In Ukraine, a US firm tests a promising tool against GPS jammers: cell phones

Networked cell phones running special software may make a cheap and easy-to-deploy counter to sophisticated Russian electronic-warfare tactics, according to an American company working to develop such a system in Ukraine.

Russian electronic warfare—signal jamming, GPS spoofing, and more—makes it harder for Ukrainian troops to operate drones and other equipment, even high-end U.S. weapons. Detecting and finding adversary jamming devices usually requires high-end software-defined radios, the sort of equipment the United States and other well-funded militaries can issue to their troops, but that smaller militaries with constrained budgets, like Ukraine’s, struggle to acquire in large numbers. 

If a cheap system can be made to locate enemy jamming gear, it could help Ukrainian operators regain some advantage against their better-armed foes. It could even change how cellphone network providers ensure their devices can stand up to sophisticated attacks. 

Around Christmas last year, the Ukrainian military reached out to Sean Gorman and his group at Zephr, a company that specializes in hardening devices against GPS-signal interference. Zephr shortly sent six Android Pixel phones loaded with their software to Ukraine, and in April began conducting field tests near the frontlines in Donetsk. 

“We strapped those phones to drones. We put them in cars. We set up stationary stands for them, and then we've also been doing controlled experiments where they use their own jammer, so we know the location of it,” Gorman told Defense One.

The first objective: see if regular consumer phones, working together in a network, could reveal the existence of an entity trying to jam GPS location data. They found that by comparing the GPS reception of various phones, they could detect when one or more was under attack. 

“We're basically exploiting the sensors on the phone,” Gorman said. “The most helpful sensor on the phone is the raw [global navigation satellite system, or GNSS] measurements that the phone provides. So you have AGC, the automatic gain control, you have Doppler [sensor], you have carrier phase, you have code phase,” and other data cell phones collect about their distance from satellites, cell towers, and other pieces of network equipment. 

Manufacturers put these sensors in cell phones so the phone can figure out how to optimize performance—for example, to locate the nearest cell tower. But these measurements all play a role in the GPS-processing software that phones use to tell the consumer where they are. When you can get that data from a wide variety of phones, you can figure out which ones are under attack.

“With the computational AI we're running on the back end, and the sophistication of how we're processing most signals and software, instead of just depending on sensors or really expensive antenna arrays, there's a heck of a lot you can do networking phones together as one big distributed antenna,” Gorman said. 

The tests also revealed new insights into Russian electronic warfare. 

Russia is spoofing GPS signals in the Baltic Sea, making receivers display incorrect location data and causing what NATO officials have described as an unsafe situation for consumer aircraft. A common anti-drone tactic is spoofing GPS so that a drone believes it is over an airport, where it must land or retreat from restricted airspace. 

On the frontlines in Donetsk, Ukraine’s soldiers often report spoofing attacks aimed at their drones. But Gorman and his team found that much of this “spoofing” activity is actually just high-powered jamming attempts. Because those attempts are occurring in the same frequency bands as GPS or GNSS, they appear like satellite signals, effectively creating phantom or ghost satellites in places where they could not possibly exist and still provide signal, such as beneath the horizon. 

“This noise doesn't resemble a typical GNSS signal, but it does contain energy at the frequencies where the receiver is looking for satellite signals. The receiver's signal-processing algorithms use correlation techniques to identify and track satellite signals. When a strong jamming signal is present, it can cause false correlations, leading the receiver to ‘think’ it's detecting satellites that aren't actually visible,” Gorman explained in a write-up of the testing provided to Defense One. 

The group, now working under a contract with the Ukrainian government, is trying to take the research a step further to not just detect jammers but triangulate their positions so the jammer can be avoided or eliminated. 

“The new techniques we are building will estimate the location of signals of interest using three inputs: 1. localization by range inferred from power; 2. localization by area of effect, and 3. triangulation of jammers based on angle of arrival,” Gorman said in an email. “Each smartphone will detect the interference signal, log the last known position, and timestamp the reception, providing data points across the network. By aggregating and processing these signals centrally, the system will triangulate the jammer's position.”

The U.S. military is investing heavily in alternatives to GPS, usually referred to as alternative position, navigation, and timing—or Alt PNT—to help people, drones, etc., figure out where they are. But investments that haven’t shown a great return in many cases. 

“None of it works without an initial known position. And it's almost impossible to get an initial known position without having GPS,” Gorman said.

This networked approach won’t replace other more expensive military systems for detecting and finding jammers, he said, but it does present a solution that can be deployed quickly and cheaply for militaries like Ukraine or other groups that could be disrupte by jamming activities, such as first responders. And while it won’t replace efforts to develop alternative position, navigation and timing methods, it might make them more effective.

“Maybe there's a radically different way to think about this? Instead of throwing all the resources towards Alt PNT, GNSS resilience, which are all super helpful, maybe we can look at how this is now a reality? How well can we map all of this and understand an area back to where the emitters are so that we can dodge those areas and we can remove the emitters?” Gorman said. 

But, he said, “There isn't a [silver] bullet.”