Very-long-baseline interferometry (VLBI) is a type of
astronomical interferometry used in
radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comin ...
. In VLBI a signal from an
astronomical radio source, such as a
quasar
A quasar is an extremely luminous active galactic nucleus (AGN). It is pronounced , and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from a galaxy nucleus is powered by a supermassive black hole with a mass rangi ...
, is collected at multiple radio telescopes on Earth or in space. The distance between the radio telescopes is then calculated using the time difference between the arrivals of the radio signal at different telescopes. This allows observations of an object that are made simultaneously by many radio telescopes to be combined, emulating a telescope with a size equal to the maximum separation between the telescopes.
Data received at each antenna in the array include arrival times from a local
atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwe ...
, such as a
hydrogen maser. At a later time, the data are correlated with data from other antennas that recorded the same radio signal, to produce the resulting image. The resolution achievable using interferometry is proportional to the observing frequency. The VLBI technique enables the distance between telescopes to be much greater than that possible with conventional
interferometry
Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber o ...
, which requires antennas to be physically connected by
coaxial cable
Coaxial cable, or coax (pronounced ) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ( insulating material); many coaxial cables also have a p ...
,
waveguide,
optical fiber
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass ( silica) or plastic to a diameter slightly thicker than that of a human hair
Hair is a protein filament that grows ...
, or other type of
transmission line
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...
. The greater telescope separations are possible in VLBI due to the development of the
closure phase imaging technique by
Roger Jennison in the 1950s, allowing VLBI to produce images with superior resolution.
VLBI is best known for imaging distant cosmic radio sources, spacecraft tracking, and for applications in
astrometry
Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way.
Hist ...
. However, since the VLBI technique measures the time differences between the arrival of radio waves at separate antennas, it can also be used "in reverse" to perform earth rotation studies, map movements of
tectonic plate
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
s very precisely (within millimetres), and perform other types of
geodesy
Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure (geometric shape and size), orientation in space, and gravity. The field also incorporates studies of how these properties change over time and equival ...
. Using VLBI in this manner requires large numbers of time difference measurements from distant sources (such as
quasar
A quasar is an extremely luminous active galactic nucleus (AGN). It is pronounced , and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from a galaxy nucleus is powered by a supermassive black hole with a mass rangi ...
s) observed with a global network of antennas over a period of time.
Method
In VLBI, the digitized antenna data are usually recorded at each of the telescopes (in the past this was done on large magnetic tapes, but nowadays it is usually done on large arrays of computer disk drives). The antenna signal is sampled with an extremely precise and stable atomic clock (usually a hydrogen
maser
A maser (, an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The first maser was built by Charles H. Townes, James ...
) that is additionally locked onto a GPS time standard. Alongside the astronomical data samples, the output of this clock is recorded. The recorded media are then transported to a central location. More recent experiments have been conducted with "electronic" VLBI (e-VLBI) where the data are sent by fibre-optics (e.g., 10 Gbit/s fiber-optic paths in the European
GEANT2 research network) and not recorded at the telescopes, speeding up and simplifying the observing process significantly. Even though the data rates are very high, the data can be sent over normal Internet connections taking advantage of the fact that many of the international high speed networks have significant spare capacity at present.
At the location of the correlator, the data are played back. The timing of the playback is adjusted according to the atomic clock signals, and the estimated times of arrival of the radio signal at each of the telescopes. A range of playback timings over a range of nanoseconds are usually tested until the correct timing is found.
Each antenna will be a different distance from the radio source, and as with the short baseline radio
interferometer
Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber o ...
the delays incurred by the extra distance to one antenna must be added artificially to the signals received at each of the other antennas. The approximate delay required can be calculated from the geometry of the problem. The tape playback is synchronized using the recorded signals from the atomic clocks as time references, as shown in the drawing on the right. If the position of the antennas is not known to sufficient accuracy or atmospheric effects are significant, fine adjustments to the delays must be made until interference fringes are detected. If the signal from antenna A is taken as the reference, inaccuracies in the delay will lead to errors
and
in the phases of the signals from tapes B and C respectively (see drawing on right). As a result of these errors the phase of the complex visibility cannot be measured with a very-long-baseline interferometer.
Temperature variations at VLBI sites can deform the structure of the antennas and affect the baseline measurements.
