Maintaining Air Traffic Efficiency When GPS Signals Degrade

GPS has become the principal means of navigation for pilots operating in U.S. airspace. That means GPS interference events can have a significant impact on flight operations. And given the low power of the GPS signal, it's easily jammed.

As a result, the FAA asked MITRE, operator of its federally funded R&D center, to develop capabilities to monitor GPS signal degradation events and assess their impact on aircraft navigation nationwide. We're doing just that with the prototype Navigation Operational and Planning Agility Suite (NOPAS).

"Our expertise with GPS dates back to its inception," says Rick Niles, chief engineer for MITRE's National Airspace System (NAS) infrastructure modernization efforts. "We've used that expertise—along with our deep understanding of aircraft navigation and NAS operations—to create NOPAS."

While some NOPAS capabilities are still under development and others are being evaluated, their potential is already evident.

Understanding the Prevalence and Impact of Jamming Events

"There are a couple of drivers for NOPAS," says Wayne Cooper, NOPAS project lead. One is that GPS signal interference events are increasing. "In 2022, for example, there were GPS jamming events in the Denver and the Dallas Fort-Worth areas that caused flights to be delayed, cancelled, or diverted."

Most such events are accidental. "They’re often caused by someone leaving a GPS repeater on," Cooper says, referring to a device used to rebroadcast GPS signals in indoor environments during aircraft system testing. "However, there’s always the potential for malicious activity."

When GPS signals become unreliable, pilots must revert to using older, ground-based navigation aids. If those systems are unavailable, they must rely on ongoing voice communications from the air traffic controller to guide them safely to their destinations.

"When GPS becomes degraded, the FAA needs to know as quickly as possible," Cooper says. "They also need to know whether ground-based navigation aids are available for use in the affected region so they can determine the best means to maintain safe and efficient flight."

Secondly, the Department of Defense (DoD) has ramped up testing and training operations that involve purposely jamming GPS signals.

"We’re seeing GPS jamming happening in combat zones, so the DoD is conducting exercises to prepare warfighters for these scenarios," Niles explains. "That may require rerouting air traffic around the testing locations." The FAA is therefore seeking enhanced insight into the impact these events would have on regular air traffic so that it can better coordinate with the DoD on when and where they will occur.

NOPAS is designed to address all these scenarios.

Gaining Rapid Insight into Unplanned GPS Degradation Events

When a GPS degradation event occurs, air traffic controllers typically find out through pilot reports. The FAA can then initiate an investigation. "But that takes time," Cooper says. "With NOPAS, we’re using data from aircraft transmissions to identify these events in near-real-time."

All commercial airliners and business jets, as well as many other aircraft, are equipped with technology that captures the aircraft’s GPS-generated location information and broadcasts it to other aircraft in the vicinity and to receivers on the ground.

"That broadcast information also includes measures of GPS navigation integrity calculated on the aircraft," Cooper says. NOPAS uses the GPS navigation integrity information from each aircraft to determine whether GPS navigation is degraded in that area.

NOPAS then maps the locations where GPS is unreliable on an easy-to-understand heat map display. An overlay map shows the other navigation systems pilots can use in these regions.

For instance, if existing ground-based navigation aids provide sufficient coverage, little additional action may be needed. In other cases, a degradation event might require air traffic controllers to use voice commands to guide flights based on their radar position.

"Today, the impact of a GPS jamming event can last for several days," Cooper says. "By reacting faster, the FAA has the potential to limit the impact to a few hours." A more rapid response can also reduce the need for more disruptive options—such as flight rerouting, holding patterns, and diversions to other airports—which are costly for airlines and inconvenient for the flying public.

Scheduling Planned Jamming Events for Least Disruption

Additional NOPAS capabilities are being developed to help the FAA better coordinate with the DoD on the scheduling of planned GPS jamming events for military exercises.

The agencies already coordinate, but they currently rely heavily on institutional knowledge to determine the best times for these tests. "The FAA wanted a more objective way of measuring the tests' potential impact," Cooper says. To achieve that, our systems engineers are mining historical data about traffic volumes and the impact of previous GPS degradation events to estimate the future impact of a testing event at any given location.

"NOPAS takes into account airspace configuration, day of week and time of day, the length of the testing event, and other factors—all in one system," Cooper says.

This capability will not only help the FAA prevent disruptions to commercial aviation traffic, but it will also help the DoD.

"The FAA can stop a test if it proves to be too disruptive," Cooper explains. "That's very costly to DoD, which invests millions of dollars to plan and set up a testing event. NOPAS can help prevent those scenarios."

Next? Detecting Spoofed GPS Signals and Moving Closer to Real Time

Work to refine and expand NOPAS is ongoing. A key focus for the coming year is to develop the capability to distinguish between GPS jamming and spoofing events.

With jamming, the pilot loses the ability to navigate. With spoofing, the pilot would receive a false position that looks real.

"That’s a much more dangerous situation," Cooper notes. The pilot could be drawn off course, for example. And pilots flying at low altitudes, especially in mountainous regions, would be at risk if they're relying on an incorrect signal for navigation.

"We've already developed algorithms to detect potential GPS signal spoofing. They're currently being validated," Cooper says. "Currently, spoofing is very, very rare, but we want to be ready."

The other big NOPAS improvement MITRE researchers are working to achieve in the next year is to accelerate data processing in NOPAS.

"Right now, we can identify a GPS degradation event within a few hours of when it began," Cooper says. "Our target is to do so within 15 minutes, because the faster these events can be identified, the better it is for everyone—the FAA, airlines, private pilots, and the flying public."