In a non-peer-reviewed paper he has posted on his lab’s website, Humphreys claims to have provided the most comprehensive characterization of Starlink’s signals to date. This information, he says, is the first step toward developing a new global navigation technology that works 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 reveal any signal structure to us. We had to start from scratch, basically building a small radio telescope to listen for their signals.”
To begin the project, UT Austin acquired a Starlink terminal and used it to stream high-definition tennis videos of Rafael Nadal from YouTube, 24 hours a day. This provided a constant source of Starlink signals that a separate nearby antenna could pick up.
Humphreys quickly realized that Starlink is based on a technology called orthogonal frequency division multiplexing (OFDM). OFDM is an efficient method of coding 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 Psiaki, a GPS expert and aerospace professor at Virginia Tech. “It’s a way to pack the most bits per second into a given bandwidth.”
The UT Austin researchers did not attempt to crack Starlink’s encryption or access any user data from the satellites. Instead, they looked for synchronization sequences: predictable, repeating signals sent by orbiting satellites to help receivers coordinate with them. Not only did Humphreys find these sequences, but “we were pleasantly surprised to find that [had] more sync sequences than are strictly required,” he says.
Each sequence also contains clues about the satellite’s distance and speed. With Starlink satellites transmitting about four sequences every millisecond, “this is wonderful for the dual use of your system for positioning,” 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 sequences to figure out which satellite they came from, then calculate the distance to this satellite. . By repeating this process for several satellites, a receiver can be located within about 30 meters, Humphreys says.
