Ultra-Precise relative Astrometry with next-generation VLBI: Methods, Opportunities and Requirements

We are at the cusp of a generational shift in the power of radio interferometers, with exciting next-generation instruments, such as SKA, ngVLA and upgraded EHT, that are coming online. Next-generation VLBI observations bring a potential order-of-magnitude improvements in sensitivity and in astrometric performance, provided that the requirements for optimum calibration methods and usability are not designed out. We have been exploring the implications for astrometry of these new instruments, as sensitivity alone will not improve over the current astrometric limits - being systematic and not driven by thermal noise. Thus using traditional phase-referencing techniques will not be able to significantly overcome the current astrometric accuracy nor extend the moderate range of operational frequencies. The developments that drive forward the field of ultra-precise astrometry constitute a very active field of research. Next-generation methods such as Source-Frequency-Phase-Referencing and/or MultiView, combined with next-generation technology, offer an order-of-magnitude increase both in accuracy and in the range of operating frequencies across the radio spectrum, for both ground and space-VLBI domains. In this talk I will report on the fundamentals, potential and recent on-going steps to confirm that these new methods will deliver on their promise and to ensure that any technological requirements are not designed out from the next-generation instruments. We foresee that the combination of the next-generation developments (on instruments, methods and technology) will have the greatest impact in the compilation of large and unbiased ultra-precise astrometric surveys, compared to current instruments, expanding the role of VLBI into new areas of research.