Very Long Baseline Interferometry
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VLBI
Multiple antenna radio astronomy is also known as interferometry. When the antennas are too far apart to allow the antennas to be connected by conventional cables, the data are recorded on magnetic tape (or recently hard disks) and shipped to a central processing location (correlator). This technique is known as Very Long Baseline Interferometry (VLBI). VLBI involves using a number of antennas linked together to create a giant antenna which can resolve features with smaller angles. The band of radio waves used depends on what they want to achieve for the particular experiment. A related topic is optical interferometry, which can be used to image astronomical objects in the optical with very high resolution using the same principles as radio VLBI.
Some of the scientific results derived from VLBI include:
- Motion of the Earth's tectonic plates
- Regional deformation and local uplift or subsidence.
- Definition of the celestial reference frame
- 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.
- 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)
VLBI is used mostly for imaging distant cosmic radio sources, astrometry (defining the celestial reference frame), geodesy and Earth rotation studies. It is essential for accurate spacecraft tracking that the positions of the antennas is known to the millimetre. This technique measures the time differences between the arrival of radio waves from distant sources (such as quasars) at two separate antennas. Using large numbers of time difference measurements from many quasars observed with a global network of antennas over a period of time, it is possible to map movements of tectonic plates to within millimetres.
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. 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.
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 consists of an eight meters radio telescope. It was launched in February 1997 and is orbiting the Earth in an elliptical orbit to enable VLBI observations on baselines between space and ground telescopes. The primary targets are active galactic nuclei, 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 predicts, 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 is required for all supports on this system.
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