December 6, 2022

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Technological development

Starlink signals can be reverse-engineered to work like GPS—whether SpaceX likes it or not

Starlink's satellite signals, and joyless tech

In a non peer-reviewed paper that he has posted on his lab’s site, Humphreys statements to have furnished the most full characterization of Starlink’s signals to day. This details, he says, is the very first action toward developing a new worldwide navigation technological know-how that would work independently of GPS or its European, Russian, and Chinese equivalents. 

“The Starlink technique sign is a carefully guarded secret,” states Humphreys. “Even in our early conversations, when SpaceX was staying additional cooperative, they did not reveal any of the signal construction to us. We had to begin from scratch, setting up mainly a small radio telescope to eavesdrop on their alerts.”

To get the job commenced, UT Austin acquired a Starlink terminal and applied it to stream large-definition tennis videos of Rafael Nadal from YouTube. This offered a constant source of Starlink signals that a separate close by antenna could listen in on.

Humphreys swiftly realized that Starlink relies on a technology referred to as orthogonal frequency-division multiplexing (OFDM). OFDM is an successful approach of encoding electronic transmissions, initially produced at Bell Labs in the 1960s and now employed in Wi-Fi and 5G. “OFDM is all the rage,” states Mark Psiaki, a GPS professional and aerospace professor at Virginia Tech. “It’s a way to pack the most bits for every second into a provided bandwidth.” 

The UT Austin researchers did not try out to crack Starlink’s encryption or access any person facts coming down from satellites. Rather, they sought out synchronization sequences—predictable, repeating signals beamed down by the satellites in orbit to help receivers coordinate with them. Not only did Humphreys come across this sort of sequences, but “we ended up pleasantly amazed to obtain that they [had] additional synchronization sequences than is strictly needed,” he says.

Every single sequence also consists of clues to the satellite’s distance and velocity. With the Starlink satellites transmitting about 4 sequences each millisecond “that’s just fantastic for dual use of their procedure for positioning,” suggests Humphreys. 

If the terrestrial receiver has a fantastic thought of the satellites’ movements—which SpaceX shares on the web to decrease the danger of orbital collisions—it can use the sequences’ regularity to get the job done out which satellite they came from, and then determine the distance to that satellite. By repeating this system for several satellites, a receiver can track down alone to inside of about 30 meters, says Humphreys.