Experts Crack Signal Structure Of Starlink As Musk Refuses To Help Pentagon

Todd Humphreys’s offer to SpaceX was simple. With a few software tweaks, its rapidly growing Starlink constellation could also offer precise position, navigation and timing.

Photo Insert: To get the project started, UT Austin acquired a Starlink terminal and used it to stream high-definition tennis videos of Rafael Nadal from YouTube.

The US Army, which funds Humphreys’s work at the University of Texas (UT) at Austin, wanted a backup to its venerable and vulnerable, GPS system. Could Starlink fill that role? Mark Harris wrote for MIT Technology Review.

When the idea was first proposed in 2020, executives at SpaceX were open to the idea, says Humphreys. Then word came from on high.

“Elon told the leaders we spoke to: every other LEO [low Earth orbit] communications network has gone into bankruptcy,” Humphreys told MIT Technology Review. “And so we [SpaceX] have to focus completely on staying out of bankruptcy. We cannot afford any distractions.”

But Humphreys wouldn’t take no for an answer. For the past two years, his team at UT Austin’s Radionavigation Lab has been reverse-engineering signals sent from thousands of Starlink internet satellites in low Earth orbit to ground-based receivers.

Humphreys says his team has cracked the problem and he believes that regular beacon signals from the constellation, designed to help receivers connect with the satellites, could form the basis of a useful navigation system. Crucially, this could be done without any help from SpaceX at all.

To get the project started, UT Austin acquired a Starlink terminal and used it to stream high-definition tennis videos of Rafael Nadal from YouTube. This provided a constant source of Starlink signals that a separate nearby antenna could listen in on.

Humphreys quickly realized that Starlink relies on a technology called orthogonal frequency-division multiplexing (OFDM). OFDM is an efficient method of encoding digital transmissions, originally developed at Bell Labs in the 1960s and now used in Wi-Fi and 5G. “OFDM is all the rage,” says Mark Psiaki, a GPS expert and aerospace professor at Virginia Tech.

“It’s a way to pack the most bits per second into a given bandwidth.”

In a non-peer-reviewed paper that he has posted on his lab's website, Humphreys claims to have provided the most complete characterization of Starlink’s signals to date. This information, he says, is the first step toward developing a new global navigation technology that would operate 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.”

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