For the interested reader, who might want to know more about VLBI astrometry, maser trigonometric parallaxes, and, more generally, about Galactic distances determined at radio frequencies,  we refer to the following papers:

• Microarcsecond Radio Astrometry 

M. J. Reid & M. Honma, 2014,  Annual Review of Astronomy and Astrophysics, 52, 339

• Techniques for Measuring Parallax and Proper Motion with VLBI 

M. J. Reid, 2022, Publications of the Astronomical Society of the Pacific, 134, 1042

• Mapping the Far Side of the Milky Way 

M. J. Reid, 2024, Proceedings of the International Astronomical Union, 380, 111