very-long-baseline interferometry

The term very-long-baseline interferometry (VLBI) is self-explanatory, but the term is generally used for radio interferometry with baselines sufficiently long to make real-time processing of the signals unwieldy, which is generally true of interferometry using receivers at separate sites. Among the longest such baselines have been between sites on opposite sides of the Earth (Earth-sized VLBI), and space telescopes (e.g., Spektr-R) have been included for even longer baselines.

The signal data as well as its timing are stored at each telescope and later processing of the collected data determines the exact direction from which the signal came, allowing telescopes far apart (e.g., thousands of miles) to be used. Its use has achieved angular resolutions in the 10 microarcsecond range. VLBI arrays often use existing distantly-separated telescopes, and may be short-term projects, such as for specific surveys. Some VLBI arrays:

• Very Long Baseline Array (VLBA).
• European VLBI Network (EVN).
• VERA.
• Event Horizon Telescope (EHT).

A more recent development is electronic very-long-baseline interferometry (e-VLBI), which is near-real-time transmission of the data for immediate analysis. (The challenge is that the amount of observation data necessary to carry out the analysis can be so large as to make this difficult or impossible.)

The purpose of VLBI is to observe radio sources in detail, but a byproduct is the measurement of the distance between receivers down to the millimeter range, precise enough to reveal the effects of plate tectonics.