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Keeping NextGen on the airKeeping NextGen on the air

As GPS navigation grows, so does GPS interferenceAs GPS navigation grows, so does GPS interference

An AOPA-co-chaired task group that studied how to keep civil aircraft flights on course when military training and testing interfere with GPS services used for navigation has issued a wide-ranging set of recommendations to the FAA.

Satellite-based navigation is becoming the norm. iStock photo.

GPS interference has many causes, but the task group focused on GPS interference resulting from Department of Defense activity.  

GPS is rapidly becoming the dominant air-navigation technology under the FAA’s NextGen modernization program, and the pace of the advance is sure to accelerate as more aircraft take on Automatic Dependent Surveillance-Broadcast (ADS-B) Out systems before a mandated compliance date of Jan. 1, 2020.

By their nature, signals from GPS are fragile due to their very low power, so as the FAA modernizes the National Airspace System, it is essential to ensure that alternate navigation aids and capabilities are available if GPS becomes unavailable. 

National Security Presidential Directive 39 directs the Department of Transportation to, “… develop, acquire, operate, and maintain backup positioning, navigation, and timing capabilities that can support critical transportation, homeland security, and other critical civil and commercial infrastructure applications within the United States, in the event of a disruption of the Global Positioning System …” The same U.S. policy directs the Department of Defense to “train, equip, test, and exercise U.S. military forces and national security capabilities in operationally realistic conditions that include denial of the Global Positioning System.”  

When military exercises intentionally degrade GPS signals—necessary to simulate “operationally realistic conditions” in training—the impact on civilian aircraft has been mixed. Sometimes the impact of a GPS signal outage on an aircraft is nothing at all. Much depends, say experts, on the aircraft’s altitude, the terrain, and the navigation equipment on board. However, sometimes the impact is more significant. 

One incident from April 2016 has come to exemplify what can happen: It documented an Embraer Phenom 300 entering a Dutch roll and emergency descent after its yaw damper disengaged when the aircraft’s dual attitude and heading reference systems responded differently to the GPS signal outage.

Numerous aircraft have reported the loss of navigation signals in affected airspace; some have been observed to “disappear” from their ADS-B-plotted tracks in dead spots, to reappear in zones of better signal reception.

And because it is very difficult to predict how loss of GPS signals may affect an aircraft’s navigation and flight control systems, this is the dilemma: Is it better for an aircraft to reroute to avoid the frequently expansive swaths of airspace depicted graphically on GPS-testing notices to airmen? Can better methods be found to quantify the risks?

Degraded navigation, the loss of ADS-B, and the failure of GPS-dependent control systems aren’t the only impacts on civil aviation from what the FAA calls “intentional interference events.” There’s also economic risk for businesses like aerial surveying companies that fly GPS-based grids, and who may be unable to operate during an interference event.

The uncertainties also highlight the need for the FAA to keep NextGen’s VOR-based backup system in good working order, urged the panel that studied the problem.

Quantifying the problem

Its report, Operational Impacts of Intentional GPS Interference, is a must-read for aircraft operators who want to understand the knowns, the unknowns, and the future planning designed to make interference events—which tripled in number from 2012 to 2017—non-issues for civilian aircraft, said Rune Duke, AOPA senior director of airspace, air traffic, and security.

Duke co-chaired the Radio Technical Commission for Aviation’s GPS Interference Task Group with Capt. Wes Googe, technical pilot and manager of airspace optimization for American Airlines. Robert Sweet, a senior manager in the Air Traffic Organization, served as the coordinator for input by the FAA’s technical operations and technical experts.

The task group came into being in May 2017 after the FAA requested that RTCA, which studies technical questions for government agencies, recommend ways to accommodate aviation’s “increased reliance” on GPS simultaneously with the Department of Defense’s increased need to hold exercises that include GPS interference for national security and defense training.

The FAA requested that the task group focus on quantifying the impact of interference events on the National Airspace System and recommend follow-up actions ranging from better depictions of the events, based on their likely interference profile, to analyzing how pilots are alerted to the activity, to creating new training materials for pilots and air traffic controllers.

In its letter to RTCA, the FAA cited an AOPA survey that found that more than a third of general aviation pilots had experienced a GPS outage, and more than 60 percent “were concerned about the impact” of intentional GPS interference.

