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GPS Disruption is a Growing Problem for Aviation, Reports Show
Incident reports filed by aviation professionals to the online Aviation Safety Reporting System suggest GPS interference is getting worse.
Two Airplanes Taxiing Next To a Control Tower

It’s a site that fans of unexplained mysteries like to trawl for reports of UFO sightings. But for those of us who work in GPS cybersecurity, the Aviation Safety Reporting System (ASRS), an online database of incidents experienced by aviation professionals, has a very different story to tell.

And right now, it’s telling us that GPS interference is a big and growing problem for commercial and private aviation in the United States and elsewhere.

GPS-related incidents are increasing

My job at Spirent is to study threats to GPS and other global navigation satellite systems (GNSS). By understanding real-world conditions that affect GPS reception, we can create test scenarios that help manufacturers and end-users understand how GPS-dependent systems cope with interference.

When I started looking through the NASA-run database, I was expecting to come across a handful of incidents that we might be able to turn into test cases. I was entirely unprepared for the sheer number of GPS-related incidents that I found - and very intrigued to note that they have generally been increasing year on year.
  • In 2013, pilots reported 11 incidents of GPS interference or malfunction
  • In 2014, the number jumped to 24
  • In 2015, it was 28
  • In the first half of 2016, 13 incidents had been reported

That’s 77 occurrences of GPS failure in three and a half years - each one of a magnitude that made the professionals involved feel it was worth reporting to a voluntary, anonymous database.

Most incidents are unexplained

So what kind of incidents are we talking about, and what impact are they having on modern commercial and private aviation?

The vast majority of incidents involve GPS-based navigation systems either experiencing a total loss of signal or - more alarmingly - misreporting the aircraft's position. In around 50 cases, no immediate explanation is given for the malfunction. Many reports simply say the signal was lost for a time and then returned - with flight crew either falling back on other navigation techniques or asking air traffic control for guidance.

Here’s a Cessna pilot reporting an unexplained incident in 2014, for example: "While established on the localizer for the approach to Runway 22L in Boston, the GPS signal was lost numerous times (more than 3) from 20 miles out from the runway to 5 miles out. Each time the GPS failure message remained for approximately 10-15 seconds before signal returned. The weather was VFR and navigation was not affected."

(You can download a PDF guide to the abbreviations used in the incident reports here.)

Military jamming exercises cause significant disruption

But in the remaining reports, we start to get some good insight into what's causing the disruption to navigation systems. A good 20 of them, for example, specifically cite nearby GPS jamming exercises conducted by the US military.

These tests are conducted in open-air ranges over a wide geographical area. They're usually designed either to test how military equipment copes with signal jamming, or to train military personnel in hostile environments where navigation signals may be being jammed by an enemy.

Such testing is vital to the effective operation of the US military, but it can have a major impact on private and commercial aviation in or near the same airspace - even though they are usually notified in advance that the jamming is going to occur.

Consider this exasperated report, filed by the pilot of a private jet who had received an advisory Notice to Airmen (NOTAM), requesting him/her to avoid a wide swathe of air space in the southwestern United States during a military jamming exercise earlier this year:

"The recent notice - NAFB 16-03 flight advisory GPS interference testing illustrates a continuous disregard for safety of our nation's airspace navigation system. The coverage area of this test effectively grounds all GPS-equipped aircraft in the Southwest flying IFR since they may experience unknown signal loss.

[…] At the time that ground based Navaids are being decommissioned and our government is promoting GPS based systems as primary navigation sources, the repeated interruption of GPS signals by our military [is] threatening the safety of our aviation system."

Not all of the pilots reporting GPS signal loss use such strong words, but judging from the reports in the database, the disruption caused by military testing appears to be widespread. And we know that it doesn't just affect aviation: in 2015, agricultural businesses in Idaho were knocked out of operation for several hours by a military exercise they hadn't been made aware of.

Disruption from personal privacy devices

The ASRS also highlights another growing source of interference to commercial and private aviation. One of the more colourful reports filed to the database involves the tracing of an intermittent interference source to a jammer in a truck in a car park near North East Philadelphia Airport.

