GPS Interference Testing Remains Problematic

 - May 18, 2021, 12:47 PM

During an NBAA News Hour session on May 14, experts summarized the status of problems caused by military GPS jamming tests. This type of testing continues to grow and is causing GPS reception outages for all types of flight operations, and there is no plan by the military to reduce GPS jamming training operations.

“We’ve seen the number of GPS interference events have nearly quadrupled in the past decade,” said NBAA director of air traffic services and infrastructure Heidi Williams. “The number of locations across the National Airspace System [NAS] where those jamming events occur has doubled in the last two to three years. So we’re seeing a proliferation of events and locations.” She added that it’s likely the impact of this GPS jamming testing is growing. “It’s fair to say that the events are often a safety concern for operators in the NAS and are profound when they do have an impact.”

The reason for this testing and why it is growing is that the military needs to be prepared if GPS signals are compromised by enemy jamming, explained Jim McClay, director of airspace, air traffic, and security for the Aircraft Owners and Pilots Association (AOPA). “The jamming by the DoD [Department of Defense] is to allow them to be able to train in an environment deprived of GPS signal. With the threat landscape globally having changed to more of a large nation-state scenario, the military has to be able to train in an environment where another nation takes out the GPS constellation and they can no longer rely on that.”

This issue is not new, and in 2017 the FAA tasked an RTCA tactical operations committee to examine the impact of intentional GPS jamming events and to make recommendations on mitigating those impacts, according to Williams. The committee issued its report in May 2018, then the committee was disbanded. The report had 25 recommendations to the FAA, but she said, “we didn’t have a status on what the agency decided to do with those recommendations and what mitigations were in play between the FAA and the DoD. That brings us full circle to today and why we’re still talking about this.”

“We as an industry are becoming more reliant on GPS,” said McClay. GPS is a critical part of the NAS and there are more IFR procedures that rely on GPS, as well as airborne equipment that is primarily GPS-based. “This continues to be a concern for us,” he said.

Jack Allen, managing director of air traffic management for Airlines for America, explained that these GPS jamming events are flagged with notams 120 hours before the event. But these GPS notams aren’t included in standard briefings and pilots need to search separately for GPS notams.

But many of these GPS notams are textual and difficult to decipher, according to McClay. “We’re trying to expect folks to interpret this information by looking at a series of lat/longs and a description. It’s not effective.” To its credit, the FAA does publish graphical depictions of GPS interference testing advisories on its FAASafety website. 

The other issue is that there is no consistent agreement about how to collect data on interference occurrences. According to Williams, GPS outages have a significant safety impact. In one case it caused an aircraft to enter a Dutch roll and another to lose all GPS information on the final segment of an instrument approach.

“Those are big deals,” she said, especially considering the increasing reliance on GPS as the NAS moves to reduce the number of ground-based navigation facilities (mostly VORs). As well, many aircraft equipped are now equipped with GPS-dependent ADS-B Out avionics, which report position information to air traffic control and traffic information to other aircraft. “We need a resilient NAS,” she said.

The advice from the News Hour panelists for pilots experiencing a GPS outage is to let air traffic controllers know. “It’s important to do that,” said McClay. “Perhaps there is a concern with pilots not reporting it because they’re not aware what’s happening. There may be a concern because they’re not sure where this report goes.”

However, during a briefing with the FAA, the agency explained that a report to a controller goes to the facility supervisor, then to FAA headquarters and the DoD. “DoD can review these reports to help craft future events, hopefully, to avoid trouble,” McClay said.

There is also a way for the FAA to ask the DoD to stop the jamming in case of a safety issue. AOPA explained, “According to the Pilot/Controller Glossary, ‘stop buzzer’ is a term used by ATC to request suspension of ‘electronic attack activity.’ Pilots should only use the phrase when communicating with ATC or over the emergency frequency 121.5 MHz if a safety-of-flight issue is encountered during a known GPS interference event. Using this unique phrase when experiencing an unsafe condition related to GPS interference will ensure that ATC and the military react appropriately by stopping the jamming.”

Pilots can use the FAA’s GPS Anomaly Reporting Form to notify the agency of GPS issues. In any case, the FAA Aeronautical Information Manual recommends that pilots report any anomalies with navigation aids or global navigation satellite systems (generally GPS but there are other networks) per paragraph 1-1-13.

It should be noted the GPS jamming is a worldwide problem, not isolated to the U.S. Notams for GPS interference reference jamming potential in many regions, and in the case of international notams, information is provided for pilots to report jamming safety issues and incidents.

“We’re not sure how many instances [of jamming occur],” said McClay, "but the FAA does keep track of how many 'stop buzzer' events happen. It’s important for pilots, if appropriate, to call 'stop buzzer.' Pilots should be willing to speak up.”

While there hasn’t been much movement on the RTCA committee’s recommendations, the dialogue continues and the FAA wants to continue collaborating with the aviation industry, according to Williams.