Very Long Baseline Interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy, in which the data received at each antenna in the array is paired with timing information, usually from a local atomic clock, and then stored for later analysis on magnetic tape or hard disk. At that later time, the data are correlated with data from other antennas similarly recorded, to produce the resulting image. The resolution achievable using interferometry is proportional to the distance between the antennas furthest apart in the array. The VLBI technique enables this distance to be much greater than that possible with conventional interferometry, which requires antennas to be physically connected by coaxial cable, waveguide, optical fiber, or other type of transmission line. 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 most often performed at radio wavelengths; however, the technique has recently been extended to optics. An astronomical interferometer or hypertelescope is an array of telescopes or mirror segments acting together to probe structures on higher resolutions. ... Microwave image of 3C353 galaxy at 8. ... Atomic clock Chip-Scale Atomic Clock Unveiled by NIST An atomic clock is a type of clock that uses an atomic resonance frequency standard as its counter. ... Coaxial cable is an electrical cable consisting of a round conducting wire, surrounded by an insulating spacer, surrounded by a cylindrical conducting sheath, and usually surrounded by a final insulating layer. ... To meet Wikipedias quality standards and appeal to a wider international audience, this article may require cleanup. ... Optical fibers An optical fiber (or fibre) is a transparent thin fiber, usually made of glass or plastic, for transmitting light. ... A transmission line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic waves or acoustic waves, as well as electric power transmission. ... The closure phase is an observable quantity in imaging interferometry, which allowed the use of interferometry with very long baselines. ... Roger Clifton Jennison worked as a radio astronomer at Jodrell Bank under the guidance of Robert Hanbury Brown. ... This article is in need of attention. ...

VLBI is most well-known for imaging distant cosmic radio sources, spacecraft tracking, and for applications in astrometry. 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 plates very precisely (within millimetres), and perform other types of geodesy. Using VLBI in this manner requires large numbers of time difference measurements from distant sources (such as quasars) observed with a global network of antennas over a period of time. This article does not cite its references or sources. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... It has been suggested that geodetic system be merged into this article or section. ... This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous starburst ever seen in such a combination. ...

Some of the scientific results derived from VLBI include:

• Imaging high-energy particles being ejected from black holes at enormous velocities (see quasar)
• Imaging the surfaces of nearby stars at radio wavelengths (see also interferometry)
• Definition of the celestial reference frame
• Motion of the Earth's tectonic plates
• Regional deformation and local uplift or subsidence.
• Variations in the Earth's orientation and length of day.
• Maintenance of the terrestrial reference frame
• Measurement of gravitational forces of the Sun and Moon on the Earth and the deep structure of the Earth
• Improvement of atmospheric models
• Measurement of the fundamental speed of gravity
• The tracking of the Huygens probe as it passed through Titan's atmosphere, allowing wind velocity measurements

There are several VLBI arrays located in Europe, the US and Japan. The most sensitive VLBI array in the world is the European VLBI Network (EVN). This is a part-time array with the data being processed at the Joint Institute for VLBI in Europe (JIVE). In the US the Very Long Baseline Array (VLBA) operates all year round. The EVN and VLBA mostly conduct astronomical observations - the combination of the EVN and VLBA is known as Global VLBI. When one or both of these arrays are combined with one or more space-based VLBI antennas such as HALCA the resolution obtained is higher than any other astronomical instruments, capable of imaging the sky with a level of detail measured in microarcseconds. This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous starburst ever seen in such a combination. ... Interferometry is the applied science of combining two or more input points of a particular data type, such as optical measurements, to form a greater picture based on the combination of the two sources. ... This article covers the physics of gravitation. ... For other uses, see Sun (disambiguation). ... Crust composition Oxygen 43% Silicon 21% Aluminium 10% Calcium 9% Iron 9% Magnesium 5% Titanium 2% Nickel 0. ... The speed of gravity is the speed at which changes in the location of an object propagate their gravitational effects to all other objects in the Universe. ... A scale replica of the probe An artists impression of the Huygens probe as it descends through Titans murky, brownish-orange atmosphere of nitrogen and carbon-based molecules, beaming its findings to the distant Cassini orbiter. ... Atmospheric characteristics Pressure 146. ... VLBA locations. ... The HALCA satellite is an 8 meter diameter radio telescope used for Very Long Baseline Interferometry. ...

Recently it has become possible to connect the VLBI radio telescopes in real-time, while still employing the local time references of the VLBI technique. In Europe, 6 telescopes are now connected to JIVE with optical fibres at 1 Gigabit per second and the first astronomical experiments using this new technique (e-VLBI) have been successfully conducted. This article needs to be cleaned up to conform to a higher standard of quality. ...

Space VLBI

The latest development in radio astronomy observations is the Space Very Long Baseline Interferometry (SVLBI) program. This is used to perform radio astronomy with an extended baseline VLBI, of which one element is a space-based antenna.

