
VLBI is a radio astronomy technique that combines signals from multiple telescopes to achieve high-resolution imaging.VLBI is a technique in radio astronomy where signals from distant radio telescopes are combined taking into account the delay in wave arrival. Resolving power (ability to distinguish fine details) depends on the distance between antennas, not their size. With baselines of thousands of kilometers, an angular resolution of microarcseconds is achieved — one can see a lighter on the Moon. To combat atmospheric distortions, the method of closure phases is used, combining signals from three or more antennas.
The principle of interferometry in radio astronomy was developed by Martin Ryle and others in the 1940s–50s. A scheme with independent local oscillators and magnetic tape recording was proposed in the early 1960s by Soviet and American scientists. The first successful observations in 1967 were performed by groups in the USA and Canada, using rubidium frequency standards. Subsequently, the transition to digital recording and the advent of hydrogen masers increased accuracy; today, real-time data transmission (e-VLBI) via fiber optics is being introduced.
Radio telescopes, operating as a single interferometer, record the signal along with precise time stamps from atomic clocks. The correlator later aligns the data, compensating for geometric delays due to Earth's rotation and antenna positions. Resolution is determined by the ratio of wavelength to baseline projection: θ ≈ λ / B. For wavelength 1.3 mm and baseline 8000 km, θ ≈ 20 microarcseconds. For millimeter VLBI, suppression of water vapor is critical, so telescopes are built at altitudes above 3000 m.
Thanks to VLBI, the first image of a black hole's shadow was obtained, and the movement of lithospheric plates is measured with millimeter precision.
The largest virtual telescope — the Event Horizon Telescope — combined stations from Antarctica to Europe to show the silhouette of a black hole in 2019.























