In a non peer-reviewed paper that he has posted on his lab’s web site, Humphreys claims to have supplied probably the most full characterization of Starlink’s alerts so far. This data, he says, is step one towards growing a brand new international navigation expertise that might function independently of GPS or its European, Russian, and Chinese language equivalents.
“The Starlink system sign is a intently guarded secret,” says Humphreys. “Even in our early discussions, when SpaceX was being extra cooperative, they didn’t reveal any of the sign construction to us. We needed to begin from scratch, constructing mainly a bit radio telescope to listen in on their alerts.”
To get the challenge began, UT Austin acquired a Starlink terminal and used it to stream high-definition tennis movies of Rafael Nadal from YouTube. This supplied a relentless supply of Starlink alerts {that a} separate close by antenna might eavesdrop on.
Humphreys rapidly realized that Starlink depends on a expertise known as orthogonal frequency-division multiplexing (OFDM). OFDM is an environment friendly methodology of encoding digital transmissions, initially developed at Bell Labs within the Sixties and now utilized in Wi-Fi and 5G. “OFDM is all the fad,” says Mark Psiaki, a GPS skilled and aerospace professor at Virginia Tech. “It’s a option to pack probably the most bits per second right into a given bandwidth.”
The UT Austin researchers didn’t attempt to break Starlink’s encryption or entry any person information coming down from satellites. As an alternative, they sought out synchronization sequences—predictable, repeating alerts beamed down by the satellites in orbit to assist receivers coordinate with them. Not solely did Humphreys discover such sequences, however “we have been pleasantly shocked to search out that they [had] extra synchronization sequences than is strictly required,” he says.
Every sequence additionally comprises clues to the satellite tv for pc’s distance and velocity. With the Starlink satellites transmitting about 4 sequences each millisecond “that’s simply great for twin use of their system for positioning,” says Humphreys.
If the terrestrial receiver has a good suggestion of the satellites’ actions—which SpaceX shares on-line to cut back the chance of orbital collisions—it might use the sequences’ regularity to work out which satellite tv for pc they got here from, after which calculate the space to that satellite tv for pc. By repeating this course of for a number of satellites, a receiver can find itself to inside about 30 meters, says Humphreys.
