The offer submitted by Todd Humphreys to SpaceX was simple. With a few software tweaks, its rapidly growing constellation of Starlink satellites could also become an ultra-precise location, navigation and timing aid. The US military, which funds the work of Todd Humphreys at the University of Texas at Austin, wanted an alternative to its venerable but vulnerable GPS system. Could the Starlink network embody this alternative?
In 2020, when the project was first presented to them, SpaceX executives were open to the idea, says Todd Humphreys in an interview with the MIT Technology Review. Then the order came from above. “Elon Musk told the executives we spoke to that all other LEO communication networks [orbite terrestre basse] went bankrupt,” he continues. “And so we [SpaceX] we have to focus entirely on avoiding bankruptcy. We cannot afford to be sidetracked from that goal.”
However, Todd Humphreys ignored this refusal. For the past two years, his team at the University of Austin’s Radio Navigation Laboratory has reverse-engineered signals sent from thousands of Starlink Internet satellites in low Earth orbit to receivers on the ground. Today, Todd Humphreys says his team has solved the problem and he thinks the regular signals from constellation beacons, designed to help receivers connect to satellites, could form the basis of a useful aid system. to navigation. And most importantly, it could be done without any help from SpaceX.
In a non-peer-reviewed article published on his laboratory’s website, Todd Humphreys claims to have provided the most comprehensive characterization to date of the signals emitted by Starlink. This information, he says, is the first step towards developing a new global navigation technology that would work independently of GPS or its European, Russian and Chinese equivalents.
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“The Starlink system signal is a very well-kept secret,” says Todd Humphreys. “Even in our early discussions, when SpaceX was more cooperative, they didn’t tell us anything about the structure of the signal. So we had to start from scratch, including building a small radio telescope to spy on their signals.”
To launch the project, UT Austin acquired a Starlink terminal and used it to broadcast 24/7 high-definition videos of a Rafael Nadal tennis match from YouTube.
Todd Humphreys quickly realized that the Starlink system relied 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 for Wi-Fi and 5G. “OFDM is all the rage,” says Mark Psiaki, GPS expert 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 did not attempt to break Starlink’s encryption or access user data transmitted by satellites. Instead, they looked for timing sequences — predictable, repeating signals emitted from orbiting satellites to help receivers coordinate with them. Not only did Todd Humphreys find such sequences, but “we were pleasantly surprised to find that there were more synchronization sequences than strictly necessary,” he says.
Each sequence also contains indices of the distance and speed of the satellite. Since Starlink satellites transmit about four sequences per millisecond, “it’s wonderful for the dual use of their positioning system,” says Todd Humphreys.
If the ground-based collector has a good idea of satellite movements — which SpaceX shares online to reduce the risk of orbital collisions — it can use the regularity of the sequences to determine which satellite they came from, then calculate the distance to it from that satellite. By repeating this process for several satellites, a receiver can locate itself within about 30 meters, details Todd Humphreys.
If SpaceX later decides to cooperate by including additional data on the exact position of each satellite in its downlinks, this precision could theoretically increase to less than one meter, which would make it possible to compete with GPS. Contacted by MIT Technology Reviewthe company SpaceX did not respond to our requests.
Other researchers have taken a similar route. Zak Kassas is a professor in the Department of Electrical and Computer Engineering at Ohio State University and director of a United States Department of Transportation Center for Shipping Resilience. Last year, his team was the first to demonstrate that Starlink signals could be used for positioning, thanks in part to machine learning.
Zak Kassas’ approach, which he dubbed “cognitive opportunistic navigation”, analyzes the period and changing frequencies of signals from a satellite as it passes overhead. The receiver also uses the synchronization sequences, detects the orbit of the satellite and tracks it. After several passes of the satellite, the receiver ends up calculating its own position. At a recent conference, Zak Kassas claimed that his system has now achieved sub-10 meter accuracy with Starlink. “It’s such a general framework that we can apply it to any terrestrial or extraterrestrial signal,” he explains. “It will learn on the fly, tell you what’s being transmitted, and tell you where you are.”
A better understanding of Starlink signals has implications beyond navigation. For example, currently Starlink satellites do not appear to use two of the eight channels that SpaceX has a license for. According to Todd Humphreys, this could be due to the fact that Elon Musk is keen not to interfere with radio telescopes operating on neighboring frequencies. In the past, light trails from orbiting Starlink satellites have been blamed for disrupting optical astronomy.
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The UT Austin findings also highlight the possibility of deliberate interference with Starlink. Todd Humphreys notes that while timing sequences hold promise for navigation, the fact that they are completely predictable and used across the constellation is a security hole. “Todd Humphreys has done a great service to the navigation community by identifying these sequences,” said Mark Psiaki. “But any navigation system working on open-source sequences could certainly be hijacked because everyone will know how to spot these signals and create fake ones.”
At the end of September, Starlink reportedly suffered a catastrophic loss of communication in Ukraine where the network is widely used for voice and electronic communications. It helps fly drones and even correct artillery fire. While it’s unclear whether the outages were due to jamming by Russian forces, Elon Musk tweeted last week: “Russia is actively trying to kill Starlink. To protect itself, SpaceX has reallocated massive resources to defense purposes”.
Starlink has undoubtedly been a lifeline for Ukraine. However, reports of outages and ongoing confusion over who will pay for Starlink’s services there raise concerns for the future.
“As time goes by and their reliance on Starlink grows, Ukraine and its Western allies realize that they have little control over Starlkink and they don’t know much about him,” says Todd Humphreys. “But now many millions of people have a vested interest in Starlink’s security, including its resistance to jamming. Assessing that security begins with a clear understanding of signal structure.”
Article by Mark Harris, translated from English by Kozi Pastakia.
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