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In a non peer-reviewed paper that he has posted on his lab’s web site, Humphreys claims to have supplied essentially the most full characterization of Starlink’s alerts thus far. This info, he says, is step one towards creating 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 venture 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 may eavesdrop on.
Humphreys shortly realized that Starlink depends on a expertise referred to as orthogonal frequency-division multiplexing (OFDM). OFDM is an environment friendly technique of encoding digital transmissions, initially developed at Bell Labs within the Sixties and now utilized in Wi-Fi and 5G. “OFDM is all the trend,” says Mark Psiaki, a GPS professional and aerospace professor at Virginia Tech. “It’s a solution to pack essentially 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 consumer knowledge 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 stunned to seek out that they [had] extra synchronization sequences than is strictly required,” he says.
Every sequence additionally incorporates 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 scale back the chance of orbital collisions—it will possibly 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.
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