Monitoring VLBI Instrumentation for Accurate Terrestrial Reference Frames

Paper in proceeding, 2026

The International Terrestrial Reference Frame (ITRF) is realized by a global network of stations representing the four space geodetic techniques of VLBI, GNSS, SLR and DORIS. The technique-specific networks are combined using constraints such as local ties at colocation sites, and the assumption that station velocities and seasonal variations are identical for all techniques at a given site. Station motion is modeled primarily with linear trends to represent tectonic motion and glacial isostatic adjustment, complemented by post-seismic deformation models where necessary and applicable. Discontinuities are introduced to capture reference point changes associated with maintenance activities, including rail replacements at VLBI telescopes or equipment changes for GNSS and DORIS.

The ITRF2020 revealed a systematic drift in the intrinsic VLBI scale, with the VLBI legacy S/X station in Ny-Ålesund contributing roughly 50% of the effect. The remaining causes are currently being investigated by an International VLBI Service for Geodesy and Astrometry (IVS) working group, whose mandate covers several potential contributors to the drift; one of its specific areas of focus concerns station- and network-level effects. Missing or unmodeled discontinuities in station time series have the potential to degrade both the accuracy and the long-term stability of the terrestrial reference frame.

This study underscores the importance of detailed monitoring of VLBI instrumentation and demonstrates the impact of unmodeled equipment changes on scale accuracy. Using four VLBI legacy S/X stations (Tsukuba in Japan, Matera in Italy, Wettzell in Germany, and Zelenchukskaya in Russia) that underwent major repairs not represented in the ITRF2020 processing, we show that an additional 20% of the VLBI scale drift can be explained when these discontinuities are applied.

These results highlight that achieving the Global Geodetic Observing System (GGOS) goals, 1 mm accuracy and 0.1 mm/yr stability for the terrestrial reference frame, requires precise documentation and modeling of instrument behavior during maintenance and repair activities.