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 can simplify transportation and save lives. When Kevin Wells’ private jet lost GPS reception when it arrived at the Hayward Airport in California’s Bay Area in February 2019, he took out his phone And started shooting 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 if this is accidental or deliberate interference.” “And I want to track it.”
Wells was a victim of a gps jammer incident, in which rogue or malicious signals overwhelmed the weak signals from orbiting navigation satellites. Such incidents are usually related to U.S. military testing 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 jammers.
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 can’t 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 obscured 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 don’t have this model. We need more resources and more data.”
Regarding the Hayward incident, Walter’s team managed to identify a potential outage of 17 days between January 2019 and March 2019 [PDF]. During this period, of the 265 aircraft flying near Hayward, ADS-B data indicated that 25 of them may be affected by GPS jammers. Walter said that this kind of intermittent interference is not enough to isolate the signal source. “We can say that it is located in this area of Hayward, but you don’t want to search ten blocks. We want to locate it in a house or building. ”
Walter now hopes to expand his research in Hayward by using FAA’s own network of high-quality ADS-B receivers to reveal more interfering signals hidden in the data.
Using ADS-B signals is not the only way researchers can solve GPS reception problems. Earlier this month, John Stader and Sanjeev Gunawardena of the US Air Force Institute of Technology published a paper at the PTTI 2021 conference of the Institute of Navigation, describing another jamming detection system.
The AFIT system also uses free and open source data, but this time comes from a constantly operating network of ground GPS receivers around the world. These provided high-quality data of nearby GPS signals, which AFIT researchers then degraded to identify interference events. They were able to identify 30 potentially disruptive events within 24 hours within minutes of the interruption. The author writes: “The ultimate goal of this research work is to create a global automated system to detect interference events using publicly available data.”
Small systems like this have been put into use at Madrid Airport in Spain, where 11 GPS receivers are constantly monitoring for malfunctions or unexpected failures. Walter said: “Installation costs are high, and it takes a while.” “I think large airports may want something like this to protect their airspace, but small airports like Hayward will never be able to do it.”
Another problem with these two systems is the small number of sensors. Todd Humphreys, director of the Radio Navigation Laboratory at the University of Texas at Austin, said: “There are fewer than 3,000 GPS reference stations with public data in the world. These can be separated by hundreds of miles. Similarly, the global coverage of ships and aircraft is still so sparse that it is difficult to detect-positioning is almost impossible, except in the vicinity of ports and airports.”
Humphries prefers to use satellites to monitor satellites. Last year, he published an article about Syrian GPS jammers. Aireon is already monitoring the ADS-B signal of Iridium’s global midway satellite, and HawkEye 360 is building its own high-frequency low-earth orbit (LEO) acquisition satellite.
Walter said: “This is very exciting.” “If you have special equipment for these LEO satellites, in the long run, it may be more powerful than the entire series of ground sensors, and the price may be much cheaper.”
Until then, pilots like Kevin Wells need to stay witty and navigate areas where GPS jammers may occur. The FAA finally warned pilots covering Hayward and most of the Southern Bay Area, and the riots appeared to be subduing. However, Walter recently checked the 2020 ADS-B data near Hayward and found 13 other potentially congested flights.
When talking about the interference in 2019, Wells said: “It’s not a big deal for me because I got out of the clouds at the beginning of the second game. But if the weather is bad, it might be a big deal. Big event. There will be security issues.”