Neglecting atmospheric pressure and hydrological loading corrections at the observation level can also contaminate the VLBI measurements by introducing annual and seasonal signals, like in the Global Navigation Satellite System time series.
The phase of the complex visibility depends on the symmetry of the source brightness distribution. Any brightness distribution can be written as the sum of a
symmetric component and an
anti-symmetric component. The symmetric component of the brightness distribution only contributes to the real part of the complex visibility, while the anti-symmetric component only contributes to the imaginary part. As the phase of each complex visibility measurement cannot be determined with a very-long-baseline interferometer the symmetry of the corresponding contribution to the source brightness distributions is not known.
Roger Clifton Jennison developed a novel technique for obtaining information about visibility phases when delay errors are present, using an observable called the
closure phase. Although his initial laboratory measurements of closure phase had been done at optical wavelengths, he foresaw greater potential for his technique 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. This method was used for the first VLBI measurements, and a modified form of this approach ("Self-Calibration") is still used today.
Scientific results
Some of the scientific results derived from VLBI include:
*High resolution radio imaging of cosmic radio sources.
*Imaging the surfaces of nearby stars at radio wavelengths (see also
interferometry
Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber o ...
) – similar techniques have also been used to make infrared and optical images of stellar surfaces.
*Definition of the
celestial reference frame.
*Measurement of the acceleration of
the solar system toward the center of
the Milky Way.
*Motion of the 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
A terrestrial reference frame (TRF) is the reference frame as one views from Earth, or from the surface of another Earth-like planet. A TRF affects the way humans perceive almost everything from day to day because as they live on the Earth, as ...
.
*Measurement of
gravitational forces of the
Sun and
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
on the Earth and the deep structure of the Earth.
*Improvement of atmospheric models.
*Measurement of the fundamental
speed of gravity
In classical theories of gravitation, the changes in a gravitational field propagate. A change in the distribution of energy and momentum of matter results in subsequent alteration, at a distance, of the gravitational field which it produces. In ...
.
*The tracking of the
Huygens probe
''Huygens'' ( ) was an atmospheric entry robotic space probe that landed successfully on Saturn's moon Titan in 2005. Built and operated by the European Space Agency (ESA), launched by NASA, it was part of the ''Cassini–Huygens'' mission an ...
as it passed through
Titan's atmosphere, allowing wind velocity measurements.
*First imaging of a supermassive black hole.
VLBI arrays
There are several VLBI arrays located in
Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
,
Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...
, the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
,
Chile
Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the eas ...
,
Russia
Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-ei ...
,
China
China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
,
South Korea
South Korea, officially the Republic of Korea (ROK), is a country in East Asia, constituting the southern part of the Korea, Korean Peninsula and sharing a Korean Demilitarized Zone, land border with North Korea. Its western border is formed ...
,
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the n ...
,
Mexico
Mexico (Spanish language, Spanish: México), officially the United Mexican States, is a List of sovereign states, country in the southern portion of North America. It is borders of Mexico, bordered to the north by the United States; to the so ...
,
Australia
Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
and
Thailand
Thailand ( ), historically known as Siam () and officially the Kingdom of Thailand, is a country in Southeast Asia, located at the centre of the Indochinese Peninsula, spanning , with a population of almost 70 million. The country is b ...
. The most sensitive VLBI array in the world is the
European VLBI Network
The European VLBI Network (EVN) is a network of radio telescopes located primarily in Europe and Asia, with additional antennas in South Africa and Puerto Rico, which performs very high angular resolution observations of cosmic radio sources usin ...
(EVN). This is a part-time array that brings together the largest European radiotelescopes and some others outside of Europe for typically weeklong sessions, with the data being processed at the
Joint Institute for VLBI in Europe (JIVE). The
Very Long Baseline Array
The Very Long Baseline Array (VLBA) is a system of ten radio telescopes which are operated remotely from their Array Operations Center located in Socorro, New Mexico, as a part of the National Radio Astronomy Observatory (NRAO). These ten radi ...
