GPS is not as simple and safe a technology as you might think, and there are two ways in which malevolent minds can disrupt its operation to such an extent that flight safety can be put at risk.
A revolutionary military technology when it was first introduced, GPS has now invaded our daily lives via our mobile phones. A common, even anodyne technology. But that’s not the case: it presents vulnerabilities which, when exploited, can confuse pilots or even endanger a flight. And it’s much more widespread than you might think.
In this article, we’ll try to explain in as simple a way as possible how GPS works, how it can be disrupted to the point of giving pilots totally erroneous information, and when it’s most likely that your flight will be faced with such a situation.
- A brief history of GPS
- How does GPS work?
- GPS jamming or GPS receiver saturation
- GPS Spoofing or GPS hacking
- Which systems are affected by GPS Spoofing?
- Who jams GPS signals and why?
- Where are the risks of spamming and spoofing greatest?
- How to avoid GPS jamming and spoofing?
- Bottom line
A brief history of GPS
Before explaining how a GPS works, we need to look at its history.
GPS (for Global Positioning System) is a system that lets you know your exact location using data transmitted by satellites. Its deployment began in 1978, at the initiative of the US Department of Defense, to guide fighter planes and missiles, and global coverage was completed in 1995.
The technology was originally military, but was made available to civilians in the mid-2000s. It was the tragedy of Korean Airlines flight 007 that prompted then President Ronald Reagan to envisage this technology as essential for civilians once fully operational, and it was President Clinton who validated its access (sometimes in degraded mode) to non-military people.
Following a location error, this flight inadvertently entered Soviet airspace and, considering it to be a spy plane, the USSR air force shot it down. It was in response to this incident that the Americans decided they had to “donate” GPS to the civilian world.
How does GPS work?
GPS works on the principle of trilateration of signals emitted by satellites. Today, there are 31 active satellites orbiting the earth (plus a few spares ones), sending signals to receivers that can be anything from an airplane to your telephone. As the position of each satellite is known with a high degree of accuracy, as is its wave speed, the receiver calculates the distance separating it from each satellite and, by cross-checking, deduces its own position.
In concrete terms, then, we’re talking about a multi-billion dollar network of satellites that can be used not only to help locate an aircraft, but also to help you find your way to a restaurant for dinner, or to hide and search for objects in the wild if you’re a fan of geocaching.
That said, GPS isn’t always perfectly precise, or rather, certain factors may mean that the data transmitted by the satellites isn’t totally accurate, or the signal isn’t strong enough given the distance separating the satellite from the receiver. They’re good enough for you to find the pizzeria where you’ve decided to dine tonight, but they don’t have the level of precision needed to locate an aircraft very accurately, or even allow it to make an automatic landing without pilot intervention, requiring an accuracy of the order of a meter.
To improve signal quality, GPS is combined with other systems called “augmentation systems”. Based on ground stations (Ground Based Augmentation System or GBAS) or using satellites (Satellite Based Augmentation System or SBAS), they capture satellite signals, correct the various biases, and retransmit them to receivers, increasing their strength.
Indeed, GPS data can be biased by terrain, atmospheric composition, orbital shifts or a delay in the satellite’s atomic clock. These stations measure and correct wave biases and deviations at a given location, and communicate the corrected data to receivers.
So far, so good.
Until someone decides to use such technology not to improve the data, but to distort it on purpose. This is what happens in two cases: GPS jamming and GPS spoofing.
GPS jamming or GPS receiver saturation
It is technically possible for a person with the right equipment and located closer to the receiver to send signals using the same frequencies, which will be perceived as stronger by the receiver.
The result is that the aircraft’s GPS can no longer distinguish between right and wrong signals, and simply stops working. The receiver is somehow saturated with signals.
Is it serious? Yes and no.
Yes, because losing the ability to locate is problematic, as you might guess.
No, because it’s easy to detect and there are several ways to remedy it.
Easy to detect, because a pilot will quickly notice that his GPS is no longer working, simply because his clock is no longer giving any information (the aircraft’s on-board systems use GPS to get the exact time where they are) or because the position is no longer being updated.
Then there are ways to remedy the situation. Without going into too much detail, the aircraft are equipped with an inertial reference system (IRS) which captures all the aircraft’s movements and accelerations in all dimensions throughout the flight, and can therefore give its exact position. The pilot can also navigate in a conventional manner…just like before GPS.
