verylongbaseline

Very long baseline interferometry (VLBI) is a technique used in radio astronomy and geodesy that combines signals from widely separated radio telescopes to form an effective aperture equal to the largest distance between the antennas. The term highlights baselines that are much longer than those of individual telescope arrays, allowing angular resolutions that can reach tens of microarcseconds at millimeter wavelengths.

In VLBI, each telescope records the incoming radio waves with highly accurate time stamps from atomic clocks. The data are then correlated after the fact to produce interferometric visibilities, which encode how the signal combines across different baselines. Calibration procedures correct for atmospheric delays, instrumental delays, and clock errors. Data may be shipped on physical media or transmitted over high-speed networks for centralized processing.

A number of global networks implement VLBI, including the Very Long Baseline Array (VLBA) in the United States, the European VLBI Network (EVN), and regional arrays such as the Australian Long Baseline Array (LBA). Global VLBI combines antennas worldwide to achieve the longest baselines. A notable example is the Event Horizon Telescope (EHT), which operates at millimeter wavelengths to image supermassive black holes by exploiting the longest practical baselines available.

Applications of VLBI cover high-precision astrometry and geodesy, including measurements of parallax, proper motions, and plate tectonics, as well as high-resolution imaging of compact radio sources like quasars, active galactic nuclei, and masers. In astrophysics, VLBI supports tests of fundamental physics and gravity and provides detailed structural information about distant objects.

History traces VLBI to experiments in the 1960s, with ongoing improvements in recording bandwidth, timekeeping, and data processing enabling increasingly sensitive and high-resolution global networks.