“The task group started meeting in August and delivered the recommendation report to RTCA’s tactical operations committee at its March 1 meeting,” Duke said. “The committee approved the report and submitted it to the FAA for their action. The next steps are for the FAA to review the recommendations and report back on their concurrence and, if applicable, an implementation plan.”

Sweet added that the FAA “is working to carefully balance the reliance of air traffic on GPS with our military’s need to conduct GPS testing and training activities. This task group’s recommendations will help bring key challenges and potential solutions into better focus.”

In 25 recommendations, the report pinpoints improvement opportunities, and it calls out the FAA where it believes the agency has failed to make existing processes work. 

According to one recommendation for better online information distribution, “The preflight resources available online for pilots are fragmented and obscure. The FAA has failed to maintain several of these websites, yet they were still publicly available until recently.”

In some instances, it said, information is complete but hard to find.

“The Flight Advisory notices are an important resource for pilots but they are housed on an obscure website and can provide misleading information,” notes the next recommendation. “The FAA should continue publishing and emailing the Flight Advisory notices as they do provide valuable information to users; however, where they are hosted today has limited visibility for a pilot preparing for a flight. It is important the FAA relocate and integrate these notices with the NOTAM on NOTAM Search, which is the default location for NOTAM related information.”

Anomaly reporting

The recommendations also flagged conflicting FAA guidance to pilots on reporting GPS anomalies. “Paragraph E in each Flight Advisory states ‘pilots are encouraged to report anomalies only when ATC assistance is required.’ This guidance is repeated in Aeronautical Information Manual (AIM) paragraph 1-1-13, but this guidance is counter to FAR 91.187. Pilots operating under IFR are required at all times to ‘report as soon as practical to ATC any malfunctions of navigational, approach, or communication equipment occurring in flight.’ It is important paragraph E in the Flight Advisory and AIM paragraph 1-1-13 are modified to be consistent with the regulatory obligation of all pilots.”

The report recommended new language stating, “Pilots experiencing an anomaly should advise appropriate ATC facility and report online using FAA GPS Anomaly Reporting Form.”

Duke strongly advocates for pilots to use the online GPS anomaly reporting form, noting that reports conveyed by radio to ATC may not go much further than the controlling facility. By contrast, anomalies reported online become the subject of investigations, sometimes including the investigator calling back the pilot who reports an event to request more information.

ADS-B permits some anomalies to be well documented—which could help improve future GPS-testing notams. “With ADS-B flight track data, we can see the impact of the interference with images of aircraft flying into interference areas and disappearing. Leveraging the ADS-B data can help us come up with better models of where the interference is and how to inform pilots,” he said.

In some cases where problem areas were identified in the report, Duke said, the FAA and Department of Defense are already on the case, such as examining possible ways to reduce the graphical impact areas described in GPS testing notams from radii of hundreds of miles, especially in airspace where the only expected impact on GPS reception is near the notam area’s center.

Also, the FAA is developing new pilot guidance, including new language to be included in the Aeronautical Information Manual. The FAA is also considering a new Advisory Circular “that will detail the long list of mitigations the FAA has in place for these events,” Duke added.

With safety of civilian flights in mind, a key operational precaution already in place between the Department of Defense and the FAA requires a military interference event being stopped if the weather is less than a 5,000-foot ceiling and/or five miles visibility at a GPS-only airport when an aircraft needs access to that airport.

“This is an important mitigation that will become more robust and enforced based on this group’s work,” he said.

Panel co-chair Googe noted that the airline industry considers the Department of Defense’s efforts with GPS interference “critical to the security of the country yet also a complicating factor in the conduction of GPS dependent NextGen programs in the NAS going forward. The RTCA-sponsored intentional GPS interference work group provided the aviation community and FAA a forum to fully understanding the impact of these events, effectiveness of the associated NOTAM service, and the education process for all concerned to operate safely during these events.”

Moving forward, “It is imperative that the Alternative Position Navigation Timing document (APNT) describe a collaborative roadmap that defines the level of Required Navigation Performance (RNP) and associated equipage expected in order to meet the current and future needs of all operators in the National Airspace System during any GPS interference event,” he said, referring to the FAA project that investigates alternatives for high-precision backup for GPS other than the legacy navigation systems currently designated to provide coverage in an outage.

The report also notes that, although the task group focused on intentional and planned GPS interference events, other factors including “solar weather, illegal personal GPS jammers, unlicensed GPS repeaters or spoofing” must be addressed by the FAA as well.

Topics: Advocacy, NextGen, Avionics

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