Very-long-baseline interferometry
VLBI combines radio telescopes to emulate a giant telescope.
Very-long-baseline interferometry (VLBI) is a technique in radio astronomy that collects signals from a cosmic radio source—like a quasar—using multiple radio telescopes located on Earth or in space. The separation between these telescopes is determined by measuring the difference in arrival times of the radio signal at each one. By combining simultaneous observations from many telescopes, the system acts like a single, much larger telescope whose effective size equals the greatest distance between any two antennas. The resolution of this method scales with the observing frequency. Unlike conventional interferometry, which requires telescopes to be linked by physical connections such as coaxial cable or optical fiber, VLBI can use much larger separations thanks to the closure phase imaging technique.
Each antenna in the array records the incoming data along with precise timestamps from a local atomic clock, typically a hydrogen maser. These recordings are later correlated with data from other antennas that captured the same signal to produce an image. VLBI is widely used for imaging distant cosmic radio sources, tracking spacecraft, and astrometry. It can also be applied in reverse: by measuring the time differences of radio wave arrivals at separate antennas, researchers can study Earth’s rotation, map tectonic plate movements to within millimeters, and perform other geodetic tasks. This reverse application requires many time-difference measurements from distant sources, like quasars, observed over time by a global network of antennas.
**Method**
In VLBI, digitized antenna data are usually recorded locally at each telescope—historically on large magnetic tapes, now on large arrays of computer disk drives. The signal is sampled using an extremely precise atomic clock (often a hydrogen maser) locked to a GPS time standard, and the clock’s output is recorded alongside the astronomical data. The recorded media are then physically transported to a central location. More recent experiments use electronic VLBI (e-VLBI), where data are sent via fiber-optic connections (such as 10 Gbit/s paths in the European GEANT2 research network) rather than recorded locally, speeding up and simplifying the process.
- Technique developer
- Roger Jennison
- Closure phase development year
- 1950s
- First demonstration of closure phase wit
- 1958
- Wide adoption of closure phase for long
- 1974
- Largest vlbi array operating year round
- Very Long Baseline Array (VLBA)
- Vlba telescope count
- 10
- Vlba telescope diameter meters
- 25
- Vlba span miles
- 5351
Lore & Background
The VLBI technique was enabled by the development of the closure phase imaging technique by Roger Jennison in the 1950s. Although his initial laboratory measurements of closure phase were at optical wavelengths, he foresaw greater potential in radio interferometry. In 1958 he demonstrated its effectiveness with a radio interferometer, but it only became widely used for long-baseline radio interferometry in 1974. At least three antennas are required for this method, which was used for the first VLBI measurements, and a modified form called 'Self-Calibration' is still used today.
In VLBI, digitized antenna data are usually recorded at each telescope, historically on large magnetic tapes but now on large arrays of computer disk drives. The antenna signal is sampled with an extremely precise atomic clock (usually a hydrogen maser) locked onto a GPS time standard. The recorded media are transported to a central location for correlation. More recent experiments use 'electronic' VLBI (e-VLBI), where data are sent by fiber-optics (e.g., 10 Gbit/s paths in the European GEANT2 research network) and not recorded at the telescopes, speeding up the process. At the correlator, playback timing is adjusted according to atomic clock signals and estimated arrival times; a range of timings over nanoseconds are tested until the correct timing is found.
Temperature variations at VLBI sites can deform antenna structures and affect baseline measurements. Neglecting atmospheric pressure and hydrological loading corrections can contaminate measurements by introducing annual and seasonal signals. The phase of the complex visibility cannot be measured with a very-long-baseline interferometer due to delay errors, but the closure phase technique allows obtaining information about visibility phases when such errors are present.
Reader's Guide
VLBI is best known for imaging distant cosmic radio sources, spacecraft tracking, and applications in astrometry. It can also be used 'in reverse' to perform Earth rotation studies, map tectonic plate movements very precisely (within millimetres), and perform other types of geodesy, requiring large numbers of time difference measurements from distant sources like quasars observed with a global network of antennas over time. Scientific results from VLBI include high resolution radio imaging of cosmic radio sources including active galactic nuclei, imaging the surfaces of nearby stars at radio wavelengths, definition of the celestial reference frame, measurement of the acceleration of the Solar System toward the center of the Milky Way, motion of Earth's tectonic plates, regional deformation and local uplift or subsidence, Earth's orientation parameters and fluctuations in the length of day, maintenance of the terrestrial reference frame, measurement of gravitational forces of the Sun and Moon on Earth and the deep structure of Earth, improvement of atmospheric models, measurement of the fundamental speed of gravity, tracking of the Huygens probe as it passed through Titan's atmosphere allowing wind velocity measurements, and first imaging of a supermassive black hole. The most sensitive VLBI array is the European VLBI Network (EVN), a part-time array. The Very Long Baseline Array (VLBA) is the largest VLBI array operating all year round. The combination of EVN and VLBA is known as Global VLBI, and when combined with space-based VLBI antennas like HALCA or Spektr-R, the resolution obtained is higher than any other astronomical instrument, capable of imaging the sky with a level of detail measured in microarcseconds. This technique is currently used by the Event Horizon Telescope to observe supermassive black holes at the centers of the Milky Way Galaxy and Messier 87.
Did You Know?
- The closure phase technique was developed by Roger Jennison in the 1950s and first demonstrated with a radio interferometer in 1958.
- VLBI can measure tectonic plate movements within millimetres.
- The Very Long Baseline Array (VLBA) uses ten dedicated 25-meter telescopes spanning 5351 miles across the United States.
- The Event Horizon Telescope uses VLBI to observe supermassive black holes.
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