Test Flight Demonstrates Navigation by Cellphone Signals

This article is part of our exclusive IEEE Journal Watch series in partnership with IEEE Xplore.

In March of 2020, a team of U.S. Air Force pilots took to sunny skies over California to conduct a unique experiment, exploring a possible countermeasure if the plane’s GPS system, which determines the plane’s position so that it can be transmitted to ground controllers, were suddenly cut off. On board the plane, a completely novel backup navigation system that utilized cellphone signals was put to the test. The experiment proved to be a huge success, pointing to a new and much-needed potential backup for GPS.

Amazingly, the novel navigation system could track the plane for more than a hundred kilometers, and at various altitudes, with precision in the single-digit meters. The results are described in a study published 20 June in IEEE Transactions on Intelligent Transportation Systems.

It’s hard to imagine life without global navigation satellite systems (GNSSs), including GPS. But there are many natural and man-made threats that could knock out this vital resource, used in indispensable applications such as Google Maps and in countless military systems—underscoring the need for backup technology.

For instance, GPS signals tend to be weak in urban areas with densely clustered, tall, concrete buildings, and just one solar flare could knock out GNSSs for days if it’s severe enough. But perhaps most concerning of all are malicious attackers, who purposely interfere with GNSS signals.

“GNSS jamming and spoofing incidents have been bubbling over the past decade, reaching an outburst in 2024 with numerous aviation-related incidents from the Baltic to the Atlantic to the Mediterranean Sea,” notes Zak Kassas, professor of electrical and computer engineering at Ohio State University and director of the U.S. Department of Transportation’s Center for Automated Vehicles Research with Multimodal AssurEd Navigation (CARMEN).

In one test flight spanning a distance of about 43 kilometers, the radio SLAM system was able to track the plane with a statistically calculated error of just 7 meters.

Earlier this year, Russia reportedly launched an intense GPS spoofing campaign that forced civilian airlines in the Baltic region to ground their planes for more than 63 hours. Meanwhile, alleged Israeli GPS spoofing has disrupted the ability of civilian pilots on commercial airlines in Lebanon to navigate with GPS, prompting them to abandon the technology and fly with a compass and paper maps. In 2021 alone, more than 10,000 spoofing and jamming attacks took place, according to data from an aircraft manufacturer.

Kassas has been working for more than a decade to devise alternatives to GNSS, which could mitigate these threats, potentially saving lives. Previously, he developed a novel technique using the relative position of Starlink satellites to enable a moving car to determine its position. More recently, he and his team piloted their novel technique in partnership with the U.S. Air Force, called radio simultaneous localization and mapping (SLAM).

This radio SLAM system works by detecting the signals emitted constantly by surrounding cellphone base station towers. Importantly, these signals are a complex array of different omnidirectional radio subsignals that the SLAM system can use to create a map of its surroundings.

It does this by looking at the code and also the carrier phases of the signals, which indicate the various delays of the radio subsignals as they arrive at the receiver on the moving vehicle, in this case a plane. Kassas’s system also measures the Doppler effect as the signals arrive at the receiver, which reveal its movement relative to the surrounding cellphone base stations.

“GNSS jamming and spoofing incidents have been bubbling over the past decade, reaching an outburst in 2024 with numerous aviation-related incidents.” —Zak Kassas, Ohio State University

Together, the measurements allow calculation of the plane’s position, moment by moment, and without the need for GNSS. When the cellphone tower position is not known, the system simultaneously maps the tower positions, while pinpointing the vehicle.

Along with developing the software for these calculations, Kassas and his team developed an innovative receiver that can detect many cellphone signals thousands of feet in the air. That’s a significant technological advance—while previous receivers could pick up no more than a dozen such signals while at high altitudes, their new receiver can detect more than 100 at once.

After theoretical and ground-based experiments, Kassas and his colleagues partnered with the U.S. Air Force for the test flights. The pilots flew in several different formations, varying from a straight line at a consistent altitude to going up and down like a roller coaster.

Kassas says the cellular signals were “surprisingly powerful” at altitudes up to 23,000 feet, and horizontal distances exceeding 100 kilometers from cellphone base stations. For instance, in one test flight spanning a distance of about 43 km, the radio SLAM system was able to track the plane with a statistically calculated error of just 7 meters.

Flight Demonstration of High Altitude AircraftNavigation with Cellular Signalswww.youtube.com

Notably, the data wasn’t processed in real time during these test flights, but afterward. “Our objective was to address the fundamental research questions to push the limits of what is and isn’t possible,” he says. “Once the research matures, we could start thinking of taking this into real time, which is more of an engineering design problem than research [problem].”

In particular, new computational designs will need to be developed for real-time data analysis, which could ultimately take this novel SLAM system to the skies.

This article appears in the September 2024 print issue as “In Test Flights, Cell Signals Sub for GPS.”