On the importance of closely monitoring VLBI telescope reference points

Paper in proceeding, 2024

In the new realisation of the International Terrestrial Reference System, ITRF2020, the VLBI scale factor time series shows a significant positive drift after 2013.75.
During the past two years we investigated various causes for this scale drift and concluded that mis-modelling of the IVS station position temporal evolution, as well as ignoring major IVS station events, significantly impact the VLBI scale.
This presentation focuses on the latter aspect: IVS station events, and is the result of an extensive study of possible station events that happened for the stations that observe, or observed, the most in the IVS network. We reviewed IVS annual reports and compiled a list of possible station events that could generate changes in the reference point positions of the VLBI telescopes.
Five stations in the IVS network seem to be the most concerned: TSUKUB32, MATERA, ONSALA60, WETTZELL, and NYALES20. The five station events considered for MATERA correspond to repair of the concrete pedestal under rail in 2005 and azimuth rail wheel replacements in 2008, 2009, 2015 and 2018. There is one station event considered for WETTZELL due to gear wheels repair and new elevation bearings as well as re-adjustment of the dish surface in 2010, one station event for ONSALA60 due to subreflector control electronics replacement which triggered a need for a new pointing model in 2018, two station events for NYALES20 due to the replacement of the gear box in 2013 and a broken azimuth gear in 2016. We also consider two station events for TSUKUB32, due to the repair of the subreflector supporting structures in 2012 and the repair of the antenna base in 2013.
To quantify the impact of these station events, we considered the official IVS combined solution, i.e. the IVS contribution to the ITRF2020 realisation, and our station event list, and calculated scale factors using the CATREF software with the single-technique combination strategy that was used to generate the ITRF2020.
Including these station events as station breaks in the ITRF2020 discontinuity lists results in a reduction of 36~\% of the VLBI scale drift after 2013.75, even though some of these station events happened before 2013.75, and demonstrates the impact of a reference point position change on the VLBI scale.