Astronomers realize mysterious TV signal in their data bounced off an airplane
The sky is awash with radio-frequency interference (RFI), but thanks to two astronomers who tracked down a stray TV broadcast reflected off a passing airplane, there may be a new way to wipe out some of the rogue signals that plague our radio telescopes.
“Astronomy is facing an existential crisis,” said Jonathan Pober of Brown University, Rhode Island, USA, in a statement.
Satellites, for instance, crowd the sky. The United Nations Office for Outer Space Affairs counted 11,330 satellites in Earth orbit as of June 2023, and many more have been launched since then. Most of these satellites are designed to relay various communications over radio wavelengths. This has presented the astronomy community with a problem.
“There is growing concern — and even some reports — that astronomers may soon be unable to carry out high-quality radio observations as we know it, due to interference from satellite constellations,” said Pober.
The issue is especially relevant for telescopes such as the Murchison Wide-field Array (MWA) in western Australia, on which Pober is the U.S. science lead. The MWA consists of 4,096 antennas designed to detect low-frequency radio waves between 70 and 300 MHz that carry information from the universe‘s epoch of reionization, when the first stars and galaxies were forming. Because the MWA observes the entire sky all at once, however, “there’s no way to point our telescopes away from satellites,” said Pober.
Because of the randomness of RFI and the difficulty in tracking such signals back to their sources, modeling the interference so that it can be filtered out has turned out to be a next-to-impossible task. Typically, datasets contaminated with RFI are simply thrown out — but that leads to a lot of data being lost.
However, the case of a stray television signal has given astronomers hope that there may be a way to save some of that data.
The MWA sits inside a 186-mile-wide (300-kilometer-wide) radio quiet zone, yet the telescope has consistently been picking up television broadcasts that shouldn’t be transmitted in the quiet zone. The origin of these broadcasts had been a puzzle. “It then hit us,” said Pober. “We said, ‘I bet the signal is reflecting off an airplane.'”
Teaming up with Ph.D. student Jade Ducharme, also from Brown University, Pober set about proving the airplane hypothesis. To do so, they combined two techniques for tracking down the origin of RFI — using “near-field corrections” that involves focusing the radio telescope on nearby interference-producing objects, and “beamforming” that essentially allows the telescope to sharpen its focus on a desired object.
Through a combination of these two techniques, Pober and Ducharme were able to track a television signal back to an airplane traveling at 38,400 feet (11.7 kilometers) in altitude and at a velocity of 492 mile per hour (792 kilometers per hour). They even found that the television signal was on the frequency band used by Australian digital TV channel 7. This signal was being broadcast somewhere outside the radio quiet zone and being reflected off the hull of the airplane.
Identifying the source of the RFI opens the door to the interference being modeled so that its pattern can be recognized and ultimately filtered out, keeping the data usable to astronomers.
“This is a key step toward making it possible to subtract human-made interference from the data,” said Pober. “By accurately identifying and removing only the sources of interference, astronomers can preserve more of their observations, reduce frustrating data loss and increase the chances of making important discoveries.”
Tracking the source of the RFI back to a passing airplane was just the first step, however. The next step is to learn how to remove similar signals from the astronomical data — then, after that, the goal is to expand the technique to not just identify and remove television signals bouncing off airplanes, but also to remove signals from satellites overhead too. However, given the huge numbers of satellites, that is a much heftier task.
Nevertheless, in Pober’s view it is a task signal refinement is going to be essential if radio astronomy is to survive.
“We have no choice but to invest in better data analysis techniques to identify and remove human-generated interference,” he said.
Pober and Ducharme’s analysis of how they tracked the stray television signal back to the airplane was published on Feb. 12 in a paper in the journal Publications of the Astronomical Society of Australia.
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