In a non peer-reviewed paper printed on arXiv, Humphreys claims to have delivered the most total characterization of Starlink’s signals to day. This details, he suggests, is the initially action toward creating a new worldwide navigation technological innovation that would operate independently of GPS or its European, Russian, and Chinese equivalents.
“The Starlink technique sign is a carefully guarded magic formula,” claims Humphreys. “Even in our early conversations, when SpaceX was currently being much more cooperative, they didn’t reveal any of the sign construction to us. We had to commence from scratch, constructing generally a little radio telescope to eavesdrop on their signals.”
To get the undertaking began, UT Austin obtained a Starlink terminal and utilized it to stream large-definition tennis films of Rafael Nadal from YouTube. This supplied a continuous source of Starlink signals that a separate nearby antenna could pay attention in on.
Humphreys promptly recognized that Starlink relies on a know-how called orthogonal frequency-division multiplexing (OFDM). OFDM is an successful strategy of encoding electronic transmissions, originally designed at Bell Labs in the 1960s and now made use of in Wi-Fi and 5G. “OFDM is all the rage,” states Mark Psiaki, a GPS skilled and aerospace professor at Virginia Tech. “It’s a way to pack the most bits for each next into a offered bandwidth.”
The UT Austin researchers did not attempt to crack Starlink’s encryption or accessibility any consumer knowledge coming down from satellites. In its place, they sought out synchronization sequences—predictable, repeating alerts beamed down by the satellites in orbit to help receivers coordinate with them. Not only did Humphreys find this sort of sequences, but “we ended up pleasantly astonished to obtain that they [had] extra synchronization sequences than is strictly expected,” he claims.
Every sequence also includes clues to the satellite’s length and velocity. With the Starlink satellites transmitting about four sequences every millisecond “that’s just fantastic for dual use of their procedure for positioning,” suggests Humphreys.
If the terrestrial receiver has a great idea of the satellites’ movements—which SpaceX shares on the internet to cut down the possibility of orbital collisions—it can use the sequences’ regularity to work out which satellite they came from, and then calculate the distance to that satellite. By repeating this procedure for various satellites, a receiver can find alone to inside about 30 meters, claims Humphreys.