- [GPWS] Terrain ahead. Pull up!
(radio chatter) Terrain ahead. Pull up! Terrain ahead. Pull up! All right, so what is it
that's actually going on here? Why are these pilots
getting a terrain warning while cruising at 37,000 feet? Well, I have talked about GPS jamming in some of my other videos,
but what you're seeing here is even more complicated than that, and it can sometimes even affect more aircraft systems
than just the GPS. Stay tuned. (playful chime) This is footage that was posted
on Instagram a few weeks ago by Bernardo Cantarino Féres, a pilot who kindly gave me permission to show and explain it to you guys. What it appears to show
is an Airbus A320 or A321 that isn't really sure where
or how high it is, and I'm sure you understand that this can be a
rather nasty situation to be in. So what's causing this, then? Well, I'm sure you
all know what GPS is, but to explain what you see here, we need to talk a
little bit more about how the GPS system actually works, because this is a system
that is both fascinating and has evolved way
beyond its original intended use. GPS stands for Global
Positioning System, and it's basically the first and still the most popular of many satellite navigation systems now available around the world. Collectively, all of
these systems are known as Global Navigation
Satellite Systems, or GNSS. Now, I will use the term GPS from now on, but what
I say can apply equally to all of these systems, although as far as I know,
commercial aircraft today mostly rely on the GPS system. So what are these GNSS systems then? Well, they rely on
a constellation of somewhere between 24 and 32 satellites each, and these satellites
all fly on precise orbits at six different orbital
planes around the earth. Their actual number varies,
as older satellites are retired and replaced by newer
ones, and there also tends to be a few spare ones out there, but the principle stays the same. They basically work as extremely
accurate timekeeping devices, with each of them
keeping an atomic clock on board and ground stations that verify their orbits continuously. Simply put, each satellite
then sends out a signal which says, this is
where I am at precisely, this time, plus some more information that helps the
receiver find more satellites. Now, by using this timing and positional information, user
devices like your phone, your smartwatch, your car,
or indeed an airliner can then use several of these satellites to triangulate its own position. To be more specific, what's happening is actually a little bit
more complicated than this. The satellite signal actually says, this is what my orbit is, and this is what time it
is where I am, and depending on how far that satellite
is from you, that signal will then arrive with
a specific time delay. So what your GPS device is actually doing is
comparing the time delays of multiple satellites with
the orbit of each one of them, converting them into distances from each satellite to
figure out where you are. Now, we can go even
a little bit nerdier than that if you want to involve
the theory of general relativity, but I guess you get the basic idea. Of course, in our everyday lives, we don't have to
think about any of this. Our phones and other devices may need a few seconds
to position themselves after combining all of
that information from more systems, but they generally sort
themselves out pretty quickly, and then you can find your way to that sushi restaurants
that you're looking for. Now, because we are using
it in so many parts of our normal lives today,
it's easy to forget that GPS actually started
out as a military system. And even though most GPS
devices today are in civilian hands, GPS itself still is a military system, which is now managed by
the United States Space Force. The original purpose of GPS
was to aid in the navigation of military aircraft and
to guide various missiles and other weapons that
could be pre-programmed with the coordinates of a target. The first studies on how a system like this could theoretically work and be used started way back in the 1950s and 60s, but
only on a theoretical base then. The first satellites were then launched in the late 1970s, but the GPS system wouldn't get fully operational
until much later than that. Now, initially at least, the plan was to keep GPS signals encoded in such a way that
only the military could use them. That plan might have
changed eventually anyway, but the event that
drove the US government to release it to the
public was the shoot-down of Korean Airlines
Flight 007 back in 1983. This was a 747
with 269 passengers and crew, which was shot down
by a Soviet fighter jet after it had strayed
into Soviet airspace due to a navigation error. GPS was still far from
ready at a global scale in 1983, but this tragedy convinced the United States to start offering GPS to anyone for free as a public service. The GPS signal was initially
degraded for public users, making it less accurate
but still good enough for most applications at
the time, but various companies soon started figuring out ways to get around this accuracy limitation. So in the year of 2000 that deliberate degradation
