What is GPS interference, and why should I care?

GPS is deeply embedded into our everyday lives, with applications ranging from maps on mobile phones and dating apps to navigation in aviation and shipping. Unfortunately, GPS is vulnerable to interference, which is happening as you read this around the world. But before we look at the types of interference, we need to understand the basics of how GPS works.

How GPS works

The Global Positioning System, as GPS is called by its full name, was developed by the U.S. Department of Defense in 1973. While it was initially intended primarily for military applications, it has found numerous applications in the civilian world. It is based on a constellation of satellites that broadcast a radio signal. Receivers pick up these signals to calculate their position. The basic principle is that the receiver measures the distance to each satellite. If you know that you are, say, 21,000km from a certain satellite, you know that you are on a sphere with that radius (each point on the sphere will have the same distance to the satellite at the center). If your receiver gets a signal from at least three satellites, it can estimate its own position by calculating the intersection of these spheres (in reality it is more complicated, and you need at least four satellites).

GPS position estimation.

GPS position estimate based on the distance measurements from the satellites.

By the way, GPS is not the only Global Navigation Satellite System (GNSS) constellation in operation today. There is also Galileo (EU), GLONASS (Russia), and BeiDou (China), and they are based on the same principle. The interference described below applies to some extent to all GNSS constellations, not just GPS (even though notable differences exist between them).

How can GPS be interfered with

That GPS is vulnerable to attacks has been known since the system's inception, and one of the reasons is the low signal power. GPS satellites orbit Earth at about 20,200km and send signals with a power of between 50 and 240W (significantly less than your microwave oven with between 500 and 1700W and more in the order of a couple of light bulbs). As a consequence, the signal received is very, very low. This opens the door for two ways to interfere with GPS (i.e., two ways to hack GPS): Jamming and spoofing.

 

GPS Jamming

Since the GPS signal is so weak, it is relatively simple to overwhelm a receiver with a much stronger signal. If you have a malicious actor (i.e., a bad guy) that wants to disrupt GPS reception on the ground, they can send some signals on the same frequency band as GPS into the surroundings. A receiver in the area will not be able to make out the faint GPS signal anymore and will lose its ability to calculate a position. A colleague of mine compared this to the situation where somebody is whispering into one of your ears while a baby screams into the other.

GPS jamming has been around for decades. By now, you can even buy GPS jammers on the internet for under $20 (don’t, it is illegal to operate these, and you can cause a lot of damage with them). These devices are usually low power and affect only receivers that are fairly close. Apparently, they are sometimes used by taxi or delivery drivers who don’t want their bosses to track their cars' position. By running a low-power jammer in their car, their vehicle's GPS tracker can’t calculate its position, and they can, thus, not be tracked.

GPS jammer to plug into your car charger, found here.

Of course, more powerful jammers are on the market for armed forces. These jammers can have much higher power and are more sophisticated. As a consequence, they also can have a much larger radius where GPS receivers can’t get a signal. In this case, the range is usually limited by the “line of sight”. That means that a receiver needs to have an unobstructed line to the sender to get its signal. After a certain distance, the curvature of the earth interrupts this line of sight, and the jammer becomes invisible.

Starting from about autumn 2023, another way of interfering with GPS has significantly increased around the world.

 

GPS Spoofing

GPS spoofing is much more sophisticated compared to jamming. Here, a malicious actor doesn’t just overpower the receiver but instead sends it fake signals. This signal mimics the signals coming from the satellites. Since these signals are valid, the receiver happily calculates its position, assuming everything is fine. While in reality, it is being tricked into believing it is somewhere else than it actually is.

Russian Krasukha mobile electronic warfare system. More on this here.

As you can imagine, such an attack is more complicated and is mostly done by armed forces. Unfortunately, they can also be more dangerous, as I explain next.


Why should I care about GPS interference?

The most obvious consequence is from GPS jamming. In that case, your GPS receiver, for example, in your cell phone, can’t calculate its position anymore. If you happen to be relying on GPS because you’re navigating an unknown city with Google Maps or the like, you might get lost and struggle to find your destination. This is a nuisance but usually not dangerous. If you rely on GPS for navigating a giant oil tanker through a narrow strait, it suddenly becomes more serious (keep in mind, though, that ships have other means of navigation as well). In shipping, GPS is not only used for navigation but also for collision avoidance. If GPS doesn’t work anymore, so does the collision avoidance system.

GPS spoofing has different effects in real life. For example, dating apps such as Tinder got confused by spoofing and matched Lebanon’s citizens with Israelis.  In Moscow, a journalist was charged extra on a ride with Uber because the driver’s GPS  device thought it traveled from somewhere else.

On the more serious side, spoofing can cause all kinds of problems in shipping and aviation. In modern airliners, GPS is deeply embedded into various aircraft systems. These systems can act up and significantly increase the pilot workload. Aircraft have redundant means of navigation, but the additional workload and the degradation of some aircraft systems reduce the safety margins. Aircraft are particularly susceptible to spoofing since they fly at altitude and have a larger line of sight. If an aircraft is flying at an altitude of 11km, it has a line of sight of about 430km (technically, it’s not the line of sight, but the radio horizon). Meaning if a powerful spoofer is within that range, they will be spoofed.

In many cases, knowing that you might be spoofed can make a big difference. If pilots know that spoofing can occur along their route, they can prepare themselves mentally and turn off specific systems. This reduces the workload and false warnings that the aircraft might generate otherwise.

What is happening these days

Of course, I encourage you to check out what’s happening these days. Just go to spoofing.skai-data-services.com.

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December in Disruption: GPS Jamming and Spoofing in Motion