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 indicators thus far. This data, he says, is step one towards creating a brand new world navigation know-how that will function 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 being more cooperative, they didn’t reveal any of the signal structure to us. We had to start from scratch, building basically a little radio telescope to eavesdrop on their signals.”
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 continuing supply of Starlink indicators {that a} separate close by antenna may pay attention to.
Humphreys rapidly realized that Starlink depends on a know-how known as orthogonal frequency-division multiplexing (OFDM). OFDM is an environment friendly methodology of encoding digital transmissions, initially developed at Bell Labs within the Nineteen Sixties and now utilized in Wi-Fi and 5G. “OFDM is all the rage,” says Mark Psiaki, a GPS knowledgeable 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 didn’t attempt to break Starlink’s encryption or entry any person information coming down from satellites. Instead, they sought out synchronization sequences—predictable, repeating indicators beamed down by the satellites in orbit to assist receivers coordinate with them. Not solely did Humphreys discover such sequences, however “we were pleasantly surprised to find that they [had] more synchronization sequences than is strictly required,” he says.
Each 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 just wonderful for dual 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 danger of orbital collisions—it will probably use the sequences’ regularity to work out which satellite tv for pc they got here from, after which calculate the gap 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.