The U.S. Federal Aviation Administration (FAA) has been quietly funding tests with live cellphones in light aircraft cockpits as a possible counter-measure to GPS spoofing attacks, Spectrum has learned.
The series of tests, which occurred this summer in the skies over Virginia, used commercial smartphones connecting to standard 4G and 5G wireless networks operated by AT&T, T-Mobile and Verizon, according to documents filed with the Federal Communications Commissions (FCC).
The phones were running an app developed by the Mitre Corporation's Center for Advanced Aviation Systems Development, a federally-funded research center that provides the FAA with advanced technical capabilities in systems engineering, mathematics, and computer science.
The app leverages a feature of wireless networks called timing advance, which is designed to minimize uplink collisions between cellphones in motion and the base stations to which they are connecting. But it's well known that the same data can also be used to calculate a rough range from a cell tower to a handset.
"We can then check that the range is coherent with the position from the [phone's] GPS system and not way out of whack," said Rick Niles, a Mitre project manager, in a phone interview. The app could never provide navigation or an exact location for the plane but it could issue an alert if there is a mismatch with the GPS data.
Such a warning would be useful because GPS is far from perfect. The unencrypted signals from orbiting navigation satellites are extremely weak, and relatively easy to overpower or even fake (called "spoofing"). A Spectrum investigation earlier this year discovered that GPS interference events are far more prevalent, particularly in the western United States, than had previously been thought.
"While it hasn't become a major threat yet, the biggest interference we've had in the US has been our own military," said Niles. "It's a hint as to how it could potentially be worse in the future."
Light and private aircraft, known as general aviation, are particularly vulnerable because they often lack the backup navigation technologies found on commercial jets.
"There are certain phases of flight, like approaches in bad weather, where you are relying on your GPS to keep you from finding rocks in the clouds," said Jim Chadwick, a senior Mitre researcher and a pilot himself. "You need to be warned if that GPS signal is leading you astray."
An advantage of using wireless networks to provide that warning is that cellphones are very cheap and now almost ubiquitous. A disadvantage is that turning them on in a plane, whether as a pilot or a passenger, is still illegal. Historically, there have been fears (and some evidence) that cellphones could interfere with aircraft systems or cause network congestion by connecting to multiple phone towers below.
The risk of either is considered small these days, and the Mitre Corporation eventually got permission from both the FCC and the FAA to conduct its Virginia test flights this summer.
Mitre's flights in a light aircraft, with Niles at the control and a safety pilot beside him, began in the state's capital Richmond and spanned hundreds of miles of urban, rural and mountainous terrain. "We flew relatively low and incremented it up in steps until we got to higher altitudes where we knew cell phone reception would get very weak," said Niles.
The Mitre team is still analyzing the test data but they did notice differences between urban and rural areas, and between different wireless carriers. They also found dropouts in reception at higher altitudes in "many areas."
"That's not entirely bad because the spoofing that's really dangerous is the one on approach to an airport," said Todd Humphries, director of the Radionavigation Laboratory at UT Austin and an expert in GPS spoofing. "This isn't a silver bullet but it's exactly what I would hope the FAA would look at. It's cheap, it's somewhat unconventional, and it could be effective."
At the end of September, Mitre will submit its report on the flight tests to the FAA, which will then decide whether to fund more research to develop the technology further.
"We're hopeful but first we need to figure out whether it's going to work or not," said Niles. "Even if it does, there a lot of regulatory things would have to happen in order for this to eventually become an approved system." When and if the technology is approved, it would be used by pilots—not passengers on commercial flights.
In other words, don't hold your breath for a pre-flight instruction to fasten your seatbelt, put your tray in the upright position, and ensure that your cellphone is turned… on.
This article was updated on September 2nd to clarify the types of aircraft Mitre used in its tests and that the technology would not be used by passengers on commercial flights.
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Mark Harris is an investigative science and technology reporter based in Seattle, with a particular interest in robotics, transportation, green technologies, and medical devices. He’s on Twitter at @meharris and email at mark(at)meharris(dot)com. Email or DM for Signal number for sensitive/encrypted messaging.