How can antennas on different continents work together as one telescope?

Each antenna records its signal locally along with extremely precise atomic-clock timestamps. The recordings are later shipped or transferred to a central correlator that aligns them in time and combines them, so the antennas need no direct real-time connection to act as a single instrument.

Why does VLBI give such sharp images?

Angular resolution improves as the separation between antennas grows. By placing antennas thousands of kilometres apart, or even in space, VLBI synthesises an aperture nearly the size of Earth, reaching milliarcsecond and microarcsecond resolution far beyond any single telescope.

Very Long Baseline Interferometry

Very long baseline interferometry links radio antennas separated by hundreds to thousands of kilometres, even across continents, to achieve the highest angular resolution attainable in astronomy.

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Definition

Very long baseline interferometry is an interferometric technique in which antennas too far apart for a direct link record signals locally against atomic-clock references, and the recordings are later brought together and correlated to synthesise an aperture comparable to the separation of the antennas.

Scope

This topic covers independent recording of signals with precise atomic-clock timestamps, later correlation of the recordings, fringe fitting to recover delay and rate, the milliarcsecond and microarcsecond resolutions achieved, space VLBI, and applications from imaging black-hole environments to geodesy and reference-frame definition.

Core questions

How are widely separated antennas combined without a physical link?
Why is precise timekeeping essential to VLBI?
What resolution can VLBI achieve and what sets the limit?
What science and geodetic applications does VLBI enable?

Key theories

Independent recording and correlation: Each station records its signal with timestamps from a hydrogen-maser clock, and the data are later aligned and correlated, so no real-time link between antennas is required.
Fringe fitting and atmospheric phase: Unknown clock offsets and atmospheric delays are solved for by fringe fitting, recovering the interferometric phase needed to combine intercontinental baselines.
Microarcsecond imaging of compact sources: Baselines spanning the globe and into space yield resolutions fine enough to image the immediate surroundings of supermassive black holes, as demonstrated by the Event Horizon Telescope.

Clinical relevance

VLBI delivers the sharpest images in astronomy, resolving black-hole shadows, relativistic jets, and stellar masers, and it underpins the celestial reference frame, precise plate-tectonic measurements, and spacecraft navigation.

History

Developed in the late 1960s when atomic clocks and tape recording allowed antennas to be correlated after the fact, VLBI grew into global networks and space-based extensions. Its resolution culminated in the Event Horizon Telescope's 2019 image of the shadow of the black hole in M87.

Key figures

Roger Jennison
Kenneth Kellermann

Seminal works

thompson2017
eht2019

Frequently asked questions

How can antennas on different continents work together as one telescope?: Each antenna records its signal locally along with extremely precise atomic-clock timestamps. The recordings are later shipped or transferred to a central correlator that aligns them in time and combines them, so the antennas need no direct real-time connection to act as a single instrument.
Why does VLBI give such sharp images?: Angular resolution improves as the separation between antennas grows. By placing antennas thousands of kilometres apart, or even in space, VLBI synthesises an aperture nearly the size of Earth, reaching milliarcsecond and microarcsecond resolution far beyond any single telescope.