was finally taken away. Our GPS devices are now more or less as accurate as military ones, and in fact there
are still some great tricks that we can use to make these devices even more accurate, although that can be a double-edged
sword as I'll soon explain. The accuracy and availability
of GPSs has meant that we have found more and more ways to use it, and by the way, we don't just use
it for positioning purposes. Many devices also use GPS data to correct their own time, and this includes our aircraft clocks. That's why the first indication
of a failing GPS receiver or signal in our cockpit on the 737, at least,
tends to be a failed clock. But the thing is, since
GPS still remains a military system, its owners could decide to
switch it off at any time. Or it could also be configured so that the GPS
service may be restricted in certain parts of the world, where a military campaign
is ongoing, for example. That's why a lot
of other countries like Russia, China, India and even
the European Union have developed and launched their own systems. These systems have
slightly different orbits and layouts, but they all operate using the same principles, so today,
a lot of consumer devices can actually use more
than one system at the same time, making the operation more robust. Now, I've made a video
about what was happening to GPS shortly
after Russia had escalated its attack on Ukraine back in 2022. In that video I explained how GPS was frequently getting jammed in certain parts
of the Northern Europe, the Black Sea and even in
some areas in the Middle East. Since GPS and other
such systems are military in nature, it's not difficult to understand why
someone might not want them to work during a war or in other times
of geopolitical tension, but how does
jamming differ from spoofing, and how does this relate to that Instagram video that
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tragedy was the triggering event that made GPS available to the public, it's not surprising that the aviation industry has embraced GPS widely. Today, we use it for many things, but ultimately, to improve
navigation accuracy and to add another layer of safety. The complicated grid of
airways that we use today mostly aren't defined using
ground NAVAIDs anymore, but instead, with a few exceptions, they are defined using GPS coordinates. And closer to the airport, we have also lately
started adding the benefit of RNAV LPV approaches,
who are constructed to eventually replace instrument
landing systems, or ILS, especially in smaller airports and in poorer parts of the world. But the question
is now starting to become if that will even be possible. Well, the actual raw accuracy of GPS isn't as good as the accuracy we
get from an ILS system, so for now at least, to do a full category 3
approach with an autoland, we still need the
airport to have approved and tested ILS equipment as well as procedures and power redundancies for the runway that will be used. But that could potentially soon change. There are ways to improve or augment the GPS accuracy by combining it with some other system. This process is
called GNSS Augmentation, and we can do it either
with something known as Satellite-based Augmentation System, SBAS, or Ground-Based
Augmentation System, GBAS. And despite what
these different names suggest, SBAS and GBAS
work in very similar ways. Both of these systems
use ground reference stations, who continuously measure
the accuracy and deviations of the GPS signals in each location. Then, in the case of an SBAS, the Satellite-based Augmentation System transmits this corrected information up to dedicated satellites,
who then sends it back to the aircraft or the other
platforms who are receiving it. GBAS, or Ground-Based
Augmentation Systems, do require some ground equipment, as their name suggest,
and that's basically an array of antennas that measure
the same GPS accuracy and deviation as the
satellite-based system do. But in this case,
the ground-based equipment then transmits this
information directly out to the aircraft that needs it. This is often even
a bit more accurate than the Satellite-based Augmentation, so it's already in use for Category 1 and Category 2 approaches equipped runways and for aircraft that
can carry that required receivers. Eventually, full autoland CAT 3 approaches should become possible
with this system as well, but as far as I know,
that hasn't happened yet. Let me know if I'm wrong here, and whilst you're down in the comments, remember to like and subscribe. Now, some of you are
probably wondering what these augmentation systems have to do with GPS jamming and spoofing. Well, as I said before,
ground-based augmentation used to improve GPS accuracy can be a double-edged sword, and here's why: If we can use ground-based
stations to measure the accuracy of GPS signals, improve them, and then broadcast the correction, then what if we used similar equipment, but the other way around? Well, while you let
that sink in, let's talk about the difference between
GPS jamming and spoofing. As I explained in my previous video, GPS signals need to