Bear in mind that GPS is not essential for navigation, and that an aircraft can fly completely safely without it. Pilots just need to be aware that GPS no longer works, so they can adopt the right way of navigating.
We’ll see later that GPS jamming isn’t always malicious, and can be completely accidental. But there’s worse: GPS Spoofing.
GPS Spoofing or GPS hacking
GPS Spoofing doesn’t cause the GPS, saturated with signals, to stop working, but it does give incorrect information to the GPS.
It pretends to be a real station and transmits false information to the receivers. Unlike jamming, the GPS continues to function normally but gives inaccurate information, which is more complicated to detect and potentially much more serious.
This can lead the pilot to believe he’s in a totally different location from where he is, or too close to the terrain, with the consequences you can imagine.
Fortunately, this too is detectable. On-board systems have safety features that will notice a sudden change in position or inconsistencies between what the GPS says and what other sensors measuring speed and altitude say, and of course the IRS mentioned above.
But even when warned, the pilot has to identify the problem: faced with two contradictory pieces of information, he has to find out which one is right and which one is wrong, and even corrupted in this case.
Then, of course, he’ll notice a sudden change in position. In daylight, he may notice that he’s not flying over the area he’s supposed to be flying over, or that there’s a mountain in front of him that he can’t see on his radar. But at night or in low-visibility conditions?
Let’s take the example of a plane whose GPS tells it that it’s not where it really is, and that its faux position puts a mountain on its trajectory. Totally possible, since the Grounds Proximity Warning System (GPWS) doesn’t use the aircraft’s altitude sensors, but a global terrain database loaded into its systems. It compares the GPS position with the database and then constructs a photograph of the terrain that appears on the instruments. If the plane flies over a flat area and the GPS makes it look like it’s over the Himalayas, heading straight for Everest at 900 km/h, the pilot will be alerted to increase altitude.
That’s what happened to this Airbus A320, which received a GPWS alert at 37,000 feet (over 11,000 meters).
Faced with this contradictory information, the pilot does what he has to do, i.e. analyze the situation and use common sense. And then common sense tells him that his altitude indicator is working properly and that he must have a GPS problem, especially given the area overflown.
But he could have had a doubt, and the decision time could have been fatal if the alert was right.
Had he obeyed the instructions, he could have found himself in a stall situation, as maximum pitch and increased throttle are not a good idea at this altitude. At this point, he would of course have received a stall alert and, at the same time, a GPWS pull-up instruction. Two alerts requiring completely opposite responses.
What if, on the other hand, the GPS had told him he was flying in an open area when he had a mountain in front of him? You see the problem.
In another case, he might have believed that, like the pilot of Korean Airlines 007, he was in a prohibited area, changed direction to get out of it and, in so doing, actually found himself in a prohibited area or a conflict zone.
It’s easy to imagine the confusion in the cockpit with the multiplication of contradictory information and alerts, all the more so with limited visibility and an inexperienced pilot.
One last thing to know about spoofing. If the IRS is an alternative in the event of corrupted GPS data, you should be aware that it updates itself regularly during the flight, based on data…from the GPS. So there’s a risk that if the pilot delays switching to IRS, it may have updated itself in the meantime, adopting the erroneous GPS data.
The problem lies in the fact that, in addition to location, GPS data is used by multiple systems on the aircraft.
Which systems are affected by GPS Spoofing?
To get an idea of the situation, we asked an A380 captain. We wanted to find out whether GPS spoofing was a myth or a common occurrence, and then what the consequences would be in the cockpit.
The answer is clear.
“Most of the time we encounter jamming problems (in Iraq, Turkey, Bulgaria, Romania, Saudi Arabia, Egypt, etc.)”.
These are indeed the areas where it’s most common, and we’ll see why later.
“Sometimes spoofing […] The problem with spoofing is that everything looks right on the instruments, but our position in reality is completely different. Sometimes the jamming is so heavy that we lose other systems based on the GPS signal, such as GPWS, predictive windshear, BTV (Editor’s note: a system that optimizes approach and braking on the runway specific to Airbus), the airport navigation system, etc. It can cause spurious terrain warnings, loosing TCAS (Editor’s note: in-flight collision avoidance system) and weather radar system, etc“
Well, that settles it.
For the sake of completeness, EASA (the European agency in charge of air transport) has issued a note on the subject in which it identifies, in a non-exhaustive way, the consequences known to date.
