Boffins Explore Cell Signals As Potential GPS Alternative

Faced with growing threats to aviation GPS systems, researchers at Sandia National Labs argue we're overlooking a readily available alternative that could work effectively with further research: cellular signals.

You'd be forgiven for scoffing at the idea that it took a group of world-class scientists until 2024 to figure out that using signals of opportunity (SOOs) to pinpoint location is possible, but they know that. Research into SOOs has been extensive over the years, and has even involved cracking the code on Starlink satellite signals to use as a GPS alternative, but most experiments have focused on terrestrial navigation - not determining position at altitude. 

"While we are still processing the flight data, we believe our preliminary findings indicate that we detected cell tower signal beacons at our peak altitude of about 82,000 feet," Jennifer Sanderson, Sandia's lead researcher on the project, said. "In comparison, other studies we've seen have focused on 5,000 to 7,000 feet."

What that means is that Sanderson and her team managed to pick up cellular signals thousands of feet beyond the maximum ceiling for commercial aircraft flights, which typically cruise between 30,000 and 42,000 feet, and they did it with nothing but a couple of Raspberry Pis and some other off-the-shelf electronics packed in a styrofoam cooler and attached to a weather balloon. 

It's not a stretch to imagine a passenger jet has better antennas, meaning they might be able to pick up even stronger signals, given the right hardware.

Good timing

There are two ways to disrupt the signals from any of several global navigation systems around the world: Jamming and spoofing. The former is straightforward: overpowering satellite signals with garbage data, while the latter can fool a receiver into thinking it's somewhere else entirely. 

Both spoofing and jamming are on the rise, the EU Aviation Safety Agency (EASA) said earlier this year. EASA and the International Air Transport Association have both described navigation signal interference as a significant safety challenge for the modern aviation industry, and one that needs to be addressed through GPS alternatives or backups. 

Some work has begun on resilient, spoof- and jam-resistant GPS alternatives, but those efforts have been hampered by delays, in the US at least, making any possible alternative appealing. 

• Pentagon has little to show for two decades of GPS modernization work
• ESA salutes Galileo satellite system meeting aviation standards
• How do you sing 'We're jamming and we hope you like jamming, too' in Russian? Kremlin's sat-nav spoofing revealed
• Russia spoofed AIS data to fake British warship's course days before Crimea guns showdown

Of course, that doesn't mean we're anywhere close to making a functional alternative to aviation GPS that can be used practically from the Sandia team's experiment - there are a lot of complications they need to work out. 

For one, none of the cell tower signals the team is looking at include any form of timing or location data, meaning that it has to be extrapolated from the physical characteristics of radio waves through things like the Doppler effect.

According to Sandia, the team is still processing their first batch of data. Given this is the first experiment of the type the team has conducted, processing includes the painstaking process of manually matching signal data to nearby GPS and non-GPS satellites to use as reference data for determining position, as there's no automated system around to do that yet. 

"No one to our knowledge has published any work using a fusion of terrestrial tower signals and non-GPS comms satellite signals, however it's not because it's too complicated from an algorithm standpoint," Sanderson told The Register in an emailed statement. It's actually all about the hardware.

"Each signal frequency requires a different antenna, RF front end and receiver. It becomes an engineering problem at that point and specific to your application," Sanderson added. Hardware to snag all the necessary signals was included in Sandia's flying coolers, making this experiment all about building the right algorithm to make sense of non-GPS signals for finding the location. 

If these signals are clean enough for navigation, it will significantly change what we thought was possible for alternative navigation

"This project is focused on fundamental research and development for algorithms and signal processing," Sanderson told us. "My goal is to have a robust dataset to develop algorithms for real-time systems, enabling hardware tests using actual live-sky data." 

As to when that might be, the team didn't offer any guesses, and we were told we couldn't view the findings since they're still under peer review. In other words, it'll be a while until something comes of the Sandia research, though there are efforts underway elsewhere to commercialize similar technology.

Whoever gets there first, the end result is safer flying.

"If these signals are clean enough for navigation, it will significantly change what we thought was possible for alternative navigation," Sanderson noted. ®