In a non-peer-reviewed paper, which he posted on his laboratory’s website, Humphries claims he has provided the most complete characterization of Starlink’s signals to date. He says this information is the first step towards developing a new global navigation technology that will operate independently of GPS or its European, Russian and Chinese counterparts.
“The Starlink system signal is a closely guarded secret,” Humphries says. “Even in our early discussions, when SpaceX was getting more cooperative, they didn’t reveal any signal structure to us. We had to basically build a small radio telescope to listen for their signals. Had to start from scratch.
To start the project, UT Austin acquired a Starlink terminal and used it to stream Rafael Nadal’s high-definition tennis videos 24/7 from YouTube. This provided a constant source of Starlink signals that a separate nearby antenna could hear.
Humphries quickly realized that Starlink relies on a technique 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 all the rage,” says Mark Sciaki, GPS specialist and aerospace professor at Virginia Tech. “It’s a way to pack the most bits per second into a given bandwidth.”
Researchers at UT Austin didn’t try to break Starlink’s encryption or access any user data that came down from the satellites. Instead, they sought synchronization sequences—predictable, repeatable signals beamed by satellites in orbit so that receivers could coordinate with them. Humphries not only found such scenes, but “we were pleasantly surprised to find that they were” [had] There is a dire need for more synchronization sequences than that,” he says.
Each sequence also contains clues to the satellite’s distance and velocity. “The Starlink satellites broadcast about four sequences every millisecond,” says Humphries, “that’s wonderful for their system’s dual use for positioning.”
If the terrestrial 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 sequences to figure out which satellite they came from, And then calculate the distance of that satellite. By repeating this process for multiple satellites, a receiver can find itself within about 30 meters, Humphries says.
