Designed to interfere with networks that track global GPS

A screenshot of the dashboard video on a Cessna private jet. When the plane ascended after takeoff, the signal from the GPS satellite was interrupted for a few seconds.

Troubleshooting the global navigation system simplifies transportation and saves lives

When Kevin Wells' private jet lost GPS reception when it arrived at the Hayward Airport in the California Bay Area in February 2019, he took out his mobile phone and started taking pictures of the dashboard. When Wells continued to descend and finally landed safely, the signals from a dozen GPS satellites flashed for a few seconds, and then slowly returned.

Wells quickly filmed the incident because it was not the first time he encountered GPS problems. Less than a month ago, his Cessna suffered a similar failure in almost the same place. He told IEEE Spectrum: "By then, I know very well whether this is accidental or deliberate interference." "And I want to track it."

Wells was a victim of a GPS jamming incident, in which rogue or malicious signals overwhelmed the weak signals from orbiting navigation satellites. Such incidents are usually related to U.S. military tests and may lead to dangerous situations and near misses. A recent Spectrum survey found that they are much more common than previously thought, especially in the western United States.

After the second incident, Wells contacted the local air traffic control tower and asked other pilots to drive near Hayward with a portable GPS receiver to find the cause of the malfunction by himself, but to no avail.

Fortunately, Wells has other ways to solve the problem. As the executive director of the Institute of Theoretical Physics at Stanford University, Wells met many researchers at the university. He brought the video to the Stanford Engineering GPS Laboratory, where Professor Todd Walter is already studying GPS Jammer.

The Federal Aviation Administration (FAA) and the Federal Communications Commission (FCC) have established procedures to locate people who intentionally or unintentionally interfere with GPS signals. When the pilot reported mysterious intermittent GPS interference near Wilmington Airport in North Carolina, the FAA eventually found a poorly designed antenna in the utility company's wireless control system. "But it took weeks, even months." Walter told Spectrum. "Our goal is to track down the perpetrators within a few days."

Walter's team is working on a drone that can automatically detect local signals in the GPS frequency band without relying on GPS for its own navigation. Walter said: "But we don't allow us to fly drones in Hayward's airspace, and we cannot take off there and find the source of interference.

Instead, Walter had another idea. Why not use GPS receivers on other aircraft to find a solution? All modern aircraft are equipped with ADS-B transponders-devices that can continuously transmit their GPS position, speed and heading to help air traffic control and avoid possible collisions. These ADS-B signals are not only captured by nearby aircraft, but also by many ground sensors, including the open access receiver network organized by the Swiss non-profit organization OpenSky.

Using the available OpenSky data, the first task of Stanford University researchers is to accurately identify interference events. They found that the vast majority of cases where the ADS-B receiver loses data has nothing to do with interference. Some receivers are unreliable, while others are blocked by buildings or trees on the plane.

The ability to correlate Wells' GPS signal loss with data from the OpenSky database is an important step in characterizing real interference. The integration and accuracy indicators built into the ADS-B data stream also help researchers. Walter said: "I think some interference events have characteristics that we can recognize now." "But I am worried that there may be other interference events, we do not have this model. We need more resources and more data.

For the Hayward incidents, Walter's team managed to identify 17 days of potential disruption between January and March 2019 [PDF]. Of the 265 aircraft that flew near Hayward during that time, ADS-B data indicated that 25 were likely to have had GPS interference. This intermittent disturbance wasn't enough to isolate the source of the signals, says Walter: "We can say it's in this area of ​​Hayward, but you don't want to search ten blocks of town. We want to locate it in a house or building."

Walter now hopes to expand his Hayward study by using the FAA's own network of high quality ADS-B receivers to reveal more of the interference signals hidden in the data.

Using ADS-B signals isn't the only way researchers can fix GPS reception. Earlier this month, John Stader and Sanjeev Gunawardena from the US Air Force Institute of Technology presented a paper describing an alternative interference detection system at the PTTI 2021 conference of the Institute of Navigation.

The AFIT system also uses free and open source data, but this time from a network of continuously operating terrestrial GPS receivers around the world. These provide high-quality data on GPS signals in their vicinity, which the AFIT researchers then degraded in order to identify interference events. They were able to identify 30 potential disruptive events within 24 hours within minutes of the occurrence of the disruption. "The ultimate goal of this research effort is to create a worldwide, automated system for detecting interference events using publicly available data," the authors write.

A smaller system like this is already in operation at Madrid Airport in Spain, where 11 GPS receivers are constantly monitoring for malfunctions or accidental malfunctions. "Installation is expensive and takes a while," says Walter. "I think large airports probably want something like this to protect their airspace, but smaller airports like Hayward will never be able to."

Another problem with these two systems is the low number of sensors, says Todd Humphreys, director of the Radionavigation Laboratory at the University of Texas at Austin: “There are fewer than 3,000 GPS reference stations in the world with publicly available data. These can be separated by hundreds of miles. Likewise, global coverage by ships and airplanes is still sparse enough to make detection difficult - and localization is next to impossible, except in the vicinity of ports and airports. "

Humphreys prefers the use of satellites to monitor the satellites. He published an article last year dealing with a GPS jammer in Syria. Aireon is already monitoring ADS-B signals worldwide from Iridium's mid-earth orbit satellites, while HawkEye 360 ​​is building its own high-frequency, near-earth orbit (LEO) acquisition satellites.

"It's very exciting," says Walter. "If you have dedicated equipment for these LEO satellites, it can be very powerful and potentially much cheaper in the long run than a whole range of terrestrial sensors."

Until then, pilots like Kevin Wells need to keep their wits about navigating areas where GPS interference can occur. The FAA eventually warned pilots covering Hayward and much of the southern Bay Area, and the disturbance appeared to be calming down. However, Walter recently looked at the 2020 ADS-B data near Hayward and found 13 other potentially jammed flights.

"It wasn't a big event for me because I came out of the clouds at the beginning of the second event," Wells says of the 2019 interference. "But if the weather had been bad it could have been a big deal. There would be a security problem. "