What's VERA ?
Dual-Beam VLBI : free from the atmosphere
Most unique aspect of VERA is `dual-beam' telescope, which can observe two nearby sources at the same time. While single-beam VLBI significantly suffers from fluctuation of atmosphere, VERA's dual-beam observations effectively cancel out the atmospheric fluctuations, and then VERA can measure relative positions of target sources with respect reference sources with highes accuracy ever acheived. Such VLBI observation technique is referred to as 'phase-referencing' observation.
VERA is the only VLBI array dedicated to phase-referencing astrometry.
In dual-beam mode, two adjacent sources are observed simultaneously, and atmospheric fluctuations, which are common to the two sources, are cancelled out as far as relative positions of two sources are measured.