Astronomers Puzzled By TV Signals Coming From The Sky For 5 Years Solve Mystery

This was weird, as the telescope is in a designated radio quiet zone established by the Australian government precisely to stop interference with radio astronomy in that region of Western Australia. That’s why the particular location for the telescope was chosen. 

And yet, over the course of five years, the television signal persisted. Stranger still, the astronomers looking at the data noticed that the signal was moving across the sky. So what are we looking at? Aliens reflecting back our own TV signals, a la Contact? Cloud-dwelling humans watching Home & Away reruns? 

“It then hit us,” Jonathan Pober, a physicist at Brown University and the US research lead for the Murchison Widefield Array project, said in a statement. “We said, ‘I bet the signal is reflecting off an airplane.’ We’d been seeing these signals for close to five years, and several people had suggested they were airplanes reflecting television broadcasts. We realized we might actually be able to confirm this theory for once.”

Unwanted interference like this contaminates astronomical data. Imagine trying to listen to a friend talking quietly from the opposite end of a dinner table, while your toddler screams into your ear. When telescopes pick up such signals, it may mean they have to throw out those observations, as it’s difficult to separate out what you actually want to study from the interference.

“It ends up being insane amounts of data being thrown out to not have any part of the observation contaminated,” Brown PhD student Jade Ducharme added.

In a new study, the team attempted to pinpoint a source of interference, and, in the process, determine whether it was planes that had been causing it as they guessed. To do so, they used near-field corrections, essentially adjusting the telescope to focus on closer objects, and beamforming, a technique to sharpen focus on nearby sources of interference.

“Using beamforming in combination with the near-field corrections […] allows us to efficiently obtain an estimate for the altitude of a near-field radio-emitting object such as an airplane or a satellite,” the team explained in their study.

“For our target observation, this technique allowed us to estimate the object’s average altitude to approximately 11.7±0.1 km [7.3±0.06 miles], suggesting its likely classification as an airplane since a plane’s cruising altitude can vary between 9.4 and 11.6 km [5.8 and 7.2 miles],” the team added. “To further support this hypothesis, we used the angular displacement as a function of time in combination with the measured altitude to calculate its speed, which we find to be 792±1 km/h [492±0.6 miles per hour], also consistent with an airplane’s cruising speed.”

On top of this, the team found that the frequency band of the reflected signal was associated with Australian digital TV Channel 7. Unfortunately, due to a lack of flight logs, they were unable to pinpoint the exact plane.

“This is a key step toward making it possible to subtract human-made interference from the data,” Jonathan Pober added. “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.”

While cool that they can pinpoint the source of interference, the real goal is to be able to remove signals being reflected off planes, leaving the useful astronomy data behind. The team aims to do this, as well as extend it to satellite-reflected interference. However, as the number of satellites in orbit grows, along with Earth-bound interference, astronomers are beginning to wonder whether we might have to shift radio astronomy to a quieter environment like the Moon.

“If we can’t find a quiet sky on Earth, maybe Earth isn’t the place to be,” Pober added. “No matter what we do, we have no choice but to invest in better data analysis techniques to identify and remove human-generated interference.”

The study is published in Publications of the Astronomical Society of Australia.