VLBI

Very Long Baseline Interferometry (VLBI) is a radio astronomy technique that combines observations from widely separated antennas to form an effective telescope with a diameter equal to the maximum baseline between stations. By correlating radio signals that are time-stamped with atomic clocks, VLBI attains extremely high angular resolution, often microarcseconds, enabling detailed imaging of compact objects.

Each participating telescope records the incoming radiation along with precise time markers, typically using hydrogen masers. The data are transported to a central correlator where cross-correlations are computed. The resulting fringe patterns are processed to produce images and to measure precise positions. Some networks perform real-time e-VLBI over high‑speed networks, but most observations are correlated after the fact.

VLBI has broad applications in astronomy and geodesy. It produces sharp images of active galactic nuclei, quasars, masers, and pulsars, and it enables precise astrometry, parallax measurements, and proper motions. In geodesy, VLBI contributes to the International Celestial Reference Frame and the International Terrestrial Reference Frame, tracking Earth’s orientation and plate motions. Space-based VLBI extends baselines beyond Earth’s diameter, increasing resolution further.

Typical observing bands include centimeter and millimeter wavelengths, such as L, S, X bands and higher frequencies for mm-VLBI. Major facilities include ground networks and arrays such as the VLBA, EVN, and GMVA, with occasional space antennas. The technique requires careful calibration to mitigate atmospheric and instrumental effects and generates large data rates requiring substantial processing.