Baseline Vector Repeatability at the Sub‐Millimeter Level Enabled by Radio Interferometer Phase Delays of Intra‐Site Baselines

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 at X band from two antennas at the Geodetic Observatory Wettzell, Germany, extracted from 107 global 24‐hr 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 toward the goal of global geodesy at the 1 mm level.

Plain Language Summary: 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. By using the inherently more precise observables—phase delays, a baseline vector repeatability of WETTZ13N–WETTZELL has been achieved at the sub‐millimeter level for the horizontal directions and at the 1 mm level for the vertical direction based on VLBI experiments of 107 days during 3.5 years. Position ties based on phase delays are also obtained for other short baselines between legacy antennas and nearby, newly built antennas, and they are critical to maintain a consistent continuation of the realization of terrestrial reference frame into the future of a network of these new antennas. We have evaluated the instrumental stability at the 1 mm level, which is an important step toward the goal of global geodesy at this level.

Key Points:

Baseline vectors between legacy antennas and co‐located VGOS antennas are obtained from phase delays with the highest possible precision.

Sources of error in short‐baseline observations are investigated at the 1 mm level in terms of their potential impacts.