November 30, 2022

sullivanprogressplaza

It's the Technology

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

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

In a non peer-reviewed paper that he has posted on his lab’s web-site, Humphreys statements to have furnished the most entire characterization of Starlink’s indicators to day. This information and facts, he states, is the very first stage toward creating a new global navigation technologies that would work independently of GPS or its European, Russian, and Chinese equivalents. 

“The Starlink procedure sign is a intently guarded secret,” says Humphreys. “Even in our early conversations, when SpaceX was remaining much more cooperative, they didn’t expose any of the sign construction to us. We had to start from scratch, developing fundamentally a minimal radio telescope to eavesdrop on their indicators.”

To get the venture started out, UT Austin acquired a Starlink terminal and utilized it to stream superior-definition tennis movies of Rafael Nadal from YouTube. This furnished a continuous supply of Starlink alerts that a separate nearby antenna could listen in on.

Humphreys swiftly realized that Starlink relies on a technological know-how known as orthogonal frequency-division multiplexing (OFDM). OFDM is an successful technique of encoding electronic transmissions, initially developed at Bell Labs in the 1960s and now used 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 per next into a provided bandwidth.” 

The UT Austin researchers did not try to break Starlink’s encryption or obtain any user details coming down from satellites. As an alternative, they sought out synchronization sequences—predictable, repeating alerts beamed down by the satellites in orbit to support receivers coordinate with them. Not only did Humphreys uncover this sort of sequences, but “we have been pleasantly shocked to uncover that they [had] additional synchronization sequences than is strictly needed,” he states.

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

If the terrestrial receiver has a very good strategy of the satellites’ movements—which SpaceX shares on the internet to lower the possibility of orbital collisions—it can use the sequences’ regularity to operate out which satellite they came from, and then compute the distance to that satellite. By repeating this approach for multiple satellites, a receiver can find alone to within about 30 meters, says Humphreys.