Temporary or non-recoverable failure or degradation of PNT information provided by GNSS possibly resulting in:
- Inconsistent flight guidance possibly resulting in route deviations, uncommanded turns, and potential airspace infringements;
- Loss or misleading surveillance system (e.g. corrupted Automatic Dependent Surveillance- Broadcast (ADS-B), TAWS (e.g., false PULL UP alert triggered by TAWS during cruising phase), wind shear, terrain and other surface functionalities);
- Loss or misleading time dependent systems (e.g. clock, fuel computation system, flight management system);
- Inconsistent, potentially misleading aircraft position, and ground or wind speed on the navigation display.
- Inability to use GNSS for navigation, including waypoint navigation;
- Inability to conduct or maintain GNSS based Area Navigation (RNAV) and/or required Navigation Performance (RNP) operations.
Who jams GPS signals and why?
But who would want to jam GPS signals, and why? We’ll make a distinction here between accidental and deliberate jamming.
Commençons par les involontaires. Yes, you can jam GPS signals completely unintentionally and cause panic around an airport. Or rather, it can be deliberately jammed, but without any intention or even awareness of impacting aircraft.
Let’s start with a well-known case: when, for one reason or another, one wants to jam various signals in a given area to prevent certain communications and reduce certain risks. This is what the security services do when high-profile figures such as heads of state travel. It’s a safe bet that your phone’s GPS won’t work very well near the motorcade of the President of the United States.
In the same vein, we know that jamming devices will logically be ready to intervene during the Paris Olympic Games this summer. And it’s perhaps even to avoid any damaging collateral effects that during the opening ceremony the Paris airports will be closed and traffic banned within a 150 km radius of the city, and not just to have clear skies that are easy to control.
And since the devices to do this can be purchased without too much trouble, we also have people who don’t want to be tracked. Drivers, for example, want the system to think they’ve arrived, so they can start billing before they’ve even loaded the customer. That’s not so critical, unless they’re doing it near an airport.
So do delivery drivers who don’t want their employers to track their journeys or realize that they’re taking more breaks than planned. For example, the truck driver who unintentionally caused panic at Newark airport and was fined $31,875.
And then there are cases where the effect of jamming is totally voluntary, as in conflict zones. The aim, of course, is to fool drones, missile guidance systems and enemy aircraft… and this in turn affects civil aviation. And we can’t rule out the possibility of ill-intentioned people acting in this way to cause an accident.
Where are the risks of spamming and spoofing greatest?
The above-mentioned EASA note also provides regular updates on the main areas concerned, even if of course this can happen anywhere in the world.
Unsurprisingly, these include current areas of conflict such as the borders of Russia and Ukraine, as well as the Middle East and, further north in Europe, territories over which Russia also has designs.
How to avoid GPS jamming and spoofing?
The situation is problematic, and according to the experts, it’s not going to get any better. The question is whether there is any way of combating these practices to put an end to what is a potential threat to flight safety.
Bad news: the answer is no.
Today, there is no way for airplanes to thwart GPS signal jamming techniques. As we’ve said, there are ways of identifying them, but not of blocking them. Once the problem has been identified, the pilot has alternative navigation methods, but it’s impossible to prevent the problem from occurring. And if he doesn’t identify the problem then…the situation can become critical.
The only possible future solution is to replace GPS with a new technology, which does not yet exist. Authorities have worked in the past on alternative systems to ILS for approach management, but these have been abandoned. It may be possible to reactivate the researches, but in any case it will be a very long time before a new system is deployed. And this only applies to approaches, since there is no other alternative today for navigation between airports.
Bottom line
Jamming or spoofing of aircraft GPS is a reality that aircraft encounter mainly around areas of conflict, but sometimes even elsewhere, when a jamming device deployed for other purposes accidentally disrupts their systems.
Fortunately, the problem is theoretically easy for pilots to identify, and certain on-board systems can alert them. In this case, they can use alternative navigation methods to dispense with the GPS, which is certainly useful but not vital.
In any case, for a long time to come, the only way to combat GPS jamming and GPS spoofing will be for onboard systems to check the validity of data, and for pilots to use their common sense, as there is currently no way of blocking such interference before it affects navigation systems, or any alternative to GPS.
Image : Plane cockpit by VladyslaV Travel photo via Shutterstock
