Monday, November 28, 2022
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Starlink signals can be converted to act as GPS, whether SpaceX likes it or not

In a non-peer-reviewed paper he posted on his lab’s website, Humphreys claims to have provided the most comprehensive characterization of Starlink signals to date. This information, he says, is the first step toward developing new global navigation technology that would work independently of GPS or its European, Russian, and Chinese equivalents.

“The Starlink system signal is a closely guarded secret,” says Humphreys. “Even in our early discussions, when SpaceX was more cooperative, they didn’t tell us anything about the signal structure. We had to start from scratch and build a small radio telescope to listen to their signals.

To start the project, UT Austin acquired a Starlink terminal and used it to stream high-definition tennis videos of Rafael Nadal from YouTube, 24/7. This provided a constant source of Starlink signals that a separate nearby antenna could listen to.

Humphreys quickly realized that Starlink relies on a technology called orthogonal frequency division multiplexing (OFDM). OFDM is an efficient method of encoding digital transmissions, originally developed at Bell Labs in the 1960s and now used in Wi-Fi and 5G. “OFDM is absolutely top notch,” says Mark Psiaki, a GPS expert and professor of aerospace at Virginia Tech. “It’s a way to cram the most bits per second into a given bandwidth.”

The UT Austin researchers did not attempt to break Starlink’s encryption or access user data from the satellites. Instead, they looked for timing sequences—predictable, repetitive signals transmitted by orbiting satellites to help receivers coordinate with them. Not only did Humphreys find such sequences, but “we were pleasantly surprised to find that they [had] more synchronization sequences than strictly necessary,” he says.

Each sequence also contains indices for the satellite’s distance and speed. With Starlink satellites sending about four sequences every millisecond, “it’s just wonderful for dual use of their positioning system,” Humphreys says.

If the ground receiver has a good idea of ​​the satellites’ movements – which SpaceX shares online to reduce the risk of orbital collisions – it can use the regularity of the patterns to determine which satellite they came from, then calculate the distance to that satellite.. . By repeating this process for multiple satellites, a receiver can locate itself within about 30 meters, Humphreys explains.

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