How does GPS jamming work?

For the majority of us who use a GPS – Global Positioning System – to guide ourselves on a daily basis or on the road to vacation, what a disaster when it doesn’t work! GPS is also widely used in professional and scientific settings, in the transport sector, but also in civil engineering, for geolocated services, topography, geodesy (which is the study of the shape of the Earth)… forget the military applications which are at the very origin of the GPS system.

GPS is physically based on radio signals at very specific frequencies. If for one reason or another, these are difficult to access, geolocation is disturbed, or even impossible. We are going to be interested here in a particular reason: jamming, which can be voluntary or involuntary and is distinguished in particular from “decoy” (spoofer in English).

The jamming of the GPS leads to a “denial of service”, that is to say that one is unable to calculate one’s position. This can be critical because many sensitive applications use it: civil aviation, defence, civil protection in particular. Deliberate jamming can thus be part of a cyberattack, particularly in the context of armed conflicts as is the case in Ukraine for example.

To understand what it is, let’s start by explaining the principle of GPS.

How does a GPS work?

Commonly referred to as “GPS” is the function that provides “geolocation” information, which should be understood as the physical location of a terminal in the geographical sense of the term. Its goal: to obtain mathematical magnitudes, called coordinates, which allow an object to be placed in a system of representation such as a map. In addition to geolocation, GPS allows precise synchronization of the receiver clock to a few tens of nanoseconds of universal time. It is for example used to synchronize mobile networks or banking transactions or stock market transactions. In the latter case, we can timestamp the moment of the sale of a security and therefore refer to its price on the market to better than the nearest second.

We often refer to the object that serves as our GPS by the acronym of the system, GPS. Strictly speaking, we should speak of a “GNSS receiver”. The acronym GNSS, for Global Navigation Satellite Systems, designates all the systems allowing global geolocation: the American GPS, the European Galileo, the Russian GLONASS and the Chinese Beidou. So, if you are in Europe, you should say: “ I’m coming in 5 minutes, my Galileo tells me I’m near you “.

In practice, satellites send radio signals to Earth. These are received using an antenna integrated into the receiver (the smartphone for example) then processed in such a way as to allow the geographical coordinates of the antenna to be calculated. It is important to note that there is no return from the receiver to the satellite: the link is only downlink. GNSS are therefore not tracking systems, but position calculation systems.

This leads to important information for the subject that interests us: the disturbance of the GPS concerns the reception of the signal at the level of the terminal on Earth. We can jam all the receivers on Earth, the network of satellites will continue to send its signals as if nothing had happened.

GPS jamming by disrupting GNSS signals

The jamming of a system is an action, not necessarily voluntary, consisting in emitting a signal on the frequency band of the system which will be added to that of the system, and disturb its operation. Commonly, this additional signal is called “noise” which is added to the natural noise which corresponds to the sum of the signals of all the existing radio sources, coming from the Earth, the Sun, space and even the ‘human activity. To imagine what the noise in question is, the simplest analogy is that of current language.

Imagine that you are at a restaurant table having a conversation with a person in front of you. You understand and hear what your interlocutor is saying. You hear the other conversations but you don’t understand them: it’s natural noise, which is added to the ambient noises like those of traffic for example… A person at a nearby table starts talking very loudly, to the point that you no longer understand what the person in front of you is saying. You have been scrambled!

For GPS, the nature of the waves changes but the principle is the same: everything that is emitted on the frequency band and that does not come from the system’s satellites acts as interference.

Moreover, the signals which come from the satellites have the particularity of being very low in power (a few tenths of a femtowatt in reception). They are therefore very susceptible to interference. A jammer located several kilometers away emitting a few milliwatts, or the order of magnitude of the power of a wifi transmitter, is sufficient. To spin the previous comparison, imagine that your interlocutor has a barely audible little voice, the slightest noise around you will prevent you from understanding him.

For further

Voluntary and involuntary interference

Unintentional interference is often linked to a faulty device. It can be a telecommunication device, such as a wifi router or a relay antenna, which transmits outside its operating frequency in a GNSS band. This may also concern electrical devices whose electromagnetic compatibility (EMC) is poor because they have been damaged or because they do not comply with the prescribed manufacturing or use standards, for example mobile telephones and microphone ovens -waves. For all these cases, one would say in everyday language that they are “poorly isolated”.

Deliberate jamming can amount to a cyberattack. It can be encountered in the context of electronic warfare, in the context of a conflict such as in Ukraine where mutual jamming is proven. The objective is then to disturb the tactical organization of the enemy by depriving him of information on the positions of his deployment or to counter his guided weapons.

But it is also found in civilian applications. The most common is a small device that can be plugged into a car cigarette lighter that interferes with the reception of all surrounding receivers, for example those in airplanes. This use is found among employees of transport companies who wish to defeat the tracking system with which their vehicle is equipped so that their employer cannot follow their movements.

How to protect your GPS from jamming?

There are few simple methods to protect against jamming. Military receivers are equipped with certain techniques that increase the power needed to jam them.

For certain applications having configurations (antennas, available energy source) allowing it, such as vehicles or planes, in addition to specific processing applied to the signals, there are “intelligent” antennas which attenuate the signal in the direction of reception of the jammer, reducing its effectiveness.

That said, not all applications, whether military or civilian, are equipped with rugged receivers. Soldiers’ radios, for example, have conventional GNSS chips. Russian soldiers themselves use GPS despite its regular jamming in Ukraine. Basic receivers have even been found in the cockpits of combat aircraft. But that should not lead to believe that this situation is reserved for this country.

For consumer or industrial receivers, when the signal is lost in seemingly problem-free reception conditions, there is often no choice but to move away from the area and report it to the ‘ANFR, the National Frequencies Agency.

Alexandre Vervisch Picois, Lecturer, GNSS and geolocation specialist, Telecom SudParis – Mines-Telecom Institute

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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