travel really great distances, which means that they
tend to be quite weak when they actually arrive
to the gadget, which is going to use them. This means that it's
relatively easy for someone to transmit a competing signal, which effectively acts as white noise, blanketing out all of the GPS reception. There are several devices
used to do this deliberately, especially when, for example, a key politician are traveling around in a motorcade, for example. But there have also been people who have tried to use these types of devices for their
own purposes, for example, to try and hide their position
from their employers while driving company
cars with trackers. But as ideas go, this one is really bad and highly illegal, and because of that, it can also become very expensive. In 2013, a guy driving
a company pickup truck in New Jersey was fined $32,000
because he used one of these illegal devices to
hide him from his boss. As it turns out, he had chosen
to park within Newark Airport to do this, which
obviously wasn't ideal. Newark was one of the
first airports implementing a version of ground-based augmentation system, which this guy's trick
device ended up blocking, and unfortunately for him, the device transmissions also
made him very easy to track. Now, what this guy was doing
is an example of GPS jamming. It is basically a denial of service, and at a local
level it's relatively easy to do, again because of
how weak the GPS signals are. But GPS spoofing, on the other hand, is much more than just white noise and a bit more sophisticated. A spoofing signal
is basically something masquerading as a proper GPS signal, or a related signal
from an augmentation system, which still allows a GPS receiver to think that it's
receiving a proper signal. Now, aircraft generally have
more than one GPS source on board in case
one of our onboard systems would fail, but in this
case, obviously, since both GPSs will then be fed the same dodgy signal, they may well agree with each other, and not detect that
something fishy is going on. This is a much more advanced form of interference that
can be considerably harder to identify quickly, and this
is likely what was happening to that aircraft that we saw
in the beginning of the video. Identifying that these signals are indeed spoofing quickly
is really important, because the ways that
we have integrated GPS into other aircraft's functions has become much deeper and more complicated than
just the GPS navigation system. In theory, if the GPS stops working in our aircraft, we just switch over to another form of navigation, like, for example,
conventional navigation with VR and NDB beacons,
but typically our first alternative to GPS is our inertial
reference system, or IRS. Now, we have more
than one of these systems as well, and they align themselves before the flight while we're
still sitting at the gate. The IRSs use laser gyros,
and they are so sensitive that they can
sometimes even be difficult to align if it's
particularly windy outside when we're starting our day. These systems can sense every turn and every acceleration
that the aircraft makes in all three axes, and by
taking those accelerations into consideration, they can
then track our position all the way through the flight. But the problem is that the IRS
will progressively become less and less accurate over the course of a day, which is
why its data can be updated from other aircraft sources. These updates can come
from conventional VOR/DME NAVAIDs or they can come from the GPS. That is where the GPS spoofing has the potential to become a seriously big headache
compared to GPS jamming. You see, if we simply
lose the GPS due to jamming, for example, then the
IRS will just take over, working from the latest known update of the GPS-derived position. But if a fake GPS signal continues to come in for a while and we
don't immediately figure it out, well then our IRSs could get updated with corrupted positioning data, and then suddenly we have
neither GPS nor IRS to rely on. Now in practice, in many cases, the aircraft flight management
computers are clever enough to figure out that
there is something wrong here, because the shift between the good and the bad GPS data
won't make any sense, given the speed and altitude
of the aircraft, for example. This means that the
pilots should get a warning, or possibly multiple warnings, that something isn't right
when this is happening. And that brings us back to that video of the Airbus A320 cockpit that was cruising merrily
at 37,000 feet. Now, I am not an Airbus
pilot, but from what I've been able to find, the Airbus A320 family has three air data
inertial reference units, or ADIRUs. These units compare data from
redundant GPS and IRS sources, also comparing them with
the aircraft's air data sensors, that determines its
speed and its altitude. Now there are different failure modes to these systems that
I won't start to speculate on here but one result of
this GPS failure that we can see is a glaringly false
enhanced ground proximity warning, at an altitude where there
clearly can't be any obstacles. The EGPWS gets its information from a ground elevation database,
