Obtaining Local-Tie Vectors from Short-Baseline Interferometry

Paper in proceeding, 2023

With the VLBI Global Observing System (VGOS) being the next step in the development of geodetic VLBI, it is necessary to connect the new VGOS network to the existing legacy S/X telescopes. At the Onsala Space Observatory (OSO), this is being done by short-baseline interferometry between the VGOS Onsala twin telescopes ONSA13SW and ONSA13NE and the legacy antenna ONSALA60.
The main aim of these sessions, referred to as ONTIE, is to obtain local-tie vectors between these three OSO telescopes that all take part in regular geodetic VLBI observations. Each ONTIE session is about 24 h long, during which all three telescopes observe simultaneously the same sources at X-band. A total of 37 ONTIE sessions have been observed since April 2019. In November 2021, the ONTIE sessions were for the first time observed with alternative observation frequency setups in order to mitigate the influence of known RFI. Additionally, scheduling was done — also for the first time — with VieSched++ instead of sked.
Interesting findings of the ONTIE sessions include unexpected offsets in the results of group and phase delays, jumps in the coordinates of the twin telescopes, and apparent yearly trends that might be an artifact of unmodeled thermal expansion of the telescopes that is left in the data.
Future ONTIE sessions are envisioned to happen on a regular basis and could, as a by-product, also serve as quasar flux-monitoring sessions by investigation of the recorded system temperatures during observation.
This paper summarizes the current status and results of the ONTIE sessions.