Monday, March 01, 2021

Friendly Jammers: The U. S. Military's Threat to Commercial Aviation


The Global Positioning System (GPS) that allows a few dollars' worth of electronics to determine your position within a few feet almost anywhere in the world has proved to be a great boon to aviation, which previously relied on an expensive and not that reliable network of ground-based technology for electronic navigation.  Newer aircraft use GPS as an essential part of their autopilot system, for example.  A report in the February 2021 issue of IEEE Spectrum shows that this happy situation is frequently being disrupted by the U. S. military in test exercises that use GPS jammers and spoofers, especially in the western United States.  Despite a formal requirement to warn pilots that such tests may be occurring, these tests have caused numerous problems to pilotes and even near-accidents over the last decade or so. 


In fairness to the military, GPS is their baby.  Deployed initially with a secret feature that degraded its accuracy for non-military users, it was designed primarily for combat uses, and commercial uses began as a kind of afterthought.  In 2000, this "selective availability" option ended, and now anybody, military or commercial, can get the highest accuracy possible out of the system.


Understandably, pilots and airframe makers have begun to rely on GPS almost exclusively for routine navigation.  The bad old days of shooting the stars with a sextant ended when various radiolocation technologies such as VOR (VHF omnidirectional range) were deployed after World War II, but in the last few years the Federal Aviation Administration (FAA) has been decommissioning those stations in preference to GPS.  And GPS works fine, except when it doesn't.


Because GPS is now an essential part of military operations, the U. S. military is now coming up with ways to deprive the enemy of it.  By the time satellite-transmitted GPS signals reach the ground, they are weak enough that suitable ground-based transmitters can either simply overwhelm them with interference (jamming) or worse, imitate them to deceive any GPS receivers in the vicinity (spoofing).  By law, any such military tests have to be announced to the aviation community through Notices to Airmen (NOTAM).  But these notices tend to be very broad, blanketing multiple states for days at a time, and most notices do not result in pilots experiencing any interference.  The net effect is that when GPS jamming happens, it tends to take pilots by surprise, and you don't want to startle a pilot when he's doing his job, or even when she's doing her job.


One of the worst such incidents happened in May of 2020 as a commercial airliner came in for an approach to El Paso International Airport in West Texas.  It was early morning, still dark, and that airport is surrounded by high mountains that pose threats to airplanes that are not where they're supposed to be.  Suddenly the pilot lost his GPS position, and rather than attempt to land blind, he declared a missed approach, went around, and landed on a different runway guided only by air-traffic control.  In a report he wrote about the incident, he remarked dryly that the runway he used "has a high CFIT threat due to the climbing terrain in the local area."  CFIT stands for Controlled Flight Into Terrain.


Investigation by Spectrum reporter Mark Harris revealed that such incidents are much more common than previously believed.  In one six-month period in 2017, for example, 96 GPS disruptions occurred in commercial aviation.  This led the FAA to ask the nonprofit Radio Technical Commission for Aeronautics (RTCA) to investigate the problem.


The RTCA's report recommends several things, but it's not clear that many of them have been implemented.  Better reporting of GPS failures due to military interference was one of them, and now the FAA wants pilots to report any such incident, not just if it required the intervention of air traffic control.  But many other recommendations, such as the military making more specific NOTAMs that describe exactly when and where the interference will happen, have apparently not been acted upon.


According to Harris, the situation is only going to get worse, as GPS manipulation becomes a more important feature of war games in military reservations, and as GPS becomes increasingly relied on as the main navigational technology used by airlines. 


This kind of situation is very familiar in engineering ethics, and can be characterized as the passed-out canary in the coal mine.  As you may know, in the days before technology was available to detect methane that could lead to an explosion, coal miners carried along canaries, whose respiratory systems are more sensitive than those of humans.  If the canary showed signs of distress, it meant there was enough gas to be dangerous, and the miners took steps to avoid igniting the gas, such as stepping outside of that part of the mine.


The near-misses and other distressing but so far harmless incidents that military GPS jamming and spoofing have caused are the kind of warnings that a responsive, on-the-ball organization will seize upon for appropriate preventative action.  Nobody wants people to die because an avoidable accident wasn't avoided. 


It's a matter of judgment as to how much effort should be expended to avoid it, but a good measure of that effort is to monitor the frequency of near-misses to see if remedial actions are making them less frequent.  If they are, the changes are doing their job.  But according to Harris, GPS failures are only increasing.  And as the aviation industry relies more exclusively on GPS for takeoffs and landings, it is just a matter of time before something really serious happens.


This situation is made worse by the distributed nature of the responsibilities involved:  the U. S. military, the FAA, airline operators, airline manufacturers, and pilots.  With so much opportunity for finger-pointing, it's no surprise that not much substantive has been done.  It would be a tragedy if effective steps were taken to fix this problem only after somebody gets killed.  But sometimes it takes a tragedy to get people to do something.


Sources:  Mark Harris's article "Lost in Airspace" appeared on pp. 22-27 of the February 2021 issue of IEEE Spectrum.  Portions of it can be viewed at  I also referred to Wikipedia articles on GPS and VOR.

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