Researchers secretly converted Starlink signals into GPS systems despite Musk’s objection

Researchers claim they are doing Starlink a favor in Ukraine, where Russia is trying to take down the satellites.
Loukia Papadopoulos
Stock photo: Communication satellite transmitting signal, 3d rendered image.
Stock photo: Communication satellite transmitting signal, 3d rendered image.

BorisRabtsevich/iStock 

In 2020, Todd Humphreys and his team at the University of Texas’s Radionavigation Lab had an interesting offer to SpaceX, according to an article by MIT Technology Review published on Friday. With a few software modifications, its Starlink constellation could also offer precise position, navigation, and timing that could serve as a backup to the army’s GPS system.

Bankruptcy interfering with research

At first executives at SpaceX were open to the idea but then then “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.”

This did not stop Humphreys who for the past two years, has been reverse-engineering signals sent from thousands of Starlink internet satellites in low Earth orbit to ground-based receivers. Now, the researcher says his team has figured out a system where regular beacon signals from the constellation could form the basis of a useful navigation system without any input from SpaceX at all.

“The Starlink system signal is a closely guarded secret,” said 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.”

Starlink relies on a technology called orthogonal frequency-division multiplexing (OFDM), an efficient method of encoding digital transmissions. “OFDM is all the rage,” said 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.”

The key here was that the UT Austin researchers did not try to break Starlink’s encryption but instead focused on synchronization sequences. Humphreys said they were able to spot such sequences, but “we were pleasantly surprised to find that they [had] more synchronization sequences than is strictly required.”

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Calculating the distances to satellites

“If the terrestrial receiver has a good idea of the satellites’ movements—which SpaceX shares online to reduce the risk of orbital collisions—it can use the sequences’ regularity to work out which satellite they came from, and then calculate the distance to that satellite. By repeating this process for multiple satellites, a receiver can locate itself to within about 30 meters,” explains MIT Technology Review.

Now Humphreys believes he is doing Starlink a favor particularly in Ukraine where Russia is trying to take down the satellites that the country so clearly needs in these dire times.

“As time goes on and their dependence on Starlink deepens, Ukraine and its allies in the West are coming to appreciate that they have little control over Starlink and know little about it,” said Humphreys. “But now many millions have a vested interest in Starlink security, including its resilience to jamming. Assessing that security starts with a clear understanding of the signal structure.”

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