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Very Long Baseline Interferometry
Very Long Baseline Interferometry (VLBI) is a type of interferometry 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 is correlated with data from other antennas similarly recorded, to produce the resulting image. The resolution achievable using interferometry is inversely proportional to the distace 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. VLBI can therefore produce images with superior resolution. VLBI is most often performed at radio wavelengths; however, the technique has recently been extended to optics.
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 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.
- 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. This provides the highest resolution of all astronomical instruments, capable of imaging the sky with a level of detail measured in milliarcseconds.
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.
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 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 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
In VLBI interferometry, the data is 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 tapes.
The data tapes (or disk drives) are then transported to a central location. In the future the data will be sent in real time using fibre-optics - this cannot be done over normal internet connections as the data-rate in a VLBI observation is so high (far higher than the total global internet traffic - however experiments have already been done with E-VLBI in Europe using the GEANT fibre-optic network). Under the present scheme, the data is usually physically transported and then played back at the location of the correlator. The timing of the playback is adjusted according to the atomic clock signals on the tapes, 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 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 tapes are played back in synchronous 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 EB and EC 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.
Further details can be found here.
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