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Radio interferometry steps off the earth.

Radio interferometry steps off the earth

Radio interferometry has shown astronomers fine structural details of many kinds of celestial objects, particularly active galaxies and quasars (see story above). Interferometry works by combining signals from a given source recorded by widely spaced receivers; the more widely spaced the receivers, the finer the detail observed. Until recently the technique was limited to linking together radiotelescopes located on the earth. Now, for the first time, astronomers have linked earth-based radiotelescopes with a receiver on an orbiting satellite, a member of the Tracking and Data Relay Satellite system, and successfully observed three quasars with a resolution of detail greater than that of any solely terrestrial combination of telescopes.

the experiment, reported in the Oct. 10 SCIENCE, shows that a receiver moving in orbit can be succesfully combined with others fixed on earth for this kind of work. It represents a first step on what radioastronomers hope will be a march into space and even to the moon. Particularly, according to the astronomers who did it, it demonstrates the feasibility of the proposed QUASAT project now under study by NASA and the European Space Agency. This would put up a satellite dedicated to radioastronomy to work as an interferometer with radiotelescopes on earth.

Interferometry combines signals received simultaneously at different telescopes and uses the correlations and differences among them in phase, intensity, amplitude or a combination of thsoe attributes. The resolution of detail it obtains can be as fine as that of a single telescope that would extend over the distance between the linked telescopes, or the baseline of the interferometer.

the first interferometers used receivers spaced a few hundres of meters or a few kilometers apart. They had wired connections and combined the signals in a central processor in real time. Very long baseline interferometry, which can be intercontinental, has to forgo the physical link between telescopes. It records the signals and combines them later. Getting it right requires ultraprecise timing, exact knowledge of the locations of the telescopes and some sophisticated computer programs. The present experiment shows that the constantly changing location of an orbiting satellite and its relation to locations on earth can be known well enough for interferometric purposes.

Gerald S. Levy of the Jet Propulsion Laboratory in Pasadena, Calif., led a group of astronomers from the United States, Australia and Japan in the work. Observing the quasars 1730-130, 1741-038 and 1510-089, they achieved a resolution equal to that of a telescope 1.4 times the size of the earth's diameter. QUASAT would give a 25,000-kilometer baseline, or about twice the size of the earth; the moon would give almost 30 times the size of the earth.
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Title Annotation:receiver placed on orbiting satellite
Author:Thomsen, Dietrick E.
Publication:Science News
Date:Oct 18, 1986
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