SETI@Arecibo: How Do We Know the Darn Thing Works?

March 9, 2001

by Seth Shostak - Senior Astronomer


Project Phoenix is back at Arecibo, checking out nearby stars for signs of intelligent life. Astronomer Seth Shostak is reporting from the observatory once again, and will be home to his Arecibo Diaries. This is the first entry.

Sometimes I envy the folks who figure they can establish extraterrestrial contact by standing in the backyard waiting for spacecraft to land. After all, the required infrastructure is pretty simple: one – count ‘em, one – backyard.

Project Phoenix demands a bit more. To begin with, you need a large radio telescope -- a mammoth assemblage of steel and aluminum. The Arecibo antenna amply fills that role. But the list of necessary equipment continues: A low-noise amplifier poised at the focus, a series of intermediate amplifiers to boost the incoming cosmic static to useful levels, digitizers to convert the static to handy ones and zeroes, spectral analyzers to break up the band into 56 million channels, signal-detection algorithms, massive arrays of hard disks…the checklist is long and wearisome. But if you don’t expect aliens in the backyard, this is what you need.

It’s complex work, and it involves complicated equipment. What’s worse, every link in the chain has to function.

The Phoenix built-in test tone, as it appears on the project’s display screens (detail -- click here for full test tone).

This is a particularly stiff requirement for SETI research. If you study pulsars and don’t observe a signal pouring out of the ‘scope, you have good reason to suspect that something is awry. But SETI signals are elusive – indeed, we’ve yet to find a confirmed extraterrestrial broadcast. So when Project Phoenix scientists see the equivalent of blank screens, they’re not immediately alarmed.

But how do they know they shouldn’t be?

It’s a worrisome prospect, resulting in short fingernails all around. As a partial fix, the Phoenix engineers have arranged for a test tone to be injected into the signal path, one that’s a bit like a canary in a coal mine. The tone serves to verify that both hardware and software are up and running. But there are, unavoidably, places where the test tone doesn’t go and signal conditions that it doesn’t create. What’s needed is a much better canary, an "end-to-end" test capable of ascertaining whether the whole kit and caboodle – amplifiers to signal confirmation software – is up to operational snuff.

NASA was kind enough to provide us with such an end-to-end test object: the Pioneer 10 spacecraft. Now 7 billion miles from Earth and counting, Pioneer 10’s faint signal has been used for years to corroborate the correct workings of the Phoenix system. Once a day, we’d swing the ‘scope in its direction and verify an "extraterrestrial signal" from the craft’s 8-watt onboard transmitter.

Alas, Pioneer 10 has gone AWOL. No one has heard its weak whine since August. NASA theorizes that the plutonium-powered transmitter may finally have died (after two dozen years), or perhaps the spacecraft is slightly turned so that its antenna no longer points to Earth.

The Galileo spacecraft, seen here near Jupiter’s moon Europa, was a prospective test object for SETI. Unfortunately, its signal has been tweaked in a way that makes it less useful for calibrating the Phoenix system.

Irrespective of the reason, we’ve lost our best canary. So we’ve tried to press some other NASA birds into service. In particular, the Galileo spacecraft now pirouetting around Jupiter and its moons might serve as a distant, test transmitter. Unfortunately, Galileo hasn’t worked out. Its carrier signal has been deliberately suppressed. Doing this has improved the data rate for Galileo’s photo reconnaissance of the Jovian system, but makes the spacecraft hard to use as a SETI calibrator.

So now we’re trying something else: we’re going to look at the Moon. Paul Shuch and Richard Factor of the SETI League – an organization that has put together a very clever SETI search by amateurs – have been kind enough to try bouncing a test signal off the Moon when that fabled orb is overhead in Puerto Rico. They’re doing so with a quad helix array antenna near Factor’s home in Kinnelon, New Jersey and feeding it with 10 watts of power.

Ten watts bounced off the Moon may not sound like much. And it isn’t. In fact, only about 10 milliwatts of the transmitted power even hits the Moon, and this weak signal is then bounced back in all directions (not just toward Earth.) Imagine shining a laser pointer at the Moon in the hope of seeing the reflected light.

Fortunately, radio technology is good. You can work it out yourself, but the amount of signal coming back from the SETI League’s Moon bounce should be about one thousand billion-billionth of a watt per square meter. It’s not going to heat the grass. But even this incredibly weak signal is a piece of cake for the Phoenix system at Arecibo: it’s about a thousand times more powerful than the detection threshold.

It may be our best hope this observing run for a system test. There’s no doubt about it: the backyard approach to alien discovery doesn’t require too much equipment checkout. But a scientific observing program does. So in the coming days we’ll be periodically aiming Earth’s biggest "satellite" dish at, well, Earth’s biggest satellite.