An outgrowth of radio astronomy interferometry, very long baseline interferometry made possible the first high-resolution observations of distant radio galaxies, as well as new accuracy levels for global navigation, high-resolution astronomy, and geodesy.
Radio telescopes, like their optical counterparts, gather electromagnetic energy at a spatial focus in order to measure intensity, frequency, spatial distribution, polarization, and variability of single or multiple radiating sources. As was established early in the post-World War II era, in order to make radio astronomy measurements with spatial resolution comparable to that at optical wavelengths, the radio receiver must be larger by the ratio of radio to optical wavelengths. This results from the fact that the angular resolution of a radio telescope is basically set by the ratio of the wavelength of the dominant radiation received (lambda) to the diameter of the total receiving instrument D.
Precisely establishing the location and characteristics of diffuse radio objects required antennae with well-known detection patterns. To detect and measure accurately net emitted radiator flux, it is at first sight sufficient to employ a sufficiently large single antenna of geometrically simple design in order to have low side-lobes to reduce the risk of image distortion. This approach, however, is almost always precluded on meter radio wavelengths for mechanical as well as environmental and logistic reasons, since mechanically stable radio antennae cannot be constructed to sizes anywhere near ten kilometers.
The concept of interference fringe visibility, first noted in radar, was applied to radio astronomy for solar measurements in 1946. From this, the notion was developed of combining concurrent radio measurements from a number of interferometer spacings to produce a contour map of the net distribution of radio emissions. It was soon realized that many electronically linked individual antennae, with tens or even hundreds of kilometers separation, can be formed into a single net receiver having a very large total baseline that, in some cases, could rival or exceed the theoretical resolution of the best optical telescopes.
One use of VLBI is to navigate a spacecraft: By measuring the “angular separation” between a fixed radio star (such as a quasar) and the moving spacecraft, the craft’s location, orientation, and path can be precisely monitored and adjusted.
These notions were first discussed from a mathematical viewpoint by the Australian physicists L. McCready
The largest interelement or baseline separation distance of the antennae fixes the highest resolution possible, whereas the spatial distribution of measurement samples taken over the x,y plane determines the complexity of the structure that can be imaged. As developed by Ronald Bracewell and Sir Martin Ryle between 1947 and 1961, aperture synthesis in radio astronomy is based on the fact that the crosscorrelation, or mathematical similarity of shape of the signals recorded between two receiving elements i and j, is a measurement of total radio visibility. The brightness or luminosity distribution for distant radio sources whose radiation can be assumed to be spatially incoherent can be directly related to the observed interferometer fringe amplitude and phase. According to the Wiener-Khinchin theorem
The basic computations for processing long baseline interferometric radio telescope data are calculations of the crosscorrelation functions between, for example, the two data streams received from each end of a two-element baseline. The basic idea of employing a variable spacing radio interferometer to explore the then totally unknown field of radio star diameters occurred almost simultaneously in groups at Jodrell Bank, Cambridge, and Sydney, Australia. The results were published in the English journal Nature in 1952. Motivated by this methodological approach, as well as by the need better to resolve localize and resolve 1C radio survey objects, Ryle and his colleagues in England, and W. N. Christiansen
By 1960, many apparently double extragalactic radio objects had been found, almost all from measurements made by prototype interferometers employing increased baselines on the order of several tens of kilometers. As soon as adjacent radio telescopes were separated by more than this distance, however, the cables needed to connect them became prohibitively long and distorted interference patterns because of differential delays caused by differences in thermal expansion or contraction (despite the fact that the cables had been buried deep underground).
To overcome this transmission-line problem, the signals from distant radio receivers encompassing a long baseline array were sent to a central base via radio links. In such a system, the radio signal received by one radio telescope is converted to a microwave signal. This converted signal is then transmitted from the original receiver to an antenna near the main station, and in turn controls signals from the other receivers. In this way, two or more radio telescopes could be joined despite separations of even a hundred kilometers or more. In 1960, Jodrell Bank successfully operated the first microwave-linked long baseline radio interferometers over distances of up to 85 kilometers. The U.S. Owens Valley movable paraboloid antennae had a larger baseline.
Notwithstanding these improvements, the variable effects of the earth’s atmosphere on microwave transmissions on many occasions caused severe problems when baseline distances, or topographic or man-made obstacles, disrupted the transmission path between microwave links. Nevertheless, the availability of sophisticated digital recorders after the mid-1960’s led to the conception and implementation of the first successful transcontinental baselines and eventually intercontinental interferometers. In both configurations, all radio telescopes encompassing the array were equipped with accurate recorders and accurately synchronized atomic clocks establishing a common time base.
