SpaceX’s Starlink satellite constellation, which has been providing internet service via satellite in Ukraine, has been growing and improving. But it turns out that it can also serve another purpose.
An academic from the University of Texas, Todd Humphrey, presented an idea to SpaceX executives in 2020. He explained that with some adjustments to Starlink’s satellite software, it would be possible to offer an alternative to GPS in case of failure. Although SpaceX initially focused on satellite internet, Humphrey continued to work on this option.
After reverse-engineering the transmitted signals, Humphrey and his team discovered a solution to the problem. They found that SpaceX’s satellites could potentially offer a geopositioning system in addition to internet service.
Humphrey published a scientific study analyzing Starlink’s system signals, which he described as “a closely guarded secret.” While SpaceX was initially more cooperative in the early discussions, they never provided details about these signals. Humphrey had to analyze them after purchasing a Starlink terminal.
To conduct this reverse-engineering work, the researchers studied high-definition YouTube videos of Rafael Nadal playing tennis. This provided a data source to study and allowed them to decode the transmissions, which used the OFDM technique (Orthogonal Frequency-Division Multiplexing).
The researchers then searched for synchronization sequences and discovered that there were more than what was strictly necessary. Each sequence contains data about the distance and speed of the satellites. Since four of these chains are transmitted every millisecond, they can be used for geolocation.
Currently, without any modifications from SpaceX, the system can geolocate a receiver with an accuracy of 30 meters. This accuracy could be improved to less than a meter, competing with GPS, if SpaceX includes additional data about the exact position of each satellite in the downward channel.
However, there are some security concerns. Mark Psiaki, a GPS expert and professor at Virginia Tech, points out that the predictability and widespread usage of these open-source code sequences pose security vulnerabilities. Any navigation system that relies on open-source code sequences could be easily falsified, as anyone would know how to detect these signals and create false ones.
It remains to be seen if SpaceX will pursue the implementation of their satellite constellation as a GPS alternative in the future.