In one unreviewed paper which he posted on his lab’s website, Humphreys claims to have provided the most complete description of Starlink’s signals to date. According to him, this information is the first step towards the development of a new global positioning technology that can work independently of GPS or equivalent devices from Europe, Russia and China.
“The signaling of the Starlink system is a closely guarded secret,” said Humphreys. “Even in our initial discussions, when SpaceX was more cooperative, they did not reveal any signal structures to us. We had to start from scratch, building basically a small radio telescope to eavesdrop on their signals.”
To start the project, UT Austin purchased a Starlink terminal and used it to stream high-definition tennis videos of Rafael Nadal from YouTube. This provides a constant source of Starlink signal that a separate nearby antenna can hear.
Humphreys quickly realized that Starlink was based on a technology called orthogonal frequency division multiplexing (OFDM). OFDM is an efficient digital transmission coding method, originally developed at Bell Labs in the 1960s and now used in Wi-Fi and 5G. “OFDM is a rage,” said Mark Psiaki, GPS specialist and professor of aerospace engineering 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 break Starlink’s encryption or access any user data from the satellite. Instead, they look for synchronizing sequences—predictably repeating signals projected down by satellites in orbit to help receivers coordinate with them. Humphreys not only found such sequences, but “we were surprised to find that they [had] he said.
Each sequence also contains clues about the satellite’s distance and velocity. With the Starlink satellites transmitting about four sequences per millisecond, “it’s amazing to dual-use their system for positioning,” says Humphreys.
If the receiver on the ground has a good idea of the motions of the satellites — which SpaceX shares online to reduce the risk of orbital collisions — it can use the regularity of the sequences to find the satellite. to which they come and then calculate the distance to that satellite . By repeating this process for multiple satellites, a receiver can position itself within about 30 meters, says Humphreys.
