Prototype SETI Antenna Array Will Help Radio Astronomers Too

Contact: Steven Ellingson, (614) 292-5935;
Ellingson.1@osu.edu

 
Written by Pam Frost, (614) 292-9475;
Frost.18@osu.edu

 

COLUMBUS, Ohio — Engineers at Ohio State University are building a new kind of radio antenna array for the Search for Extraterrestrial Intelligence (SETI).
 
Because of interference from local radio and television stations, the array probably won’t be listening for signals from space in Columbus. It will serve as a prototype for a larger, more sensitive system elsewhere in the world.
 
But astronomers who use traditional dish-shaped radio telescopes will also benefit when the 64-antenna array goes on-line in the fall of 2000.
 
That’s because the array will be able to track earth-orbiting satellites that interfere with radio telescope signals, said Steven Ellingson, senior research associate at Ohio State’s ElectroScience Laboratory. The array will also alert astronomers to brief cosmic events such as supernovas.
 
The new system will also serve as a testbed for the development of radio telescopes that will use similar technology, such as the Square Kilometer Array of telescopes currently being planned by a consortium of universities and observatories around the world, of which Ohio State is a member.
 
Ultimately, Ellingson and his colleagues may most assist astronomers by finding ways to counteract the radio frequency interference that will plague the antenna in Columbus.
 
Local FM radio and TV stations make urban areas such as Columbus among the worst places in the world to build a radio telescope, Ellingson said — but one of the best places to study radio interference.
 
"We may not be able to do front-line radio astronomy here, but we’re very interested in engineering ways to clean up radio telescope data," he said.
 
The current incarnation of the array features 8 flat spiral-shaped antennas on the roof of a building behind the laboratory. Ellingson and Brian Baertlein, a research scientist at the ElectroScience Lab, designed the small antennas to view the entire sky all at once, from horizon to horizon. Large dish- shaped antennas can only focus on one small portion of the sky at a time.
 
Robert Dixon, director of the array project, named the array Argus after Argus Panoptes, the mythological Greek hero with 100 eyes. Dixon is also a senior research engineer in Ohio State’s Office of the Chief Information Officer.
 
The spiral antennas detect a broad range of frequencies, from 400 to 2,000 megahertz — roughly the range from two-way radios to military and weather satellites, with TV signals in-between. While commercial FM radio stations broadcast at only 100 megahertz, the harmonics generated by those signals reach Argus’s range of sensitivity, Ellingson said.
 
The researchers developed a new kind of digital receiver to handle that wide tuning range. They also developed computer algorithms for calibrating the antenna system, as well as algorithms for detecting and suppressing interference.
 
For example, they were able to remove interference from the Russian GLONASS satellite from astronomical observations taken at the Australian Telescope Compact Array (ATCA) in Narrabri, New South Wales.
 
Ellingson explained how Argus is able to take a radio "picture" of the entire sky at once. The small antennas work together to form one large image, he said, like the light sensors in the human eye. Software processes the image.
 
"The brain doesn’t scan an image like a TV to form a picture — it sees an image all at once and makes sense out of it. That’s akin to what we’re trying to do," Ellingson said.
 
Taking such a large picture has its drawbacks, however.
 
When completed, Argus’s 64 antennas will generate 2.56 gigabytes of data — the equivalent of 4 CD-ROMs — per second, and there’s no economical physical connection that can carry that much data for any great distance, Ellingson said. So the engineers will process all the data just as it emerges from Argus, and throw away all but the most important signals. Ellingson’s team of graduate students is building a mini- supercomputer out of PCs to process the data.
 
Ellingson maintains that the radio interference in Columbus will prevent Argus from ever being a front-line SETI instrument. The SETI Institute is funding the project because it hopes the array will pave the way for similar telescopes in more remote parts of the world that aren’t besieged with radio interference.
 
Because Argus can watch the entire sky at once, it will provide an early warning system for radio astronomers more interested in scanning the skies for supernovas or other short-lived astronomical events than listening for ET. Ellingson and his staff plan to alert observatories around the world when Argus detects such events, some of which last for only minutes at a time.
 
"By the time astronomers hear about these events, they might already be over with," said Ellingson, "but with Argus we can notify them that something is happening so that they can focus their telescopes on that area of the sky in time."
 
Argus will also assist radio astronomers by tracking the position of earth-orbiting satellites that interfere with telescope signals. Most satellite orbits are publicly known, Ellingson said, but a satellite can change orbit or broadcasting frequency at any time, and new satellites appear constantly.
 
Tracking these satellites isn’t just a technical issue, it’s economic as well, Ellingson said. Astronomers compete for a limited number of radio telescopes around the world, sometimes paying a great deal of money to use a telescope for only a few days. While astronomers plan their observation time to avoid known sources of interference, one rogue satellite can do a great deal of harm.
 
"When people have to throw away half their data because a satellite was in the wrong place at the wrong time, it’s heart-wrenching," he said.
 
If Ellingson’s efforts to combat interference from radio and TV stations prove successful, the Argus project may assist astronomers in yet another way. Around the world, as cities fill once-remote areas that are home to radio telescopes, interference is a big problem.
 
Ellingson cited the case of Arecibo Observatory in Puerto Rico. Since the telescope’s construction, radio and TV stations have sprung up around the island. Now interference blocks many of the signals the telescope could receive when it was built in the 1960’s.
 
The mainstream radio astronomy community has become more accepting of SETI, Ellingson said, not in small part because the privately-donated SETI Institute funds now represent a very significant portion of all U.S. funding for radio astronomy, including government sources.
 
Ellingson feels there are strong ties between the SETI and mainstream radio astronomy communities. Many SETI scientists started their careers as mainstream radio astronomers, he said, and many continue to perform basic scientific research. And all radio astronomers will benefit from new technologies to track satellites and erase interference.
 
"It’s clear that we all have the same problems. What’s good for one of us is good for the other," Ellingson said.
 
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Editor’s Note: For decades, Ohio State University engineers and volunteers conducted the world’s longest-running search for extraterrestrial intelligence at its radio observatory north of the Columbus campus. That search was abandoned when the observatory property was sold several years ago.