Invited Presentation

Multi‑frequency VLBI and Frequency Phase Transfer at Millimeter Wavelengths

Author(s): Guang-Yao Zhao (MPIfR), Taehyun Jung (KASI), Andrei Lobanov (MPIfR), Bong Won Sohn (KASI), Eduardo Ros (MPIfR), Alan Roy (MPIfR), Jan Wagner (MPIfR), Pablo Torne (IRAM), Anton Zensus (MPIfR), Maria Rioja (UWA/ICRAR), Richard Dodson (ICRAR), ...

Presenter: Guang-Yao Zhao (Max Planck Institute for Radio Astronomy)

Recent advances in multi‑frequency receivers are transforming mm‑VLBI, with Frequency Phase Transfer (FPT) emerging as a key technique for high‑frequency observations. By mitigating turbulent tropospheric phase fluctuations, FPT extends phase coherence times and significantly improves sensitivity. The Korean VLBI Network (KVN) has pioneered the routine use of simultaneous multi‑frequency observations, demonstrating the strong scientific potential enabled by FPT.
In this talk, we review recent progress in dual‑ and triple‑band VLBI capabilities across the global mm‑VLBI community. First, a global VLBI array equipped with triple‑band receivers operating at 22/43/86 GHz is rapidly maturing. Pilot studies have delivered high‑fidelity imaging and high‑precision core‑shift measurements, which are critical for probing magnetic field structures in AGN jets.
Furthermore, recent test observations have demonstrated the feasibility of extending FPT to even higher frequencies, reaching 258 GHz with APEX and the IRAM 30‑m telescope. Future additions of facilities such as JCMT, GLT, and other mm/submm telescopes are expected to enable transformative improvements in black hole imaging and studies of ultracompact regions, including AGN jets.