Starlink signals could work as GPS alternative, whether SpaceX likes it or not • TechCrunch
With nearly 3,000 satellites in orbit, the Starlink constellation is easily the largest in history and of course presents a tremendous opportunity for global connectivity. But its signals can also be analyzed and used as an alternative to traditional GPS, a new research system that claims, with or without SpaceX’s blessing.
Todd Humphreys and his team at the University of Texas Austin tapped into the “signal architecture” of the Starlink downlink, and while there’s a lot they can learn without inside information from the company itself, they’ve found a lot of useful data.
The satellites have to pass their signals back to Earth at some point, and in the case of Starlink, that’s pretty much a steady stream. That doesn’t mean just anyone can get in, though – the signal itself is structured and encrypted in a special way, which, probably, SpaceX decided was best for the kind of orbital broadband it provides.
The exact parameters of this signal are unknown, but in order to ensure that packets arrive in order and are safe, transmission – like any data transmission these days – involves very accurate timing data and other telemetry so that the recipient and sender can stay in sync.
By carefully analyzing the signal from a single satellite and combining that data with what is known about the exact location of the satellite, time down to nanoseconds, etc., Humphreys was able to decode the transmission to a certain extent. They document their results in paperWaiting for review and publication by colleagues.
A side effect of accurate telemetry from a few satellites (as well as other measurements such as Doppler radar) is that you can use it to determine your exact location, and the paper describes how to do this.
As the record indicatesThis idea isn’t new and in fact has been pursued as a possibility for years, but eventually SpaceX decided to focus on the consumer side of things. But scientists at UT Austin have tasted the possibilities and decided to pursue it independently with its rig without the cooperation of SpaceX.
As they write in the introduction to the paper:
The signal characterization presented here includes the exact values of synchronization sequences embedded in the signal that can be exploited to produce spurious range measurements. This understanding of the signal is essential for emerging efforts seeking dual-purpose Starlink signals for positioning, navigation, and timing, even though they are only designed to provide broadband Internet.
To be clear, no one is accessing Starlink’s user data here. Sync sequences are just sequences of timings and other data that devices use to stay connected – payload data is completely separate.
In the paper, due to the fact that the signal was aimed at an actual Starlink user station, the location for that station should have been too, and they were able to get it within 30 metres. It’s obviously no better than GPS, but it could be faster and ultimately more accurate if SpaceX gives the project its blessing.
A software update that finely tunes how satellites transmit their signals and a little bit of data about correcting the discrepancy between their clocks, Humphreys suggests Starlink transmissions could be used to determine a person’s location within one meter.
It will be a public service and it won’t cost SpaceX much of anything to implement, but it’s also a valuable service for which there is no business in its right mind (especially one that has just committed to implementing Very unprofitable call deal in Ukraine) will be implemented and provided free of charge. Having said that, the genie might be out of the bottle – the data in the paper “lights the path” to this use, and someone might find a way to make it work no matter what anyone says.