Researchers who decoded Starlink to function as a backup for GPS found a security flaw in the system, using a video of tennis star Rafael Nadal, which could be exploited in Ukraine.

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After SpaceX refused to continue researching Starlink as a possible military alternative to GPS in 2020, a group of researchers found a way to do so without the help of Elon Musk, or his company that created the Internet satellite constellation.

For the past two years, Professor Todd Humphreys has led a group of University of Texas researchers to reverse engineer signals from the satellite Internet constellation with the hope of forming a new navigation system that would operate separately from the Global Positioning System and its European, Russian and Chinese equivalents, MIT Technology Review reported.

In a non-peer-reviewed study, Humphreys says the group created a complete characterization of Starlink’s signals without breaching its encryption or accessing user data from the satellites.

“The Starlink system signal is a closely guarded secret,” Humphreys told MIT Technology Review. “Even in our early discussions, when SpaceX was more collaborative, they didn’t reveal any signal structures to us. We had to start from scratch, basically building a small radio telescope to eavesdrop on their signals.”

Starting with a Starlink unit programmed to broadcast high-definition YouTube videos of Spanish tennis player Rafael Nadal, the group began tracking the satellite’s synchronization sequences and detected their transmission patterns, approximately four sequences every millisecond. These sequences – repeating patterns of signals transmitted to Earth by the satellite – help the receivers coordinate with them, leaving clues to the satellite’s distance and speed.

The terrestrial receiver, using the timing of the signals received from the satellite and publicly available information about its orbit, can then calculate the distance to the satellite and approximate a position within 30 meters, Humphreys told MIT Technology Review. With tweaking, geolocation capabilities could become as accurate as GPS, which tends to be accurate to around 16 feet in commercial use.

The discovery, although a potential breakthrough for geolocation services, also revealed a possible security issue on Starlink signals – which are currently key to keeping Ukrainian communications services running while Russia invaded the country – if used as a system of navigation.

“Humphreys has done the navigation community a great service by identifying these sequences,” Mark Psiaki, an aerospace professor at Virginia Tech and a GPS expert, told MIT Technology Review. “But any navigation system that works on open source sequences could definitely be faked, because everyone will know how to spot those signals and create fake ones.”

Starlink has become so integral to wartime communications in Ukraine that recent disruptions have been described as “catastrophic” by officials. Moss tweeted Russia is “actively working” this week to destroy the satellites, but Humphreys’ discovery – that the signals are predictable and replicable – highlights the possibility of an intentional interruption of Starlink.

“As time goes by and their dependence on Starlink deepens, Ukraine and its allies in the West are realizing that they have little control over Starlink and know little about it,” Humphreys told MIT Technology Review. “But now many millions of people have a vested interest in Starlink’s security, including its resilience to jamming. Evaluating that security begins with a clear understanding of the signal structure.”

SpaceX, Musk, and Humphreys did not immediately respond to Insider’s requests for comment.

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