Starlink's satellite signals, and joyless tech

In a non peer-reviewed paper that he has posted on his lab’s web site, Humphreys claims to have offered the most finish characterization of Starlink’s indicators to day. This information, he claims, is the initially step toward developing a new international navigation know-how that would function independently of GPS or its European, Russian, and Chinese equivalents. 

“The Starlink technique signal is a carefully guarded secret,” states Humphreys. “Even in our early conversations, when SpaceX was currently being extra cooperative, they didn’t expose any of the signal framework to us. We had to get started from scratch, creating essentially a little radio telescope to eavesdrop on their indicators.”

To get the project commenced, UT Austin acquired a Starlink terminal and made use of it to stream higher-definition tennis films of Rafael Nadal from YouTube. This supplied a continuous supply of Starlink indicators that a independent close by antenna could pay attention in on.

Humphreys rapidly understood that Starlink relies on a technological know-how known as orthogonal frequency-division multiplexing (OFDM). OFDM is an successful strategy of encoding electronic transmissions, initially developed at Bell Labs in the 1960s and now applied in Wi-Fi and 5G. “OFDM is all the rage,” says Mark Psiaki, a GPS qualified and aerospace professor at Virginia Tech. “It’s a way to pack the most bits for each 2nd into a provided bandwidth.” 

The UT Austin researchers did not test to crack Starlink’s encryption or accessibility any consumer info coming down from satellites. As a substitute, they sought out synchronization sequences—predictable, repeating indicators beamed down by the satellites in orbit to assistance receivers coordinate with them. Not only did Humphreys find such sequences, but “we were being pleasantly shocked to uncover that they [had] additional synchronization sequences than is strictly demanded,” he says.

Every sequence also has clues to the satellite’s distance and velocity. With the Starlink satellites transmitting about 4 sequences each individual millisecond “that’s just superb for dual use of their procedure for positioning,” suggests Humphreys. 

If the terrestrial receiver has a great notion of the satellites’ movements—which SpaceX shares on line to reduce the possibility of orbital collisions—it can use the sequences’ regularity to function out which satellite they came from, and then calculate the distance to that satellite. By repeating this system for numerous satellites, a receiver can find alone to in just about 30 meters, suggests Humphreys.

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