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
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 https://read.nxtbook.com/ieee/spectrum/spectrum_na_february_2021/lost_in_airspace.html. I also referred to Wikipedia articles on GPS