so if it doesn't have the right information about
where the aircraft is, it can give these
kind of faulty messages, which are designed to be
really hard to ignore, of course. Now in terms of navigation, the pilots here may be able
to just explore a few options, and if they all fail, they can switch to using conventional navigation with VOR/NDB, and
so that's not really a huge problem. But as you can see, the primary navigation
isn't their only problem, they're also having to deal with
a hugely distracting warning which will make the hair
stand up on most pilot's arms. Now, I saw a lot of
comments on this video on Instagram asking why the pilots didn't respond to the warning, shouldn't all EGPWS
warnings be taken seriously? Well, the answer to
that is obviously no. If you're clearly at
37,000 feet and nothing has changed except
the indicated GPS position, you have to apply your
common sense and not overreact. But the really interesting question is, what if this happened while the crew were in a descent, in cloud or at night, or whilst approaching an
airport near high terrain? At 37,000 feet, we don't really need to think about executing
a terrain escape maneuver, but that's because we
can be absolutely sure that there is no danger there. But during the
descent and around terrain, you basically have to assume that the information that you're getting is real and react to it. I mean, you might
have made an actual mistake. So, how do we deal
with these types of problems then? Well, the first thing we do is the same thing that
we do with any other risks. We brief it together
with possible contingencies. This particular crew was flying close to Jeddah in Saudi Arabia, probably on their way to Abu
Dhabi, and this is a part of the world where
there have been many reports about possible GPS jamming and
spoofing in event before. EASA, Europe's aviation regulator, regularly publishes and updates a list of areas where these type
of activities have been observed, and they also list the possible effects that this might
have on aircraft systems. These effects typically include: incoherence in navigational positions such as GNSS FMS position
disagree or terrain warnings, abnormal differences
between ground speed and true airspeed, time shifts
or problems with INS or IRS. Now, other aircraft
facing similar problems in that part of the world
recently nearly entered the airspace of Iran because of them, and others have reported that their GPS position has shifted as much as
60 nautical miles at times. Now, these problems can, by the way, affect multiple aircraft types. Obviously, this video shows an
Airbus A320 or an A321, but other pilots have reported issues with Boeing 737s, 747s and 777s, Embraer E-series jets
and various business jets. Basically, any aircraft
equipped with an integrated GPS will be potentially
susceptible to this. Unfortunately, there
are multiple hotspots in the world today, and this problem
is constantly getting worse. Crews who are forced to fly in these areas
needs to be well aware of all possible risks,
how their aircraft might respond, and also have contingency plans for how to deal with
these problems if they should occur. So, in the longer term, we will need to think about
how future aircraft systems will deal with
these problems more effectively. Now, near-airports, GPS-based
RNAV approaches are cheap and easy for
many airports to implement, but if we need
to have ground equipment anyway for those ground-based
GPS augmentation systems and these systems are so easy to block or spoof, then maybe we should be looking at some kind of alternatives. Before RNAV approaches became the default alternative to ILS, the FAA and NASA actually
spent a lot of time and money on
developing another alternative, something known as
a microwave landing system. This system would still use
fixed airport equipment, but it was a lot smaller and much simpler than the glide slope and localizer antennas that the ILS needs, and
it also operated much further from other frequencies, making interference a lot less likely. Now, NASA actually used a version of this microwave landing system to land the space
shuttle, so maybe this would be a possible way forward if these GNSS problems
continue to become more widespread? In the end, the FAA
abandoned the MLS system in favor of GPS, but that was before these latest issues had started rising around spoofing. So, given the need for a
potential navigational alternative to GPS and other such
systems today, I wouldn't be surprised if the MLS or other systems would soon start to
emerge to fill this void. Now, of course, with the
slow rate of all developments in aviation, we probably
shouldn't hold our breath for this to happen, but it is a really interesting
thing to think about though. What do you think? I would love to hear your opinions and experience in the comments below. Now, if you really want to discuss this and basically anything
else aviation-related directly with me, then join my
inner circle of Patreons using the link below or somewhere
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and I'll see you next time. Bye-bye.