The JPL SVLBI project, funded by NASA, supports the VSOP (VLBI Space Observatory Program) mission developed by the Institute of Space and Astronautical Science (ISAS) in Japan. The VSOP spacecraft HALCA is an 8 meter radio telescope, and was launched in February 1997. It is now in an elliptical orbit around the Earth to enable VLBI observations on baselines between space and ground telescopes. The primary targets are active galactic nuclei, but water masers, OH masers, radio stars, and pulsars will also be observed. The JPL complex in Pasadena, Ca. ... NASA logo Listen to this article · (info) This audio file was created from an article revision dated 2005-09-01, and does not reflect subsequent edits to the article. ... VSOP can be: Very Superior Old Pale (a grade of cognac (drink)) Very Small Outline Package (a type of surface-mount integrated circuit package) V.S.O.P. is a 1977 Herbie Hancock album. ... The HALCA satellite is an 8 meter diameter radio telescope used for Very Long Baseline Interferometry. ... The 64 metre radio telescope at Parkes Observatory, New South Wales, Australia In contrast to an ordinary telescope, which produces visible light images, a radio telescope sees radio waves emitted by radio sources, typically by means of a large parabolic (dish) antenna, or arrays of them. ... Look up February in Wiktionary, the free dictionary February is the second month of the year in the Gregorian Calendar. ... 1997 (MCMXCVII in Roman) is a common year starting on Wednesday of the Gregorian calendar. ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... 50 cm refracting telescope at Nice Observatory. ... An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ... An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum. ... Radio stars are stars that produce by means of chemical and electrical discharges, emissions of various radio frequencies, whether constant or pulsed. ... Composite Optical/X-ray image of the Crab Nebula pulsar, showing surrounding nebular gases stirred by the pulsars magnetic field and radiation. ...

The baselines between space and ground telescopes will provide 3 to 10 times the resolution available for ground VLBI at the same observing frequencies. Four ground tracking stations are involved with the SVLBI project. The word resolution has several meanings, depending on context. ... Sine waves of various frequencies; the lower waves have higher frequencies than those above. ...

The whole system was supposed to operate automatically, needing only the observing schedule, Doppler predictions, and spacecraft state vectors to perform all the acquisition and tracking functions, with no operator inputs. This however has not yet been achieved and an operator presently is required to support this system. Sound waves emanating from an ambulance moving to the right. ... Ariane 5 lifts off with the Rosetta space probe on March 2, 2004. ...

How VLBI Works

Recording data at each of the telescopes in a VLBI array. Extremely accurate high-frequency clocks are recorded alongside the astronomical data in order to help get the synchronization correct

In VLBI interferometry, the 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 RAID arrays of computer disk drives). Alongside the astronomical data, the output of an extremely accurate atomic clock is recorded on the tape/disk media. The recorded media are then transported to a central location. More recently experiments ("E-VLBI") have been conducted where the data are sent by fibre-optics (e.g., the GEANT fibre-optic 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. In VLBI, the radio signals are recorded at each of the individual telescopes before being shipped to a central location. ... In VLBI, the radio signals are recorded at each of the individual telescopes before being shipped to a central location. ... In computing, a Gayness, also known as redundant array of inexpensive disks (commonly abbreviated RAID) is a system which uses multiple hard drives to share or replicate data among the drives. ... GÉANT is the main European multi-gigabit computer network for research and education purposes. ...

At the location of the correlator the data are played back. The timing of the playback is adjusted according to the atomic clock signals on the (tapes/disk drives/fibre optic signal), 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.

Playing back the data from each of the telescopes in a VLBI array. Great care must be taken to synchronize the play back of the data from different telescopes. Atomic clock signals recorded with the data help in getting the timing correct.

Each antenna will be a different distance from the radio source, and as with the short baseline radio interferometer 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 ε_B and ε_C 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. Data recorded at different VLBI telescopes are brought to a central location and played back with careful synchronization. ... Data recorded at different VLBI telescopes are brought to a central location and played back with careful synchronization. ... Atomic clock Chip-Scale Atomic Clock Unveiled by NIST An atomic clock is a type of clock that uses an atomic resonance frequency standard as its counter. ... Interferometry is the applied science of combining two or more input points of a particular data type, such as optical measurements, to form a greater picture based on the combination of the two sources. ...

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.

R. C. 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. A minimum of 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. Roger Clifton Jennison worked as a radio astronomer at Jodrell Bank under the guidance of Robert Hanbury Brown. ... The closure phase is an observable quantity in imaging interferometry, which allowed the use of interferometry with very long baselines. ... 1958 (MCMLVIII) was a common year starting on Wednesday of the Gregorian calendar. ... 1974 (MCMLXXIV) was a common year starting on Tuesday (the link is to a full 1974 calendar). ...

Further details can be found here.