"Last month I was contacted by a chief pilot, stating they were losing GPS satellite coverage on the RNAV (GPS) RWY 33 approach within the last mile of the approach. This would happen intermittently with different aircraft, and different avionics.

[…] A Federal Communications Commission Enforcement Bureau agent located a GPS jamming unit in a truck located within one mile from the approach end of Runway 33. The truck was in a parking lot, henceforth the intermittent interference; when the truck left the area, the GPS approach was normal. Agent confiscated the jamming unit and destroyed it with a sledge hammer. The driver had no idea he was using a device that was illegal. He was using the jammer to disable a tracking device that was placed in his vehicle by a vendor' to hide his location."

it's well documented that this kind of device can disrupt an airport's ground-based GPS-guided landing systems. The most famous incident occurred at Newark Airport in 2012, where a truck driver with a personal privacy device (PPD) installed in his cab caused "harmful disruption" to a GPS-based landing system at the airport as he passed by on the I-95 New Jersey Turnpike.

While the Philadelphia incident is the only one that's specifically attributed to a PPD jammer, the ASRS does contain several reports of signal disruptions experienced by pilots landing at Mexico City airport (code MMMX), which may suggest the intermittent presence of a jammer nearby:

Example 1: "While conducting the RNAV (GPS) OR ILS Rwy 05R approach to MMMX in VMC conditions, I noticed as we flew over MAVEK (SMO VOR) intersection the FMC alerted "GPS INVALID L, R". This occurred perhaps one mile from MAVEK. As we were in VMC we continued a stable approach to an uneventful landing. I strongly suspect GPS jamming is once again occurring in the vicinity of the Mateo VOR."

Example 2: "I have been going to Mexico City a handful of times lately and every time we shoot the ILS to 5R the GPS has had a RNP fail. This occurs before SMO to just after. The FMS for some reason does not keep a satisfactory RNP. Just seems like a safety concern since it is not getting better."

Example 3: "Taking an air carrier with GPS into MMMX- weather lousy, we were a few minutes from diverting in the hold north of SLM. We knew about the GPS problems after SMO for the ILS 05R […] The GPS went into DR about 8 DME from SMO, well before the other time I had landed on this runway. […] After landing, the GPS returned to normal during taxi in."

Understand the risks and mitigate accordingly

All of the reports show that flight crew and air traffic control are collectively able to implement workarounds for when GPS-based systems fail. But as modern flight management systems and ground-based landing systems come to rely more heavily on accurate, continuous GPS reception, it's clear that signal interference can be stressful for flight crew, costly for commercial operators, and disruptive for airport operations.

That makes it essential for manufacturers, commercial airlines, private operators, airport authorities and other ground-based GPS-reliant businesses to understand the risks (whether financial or safety-related) and implement appropriate mitigation strategies. Understanding the risks could include:
  • Using real-world conditions like those described in the ASRS to create test cases for flight management systems. It may seem an impossible task to re-create such a huge variety of conditions, but modern simulators and testing tools make it easier.
  • Monitoring the RF signal environment around an airport to understand where interference is occurring and - potentially - what’s causing it. This can be done using one or more interference detector,, and can produce some extremely enlightening results.

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About Spirent Communications plc
Spirent Communications plc. (LSE: SPT), a global leader in test & measurement, inspires innovation within development labs, networks and IT organizations to enable customers to deploy communications networks, services & applications, and devices that enrich life. Our extensive portfolio of solutions test data centers, high speed Ethernet networks and services, 3G/4G wireless networks and devices, network security, and global navigation satellite systems. With solutions used by more than 1,700 customers in 30 countries, the world's largest equipment manufacturers, service providers, enterprise and governments rely daily on Spirent to help ensure their success. Based in Sunnyvale, CA, Spirent Communications has 1,300 employees worldwide.

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By: Guy Buesnel - 11/7/2016 8:16:54 AM
Tags: GPS interference, GNSS interference, GPS jamming, Robust PNT

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