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Fig. 8.32 Some of the antennae forming the LOFAR array (Source: project site)

As with optical interferometry, the idea behind such arrays is to combine a large number of small-sized and poorly directional antennae. Combining the various antennae, through electronic delay lines allows the signals to be combined to:

• gain sensitivity (by increasing the collecting surface area),

• gain resolution (by reducing the size of the beams of individual antennae),

• pointing the instrument. The antennae are fixed (Fig. 8.32) and the array's lobe is positioned on the sky by adjustment of the different delays between antennae (the phased array principle).

One of the main technological challenges is being able to transfer, in real time, information from 77 stations (32 in the core and 45 in the rest of the array) to enable interferometric recombination, in contrast to classic long-baseline radio interfer-ometry, where the signals from various antennae are recorded and only correlated subsequently.

The LOFAR array, when it becomes operational, will be able to study the auroral emission from giant exoplanets, following the example of the UTR-2 array at Kharkov, but with an increased sensitivity relative to existing programmes.

8.2.3.2 The Square Kilometric Array (SKA)

The SKA project aims to construct an interferometer array with a surface area that will reach one million square metres, i.e., about 200 times that of a classic radio telescope like the Lovell Telescope at Jodrell Bank. The sensitivity of such an array should also allow it to capture signals as faint as those of television broadcasts at the distances of the nearest stars to the Sun (a few parsecs). Research into auroral radio emissions from extrasolar planets is, of course, only one of the scientific aims of the astrophysical instrument.

SKA, in its high-frequency mode, should be able to detect the gaps created by the migration of giant planets (or even terrestrial ones) in protoplanetary disks.

The project is about to enter a study phase, which will last about four years, with the aim of defining the concept and the associated equipment. Currently, the construction timeline of this array has not been defined. The final choice of instrumentation should be made by the end of the decade, and that of the core site (Western Australia or the Karoo in South Africa) after a site assessment, and in any case, again not before 2010.

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