Very-long-baseline interferometry
Very-long-baseline interferometry is a type of astronomical interferometry used in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth or 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, 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, 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 best 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, and perform other types of geodesy. Using VLBI in this manner requires large numbers of time difference measurements from distant sources observed with a global network of antennas over a period of time.
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 - 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 measurements.
- First imaging of a supermassive black hole.
VLBI arrays
e-VLBI
VLBI has traditionally operated by recording the signal at each telescope on magnetic tapes or disks, and shipping those to the correlation center for replay. Recently, it has become 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 are now 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 in 2011.The image to the right shows the first science produced by the European VLBI Network using e-VLBI. The data from 6 telescopes were processed in real time at the European Data Processing centre at JIVE. The Netherlands Academic Research Network SURFnet provides 6 x 1 Gbit/s connectivity between JIVE and the GEANT2 network.
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. The first SVLBI experiment was carried out on Salyut-6 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, 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 microarcseconds.
Method
In VLBI interferometry, the digitized antenna data are usually recorded at each of the telescopes. The antenna signal is sampled with an extremely precise and stable atomic clock 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 where the data are sent by fibre-optics 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.
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 and in the phases of the signals from tapes B and C respectively. 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.
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 is still used today.