aperture synthesis


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Related to aperture synthesis: Astronomical interferometer

aperture synthesis

A technique originally of radio astronomy devised by Sir Martin Ryle in the late 1950s. It is used to obtain high resolution images by combining signals from a number of relatively small antennas spaced over an area equivalent to a large aperture. Although originally used only for synthesized apertures that were ‘fully filled’ by smaller antennas, the term is also applied to instruments for which no clearly definable equivalent large aperture exists. The antennas are connected in interferometric pairs (see radio telescope) and the amplitudes and phases of the signals from a radio source are recorded on all antenna spacings necessary to cover the required area. The information is then processed in a computer where a Fourier transform is carried out to generate a radio map of the source.

In practice the rotation of the Earth is often used to move the antennas in space – a technique known as Earth-rotation synthesis. In such an arrangement, just 12 hours of observation generate all the baselines available from a particular configuration; the same baselines are repeated with the opposite sense during the next 12 hours. For a radio interferometer in which all the baselines are oriented east-west (e.g. the Ryle Telescope and the WSRT), each baseline produces an elliptical track of synthesized aperture. The major axis of the ellipse is determined by the baseline length, whereas the minor axis depends on the sine of the declination of the source. Low declination sources cannot therefore be mapped with a uniformly high resolution by an east-west interferometer. The VLA largely overcomes this difficulty by arranging its dishes in a Y-shaped configuration.

The computer processing involved in map-making can become very sophisticated, and special software (such as AIPS) is usually used. The Fourier transform of the synthesized aperture defines the synthesized beam (or dirty beam) of the telescope, with which the final image will appear to be convolved (see convolution). A synthesized aperture that is not fully filled will produce false features or artifacts in the map, and these must be suppressed using deconvolution algorithms such as Clean or MEM. An aperture consisting of only a few equally spaced strips will show bright, widely spaced concentric rings or grating responses (see array) around features in the map. Even a fully filled synthesized aperture may need to be graded, by gradually reducing the weight of contributions from the longer baselines. This reduces the side-lobe response of the telescope, which is the residual structure of the synthesized beam lying outside its central region. Other techniques such as self-calibration are used to remove artifacts generated by instrumental drifts and the effects of the atmosphere.

aperture synthesis

[′ap·ə‚chər ‚sin·thə·səs]
(electromagnetism)
The use of one or more pairs of instruments of relatively small aperture, acting as interferometers, to obtain the information-gathering capability of a telescope of much larger aperture.
References in periodicals archive ?
The transmitting antenna elements work sequentially and an aperture synthesis period is acquired until all the transmitting antenna elements have worked once.
As a cost-effective technique with high imaging resolution, aperture synthesis imaging would be extensively used in near-field applications.
Good, "Aperture synthesis for microwave radiometers in space," NASA Tech.
R., "Two-dimensional aperture synthesis radiometers in a low earth orbit mission and instrument analysis," IEEE International Geoscience and Remote Sensing Symposium, Vol.
Vall-llossera, et al., "The visibility function in interferometric aperture synthesis radiometry," IEEE Trans.
But Martin Ryle, a British astronomer at the University of Cambridge, developed a technique called aperture synthesis that takes advantage of what happens when radio waves interfere with each other.
The Very Large Array (VLA), a collection of 27 dishes, each 25 meters (82 feet) in diameter, performs aperture synthesis on the Plains of San Agustin, in central New Mexico, about 80 kilometers (50 miles) from Socorro and close to US Route 60.
Antenna aperture synthesis was first introduced radio astronomy.[2,6] A linear-array antenna of N elements would produce an output, after square-law detection and integration over time T, of
To face this point, the synthesis is performed by a pre-synthesis stage, henceforth the Aperture Synthesis (AS) stage, in which the amplitude and phase distributions of a continuous aperture are efficiently and effectively determined from the design specifications.
DLSLA 3D SAR acquires full 3D microwave images by wave propagation dimensional pulse compression, along-track dimensional aperture synthesis with flying platform movement and cross-track dimensional aperture synthesis with a sparse linear array [4].
Corbella, "Extension of the clean technique to the microwave imaging of continuous thermal sources by means of aperture synthesis radiometers," Progress In Electromagnetics Research, Vol.