A Darn Site BetterMar. 10, 2002
by Seth Shostak - Senior Astronomer
Once again, the SETI institute has returned to the world's largest telescope to continue its research. Follow the institute's progress in Puerto Rico here at SPACE.com with Project Phoenix astronomer Seth Shostak's reports from the front. This is the second installment.
“One-site observing is like banging your head against a wall,” intones Project Phoenix software manager Jane Jordan. “I look forward to some relief when it stops.”
Project Phoenix is definitely in one-site mode. Our second telescope at Jodrell Bank – just south of Manchester, England – sprung a busted wheel girder about a week ago, and is laid up for repairs. Bad news. But while having this second instrument is extraordinarily valuable, it’s not quite essential.
Jane gestures casually toward the glowing Phoenix work stations, carpeted with data. “When we designed this stuff, a decade ago, everyone thought the universe would be quiet. To find extraterrestrial transmissions, all we had to do was point the telescope at likely stars and search the spectrum for narrow-band signals. Just like in the movies.”
In fact, the universe is quiet, especially at the microwave frequencies used for SETI. Unfortunately, Puerto Rico isn’t. This isn’t a slam against the Isle of Enchantment. No place on Earth is free of man-made interference. No place. Even if you’re in a deep valley curtained by mountains, noisy satellites will sail overhead, raining radio signals on your head and your antenna.
One way to beat this rap is to use two telescopes, far apart, which are simultaneously trained on the same patch of sky. Most earthly interference will show up at one site, but not both. And those signals that do can usually be sorted out on the basis of their frequencies. This is the scheme normally used by Project Phoenix: all promising signals are checked out at both Arecibo and Jodrell Bank.
But when you’re working with only one antenna, like we are now, you’ve got to be inventive. It’s an old problem. In 1992, when the former NASA SETI program began observing at Arecibo, our receivers were clogged with so many signals it was obvious that a reliable one-scope method was needed for separating ET from AT&T. Fortunately, there’s a way to do this. Terrestrial interference is generally so strong that it doesn’t really matter where you point your antenna. You can pick up interfering signals in all directions. An extraterrestrial signal, on the other hand, is going to be weak. Finding one requires aiming your scope at the star system from which it’s coming.
So separating the wheat from the chaff is simple: Point at the star of interest, and note all the signals. Then train your scope on another patch of sky a few degrees away – a so-called “comparison field” – and tally up the signals there. Any signal that’s in the first group and not the second is a candidate transmission from deep space, …ace, …ace.
In 1996, Jane Jordan turned this idea into a hunk of happening software. She made it possible to have a “two-star” observing mode for Project Phoenix. For five minutes, the telescope is trained on Star 1, collecting data. Then the telescope is slewed a few degrees to Star 2, and data are collected again. Each star serves as the comparison field for the other: vive la difference. After each cycle, the receivers are tuned up the dial a bit, and the process repeats.
It’s not as efficient as two-site observing because of the constant nodding back and forth, and it’s not as speedy at recognizing some types of interference. It’s also not nearly as automated, which gives both Jane and observers (such as your faithful correspondent) cranial pain. But you go with what you’ve got, and so far this observing run, it’s been one-site observing all the way.
In another three years, the new Allen Telescope Array will be gearing up for SETI observations. It will initially be capable of checking out three stars simultaneously, so each can serve as an instantaneous comparison field for the others. Jane Jordan says this will make her head feel a lot better.