Very Long Baseline Interferometry
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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 are 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 observing frequency and 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 the optical regime.
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.
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 interferometer ) -- similar techniques have also been used to make infrared and optical images of stellar surfaces
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 measurementsThere 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.
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.
[Edit] 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 baselines between space and ground telescopes 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 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.
: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 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 RAID arrays of computer disk drives). The antenna signal is sampled with an extremely precise and stable atomic clock (usually a hydrogen maser) that is additionally locked onto a GPS time standard. Alongside the astronomical data samples, the output of this clock is recorded on the tape/disk media. The recorded media are then transported to a central location. More recently experiments have been conducted with "electronic" VLBI (e-VLBI) where the data are sent by fibre-optics (e.g., 10 Gbps fiber-optic paths in the European GEANT 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 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 εB and εC 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.
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.
