates of this baseline vector when atmospheric delays were estimated. Although the sub-millimeter repeatability has been achieved for the baseline vector WETTZ13N–WETTZELL, we conclude that long term monitoring should be conducted for more short baselines to assess the instrumental effects, in particular the systematic differences between phase delays and group delays, and to find common solutions for reducing them. This will be an important step towards the goal of global geodesy at the 1 mm level.

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.