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Ask Dr. SETI ®

Chapter 6: Technology

Phasing Several Smaller Antennas

Dear Dr. SETI:
I heard some time back that the way some radio telescopes work is they have two moderately sized dishes a fair distance apart and this creates a dish virtually the diameter of the distance between the two. One guy suggested a satellite orbiting the earth with a dish attached and a large dish on the ground to create an antennae thousands of miles in diameter. I heard this while visiting the Australia Telescope (the big array in New South Wales). Would it therefore be possible to get a bunch of DSS 1m dishes and put them, say 50 meters apart to create a larger dish to use on a SETI station?

DS, Australia

The Doctor Responds:
What you have described is an interferometer. It is the technique used not just by the Australia National Telescope Facility (ATNF), but also at the Very Large Array (VLA) in New Mexico, which you saw in the film "Contact." (That was misleading, because interfometers in general, and the VLA in particular, are somewhat less than optimal for SETI.). The technique allows you to use small antennas to get the resolution of a larger one. This is very useful for traditional radio astronomy, where the objective is to accurately determine where a particular signal has originated. It is not, however, ideal for the initial stages of SETI, where the challenge is simply to detect a very weak signal in the first place. For that, we need antenna gain, which is a function of capture area.

Regardless of their baseline (the distance between them), two identical antennas in an array will have never more than twice the capture area of a single one (hence no more than 3 dB more gain). So two antennas with a long baseline will have the beamwidth and resolving power (but not the sensitivity) of a single antenna as large as that baseline.

On the other hand, interferometry holds much promise for the second stage of SETI, signal verification. When a single antenna has detected a candidate signal, an interferometer array can be used to verify that it is indeed of extra-terrestrial origin, and to help identify the specific star from which it emanated. Once The SETI League's Project Argus network achieves critical mass (that would be at perhaps 1000 participating stations; we have only 57 right now) we can start planning our global inerfometer array.

Project ELBA (Extremely Long Baseline Array) will ultimately interconnect all participating Argus stations into the world's largest phased array. This will require a common time and frequency reference for all participants, to atomic-clock accuracy. We expect that oscillators slaved to the constellation of Global Positioning Satellites (GPS) will do the trick, and are in fact developing the technology right now. Stay tuned for details.

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