The first very long baseline interferometer (VLBI) was built in 1967 by the U.S. National Radio Astronomy Observatory
The development of VLBI radio telescopes is an example of a more cumulative than localized scientific discovery. One of the problems in making distant extragalactic observations using intercontinental baselines was ensuring accurate time synchronization in view of logistic and political problems (for example, entering the pre-Glasnost Soviet Union with sophisticated black boxes of electronics). Much higher synchronization was obtained from joint monitoring of radio broadcast time signals such as WWV, and later from the Long-Range-Navigation (LORAN C) system. With the emergence of wide-band satellite communication systems, the tape recording system can be replaced, since video signals from each antenna can be transmitted directly to the central processing center via satellite and correlated in real time.
Perhaps the main interest of radio astronomers developing VLBI arrays has been mapping of radio sources associated with distant galaxies. VLBI units provide a wide range of selectable and much finer resolutions, greater flexibility, and greater weak-signal detection capability. Many maps of extragalactic radio sources have been produced since the construction of the 27-movable-unit Very Large Array
One of the unique capabilities of VLBI systems is the possibility of making very short or snapshot observations. Snapshot mode makes it possible not only to record very short-lived features but also to map hundreds of strong sources in a single day. Even within the solar system, VLBI arrays have provided new data on the complex radiation belts and emissions surrounding Jupiter. VLBI radio astronomers have observed much deeper into the solar atmosphere than was previously possible, measuring, for example, the conditions leading to formation of major solar flares.
The first application of VLBI to very accurate satellite orbit measurements was conducted in 1969. Employing lunar-based radio transmitters, VLBI provided Irwin Shapiro with some of the first highly accurate measurements of the Moon’s libration. In addition, ray-bending predictions of the theory of general relativity were confirmed by a similar method. VLBI observables are affected not only by the radio propagation properties of the atmosphere and outer space, timing clocks, and baseline sites but also by Earth’s motions such as Earth tides, plate tectonics, and vertical orogenic/lift motions, and by Earth’s speed variations in the rate of rotation, nutation, and precession. Important estimates of these geodetic and geophysical parameters can be obtained from analysis of VLBI observations, and since then, this work has continued. Since 1978, ground site velocities in California and elsewhere caused by continental drift have been measured using a mobile VLBI system.
Since the beginnings of radio astronomy in the 1930’s with Karl Jansky’s small mono-element radio telescope, angular resolution has progressively increased from tens of degrees to submillisecond of arc resolutions using very long baseline interferometry. Further increases in angular resolution, by extending the terrestrial baseline via antennae in space, are further developments in a long chain of improvements, all based in radio interferometry and the notion of movable baselines.
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Bracewell, Ronald N. The Fourier Transform and Its Applications. 3d ed. Boston: McGraw-Hill, 2000. The principles and uses of the Fourier transform in image synthesis are discussed. -
Brown, Robert Hanbury, and A. C. B. Lovell. The Exploration of Space by Radio. New York: John Wiley & Sons, 1958. A balanced and detailed account. Includes discussions of early English and Australian work. -
Dudgeon, Dan E., and Russell M. Mersereau. Multidimensional Digital Signal Processing. Englewood Cliffs, N.J.: Prentice Hall, 1984. Although somewhat advanced, it contains several tutorial sections on correlation, Fourier transforms, and aperture synthesis. -
Kellerman, Kenneth L., and A. Richard Thompson. “The Very-Long Baseline Array.” Scientific American 258 (January, 1988): 54-61. Gives a good general overview of the New Mexico VLA, related instruments, and ongoing observational work. -
Mills, B. Y. “Apparent Angular Sizes of Discrete Radio Sources.” Nature 170 (1952): 1063-1065. The primary reference for the initial radio interferometer having somewhat movable interelement separation. -
Thompson, A. R., James M. Moran, and George W. Swenson, Jr. Interferometry and Synthesis in Radio Astronomy. 2d ed. New York: Wiley, 2001. The best overall account of movable interferometric aperture synthesis in radio astronomy. Balanced between methods and results. -
Verschuur, G. L. The Invisible Universe: The Story of Radio Astronomy. New York: Springer-Verlag, 1974. A good general introduction focusing on European results in galactic radio astronomy.
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