(VLBA), which uses ten dedicated, 25-meter telescopes spanning 5351 miles across the United States, is the largest VLBI array that operates all year round as both an astronomical and
geodesy
Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure (geometric shape and size), orientation in space, and gravity. The field also incorporates studies of how these properties change over time and equival ...
instrument. The combination of the EVN and VLBA is known as
Global VLBI. When one or both of these arrays are combined with space-based VLBI antennas such as
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
microarcsecond
A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...
s. VLBI generally benefits from the longer baselines afforded by international collaboration, with a notable early example in 1976, when radio telescopes in the United States, USSR and
Australia
Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
were linked to observe
hydroxyl-maser sources. This technique is currently being used by the
Event Horizon Telescope, whose goal is to observe the
supermassive black holes at the centers of the
Milky Way Galaxy
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
and
Messier 87.
e-VLBI
VLBI has traditionally operated by recording the signal at each telescope on
magnetic tape
Magnetic tape is a medium for magnetic storage made of a thin, magnetizable coating on a long, narrow strip of plastic film. It was developed in Germany in 1928, based on the earlier magnetic wire recording from Denmark. Devices that use magnet ...
s or
disks, and shipping those to the correlation center for replay. In 2004 it became possible to connect VLBI radio telescopes in close to real-time, while still employing the local time references of the VLBI technique, in a technique known as e-VLBI. In Europe, six
radio telescopes of the
European VLBI Network (EVN) were connected with Gigabit per second links via their National Research Networks and the Pan-European research network
GEANT2, and the first astronomical experiments using this new technique were successfully conducted.
The image to the right shows the first science produced by the European VLBI Network using e-VLBI. The data from each of the telescopes were routed through the
GÉANT2
GÉANT is the pan- European data network for the research and education community. It interconnects national research and education networks (NRENs) across Europe, enabling collaboration on projects ranging from biological science, to earth obse ...
network and on through
SURFnet
SURF is an organization that develops, implements and maintains the national research and education network (NREN) of the Netherlands, It operates the national research network formally called SURFnet.
SURF as a network is a backbone computer net ...
to be the processed in real time at the European Data Processing centre at
JIVE.
Space VLBI
In the quest for even greater angular resolution, dedicated VLBI satellites have been placed in Earth orbit to provide greatly extended baselines. Experiments incorporating such space-borne array elements are termed Space Very Long Baseline Interferometry (SVLBI). The first SVLBI experiment was carried out on
Salyut-6
Salyut 6 (russian: Салют-6; lit. Salute 6), DOS-5, was a Soviet orbital space station, the eighth station of the Salyut programme. It was launched on 29 September 1977 by a Proton rocket. Salyut 6 was the first space station to receiv ...
orbital station with KRT-10, a 10-meter radio telescope, which was launched in July 1978.
The first dedicated SVLBI satellite was
HALCA, an 8-meter
radio telescope
A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency ...
, which was launched in February 1997 and made observations until October 2003. Due to the small size of the dish, only very strong radio sources could be observed with SVLBI arrays incorporating it.
Another SVLBI satellite, a 10-meter radio telescope
Spektr-R, was launched in July 2011 and made observations until January 2019. It was placed into a highly elliptical orbit, ranging from a perigee of 10,652 km to an apogee of 338,541 km, making RadioAstron, the SVLBI program incorporating the satellite and ground arrays, the biggest radio interferometer to date. The resolution of the system reached 8
microarcsecond
A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...
s.
International VLBI Service for Geodesy and Astrometry
The International VLBI Service for Geodesy and Astrometry (IVS) is an international collaboration whose purpose is to use the observation of astronomical radio sources using VLBI to precisely determine
earth orientation parameters
In geodesy and astrometry, earth orientation parameters (EOP) describe irregularities in the rotation of planet Earth.
EOP provide the rotational transform from the International Terrestrial Reference System (ITRS) to the International Celestial ...
(EOP) and celestial reference frames (CRF) and terrestrial reference frames (TRF).
IVS is a service operating under the
International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...
(IAU) and the
(IAG).
References
External links
E-MERLINfibre-linked radio telescope array used in VLBI observations
EXPReSExpress Production Real-time e-VLBI Service: a three-year project (est. March 2006) funded by the European Commission to develop an intercontinental e-VLBI instrument available to the scientific community
JIVEJoint Institute for VLBI in Europe
The International VLBI Service for Geodesy and Astrometry (IVS)IVSOPAR: the VLBI analysis center at the Paris Observatory"VLBI - Canada's Role"
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Astronomical imaging
Interferometry
Radio astronomy
Geodesy
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