Arecibo Diary: Second Entry: Scanning for Signals

By Seth Shostak

Saturday, October 14, 2000 10:10 p.m.

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 with Project Phoenix astronomer Seth Shostak's reports from the front.

I don’t imagine that Columbus spent a lot of time at the bow of his ship, squinting at the horizon for a sign of land. He left that tedious job to some miserable hireling, sent into the rigging.

Project Phoenix has its own hireling: a custom lash-up of computers and digital electronics known as "The System." It’s impossible for humans to sort through all the radio noise washing through the Arecibo telescope. Instead, The System does the searching, and frankly, it’s this sophisticated amalgam of electronics and software that would first find ET. Not to worry: as far as we can tell The System has no interest in winning the Nobel Prize.

Using The System is, in principle, fairly simple. Although the working guts of this cybernetic beast are actually housed in a trailer parked a few dozen feet away, all operations are directed from a small alcove butted up to the main Arecibo control room. The SETI astronomer sits in front of three computer work stations; each outfitted with monitor and keyboard. Nothing special there.

Typing in a series of arcane commands fires up the computers, and the monitors glow with displays that indicate where the telescope is pointing, which star system is being scrutinized and the shape, or spectrum, of the incoming radio noise. When everything is working, and the telescope is on target, The System will analyze 56 million frequency channels every four minutes, looking for signals that poke up above the sea of natural noise produced by the receivers and the cosmos.

The System, and the carbon-based life form looking on, Peter Backus.

Finding signals is actually not a problem. The antenna, after all, is the size of the Rose Bowl or, if you prefer, 373 tennis courts. The problem lies in sorting the signals out; which might be extraterrestrial and which are merely random radio detritus from our own civilization? Puerto Rico is hemmed with radar sets, and GPS and telecommunications satellites (including those from the now-defunct Iridium enterprise) routinely glide through the tropical sky. They all make radio noise.

So The System keeps a list of terrestrial noisemakers. The first night of SETI observing is always spent compiling an inventory of the human-made signals that litter the local radio dial. If these signals are encountered during observations, The System is clever enough to ignore them. Some parts of the spectrum are so massively choked by signals that entire bands need to be blocked out. Interference is a cross that SETI must bear.

When searching for ET, all detected signals are first checked against the interference list. Those that are not obviously human handiwork are automatically re-observed by a second antenna – the 250-foot (76-meter) Lovell Telescope in Jodrell Bank, England. This instrument, rented by the SETI Institute while we’re at Arecibo, works in tandem with its heftier Puerto Rican brother. It follows up all interesting signals within minutes of their detection, offering its "second opinion" about their possible extraterrestrial nature.

Every few hours, a signal will have all the earmarks of an alien beacon; it will pass muster both in Arecibo and Jodrell Bank. At that point, the Arecibo dish goes to an "off" position: it moves about 3 degrees from the star under scrutiny, to see if the signal goes away (as it should, if it’s ET). If the signal disappears, then the antenna swings back to the star to ascertain whether the signal has returned.

By nodding back and forth, the reality of an alien transmission can be confidently checked. So far, no Arecibo signals have passed this automated "on-off" test.

Since The System is so good, you might wonder why it needs babysitting by the astronomers. Indeed, the carbon-based life forms hunkered down in the swivel chairs are seldom necessary. But if the hardware goes kaplooey (which it occasionally does) or the software goes nuts (which also happens), the humans can arrange for a fix.

And there’s one other situation in which The System needs help: detection of a signal that makes it through the automated gauntlet and appears to be from deep space. When and if this happens, the SETI researchers don’t want to be down at the pool or locked away in their cubicles. The System is great, but only people can really decide when a big discovery has been made. It’s nice to have a digital hireling, but the boss is still human.