"The effects of jamming on a GPS receiver can vary widely depending on factors such as the jammer's distance from a receiver, surrounding physical environments, or RF characteristics."
GPS jamming: is it straight out of your favorite action film or something that happens every day here in reality? While not everyone has a reason to jam GPS signals, it’s far more common than some think.
GNSS systems like GPS, GLONASS, and Galileo are embedded in our everyday lives, powering everything from Wall Street trades to commercial shipping navigation. The value of using GNSS systems is evident, but on the flip side is the value for bad actors with ill intentions. While some GPS interference is unintentional or simply done for the sake of personal privacy, many GPS jamming occurrences are done with tact and purpose and can threaten an entire industry’s operations or something as severe as national security and public safety.
Here, we explore a few types of GPS jammers and the signal characteristics they display to understand more about how GPS jamming works and, hopefully, help mitigate some of the fallout.
What Are GPS Jammers?
GPS jammers are exactly what they sound like - devices used to block or interfere with GPS or other types of GNSS signals. Most often, a GPS jammer is a small, compact device - but different types of jammers can take on a variety of physical structures or morphologies.
The most common GPS jammers are considered “chirp jammers.” Chirp jammers quickly change their frequency over time, sweeping across the frequency range to overpower a GPS signal. Since the chirp signal ‘sweeps’ across a frequency range, typically between 1565-1585 MHz, they can often overpower an entire range of signals rather than a single frequency.
GPS jammers can be as simple as a device that plugs into a car’s cigarette lighter port or more bulky, complex devices with dozens of antennas.
Simple GPS Jammers
The most simple types of jammers, often the cigarette lighter jammers, are used to disable tracking in trucks and cars - broadcasting RF signals on the L1 band at approximately 10mW.
These are typically used by truck drivers or civilians who have concerns over tracking, either by their employer or the government.
Complex GPS Jammers
More complex jammers (sometimes called hedgehog jammers due to their many antennas) are often used by criminals looking to jam multiple radio signals at the same time - such as Wifi, GPS, and cellular signals. Criminals may want to use these types of jammers to disable alarm systems, hide the location of contraband or stolen materials, or prevent satellite-based navigation services.
These types of jammers are becoming more common and readily available and present a growing threat to industries and the public. Hedgehog jammers typically broadcast on the L1 or L2 band at around 10W.
How Do Jammers Affect a GPS Receiver?
The effects of jamming on a GPS receiver can vary widely depending on factors such as the jammer's distance from a receiver, surrounding physical environments, or RF characteristics.
One common way to understand if a receiver is experiencing interference is to simply read the operations manual, which typically provides normal operating parameters. If the receiver is not operating within the parameters outlined by the manufacturer, it could be a sign of jamming.
Since the same type of jammer or jamming signal can affect a receiver in different ways, it’s important to understand how a GPS jamming device may typically be affected by a jammed signal. The effects of jamming can vary or not exist at all.
No Effect: If the jammer is out of range of the receiver or the center of frequency is not aligned with the targeted frequency, the receiver does not experience any disruption.
GPS Signal Degradation: As the carrier-to-noise ratio of the received signal drops, so does the quality of the GPS device. This is often signified by inaccurate or low-precision navigation.
No GPS Signal Received: If the jammed signal is within range of the receiver and on the correct center of frequency, the receiver may not receive any GPS signal and completely fail to function until a signal is picked up again.
A Study of GPS Jammers and their Signal Characteristics
The following GPS jammer types were studied by the folks from the Radionavigation Lab at the University of Texas at Austin. The 18 commercially-available jammers included in the study were categorized based on morphology (physical structure) but do not include every time of GPS jammer in existence.
The study’s aim was to analyze the signal characteristics of some of the most commonly used GPS jamming devices - allowing them to better understand and strategize mitigation against GPS jamming occurrences.
The three categories of GPS jammers in this study include:
Cigarette-Outlet Jammers (L1 Band Only)
Rechargeable Battery/External Antenna Jammers (L1 and L2 Bands)
‘Cell Phone’ Jammers (L1 and L2 Bands)
The study utilized two types of live experimental tests.
The first test examined the frequency structures and power levels of the jammer signals.
The second test gave an estimate of the effective ranges of the jammers when used against common commercial GPS receivers.
Study Results
The tests presented the following findings across the 18 commercially available GPS jammers under observation when tested against some of the most common commercial GPS receivers.
Test 1: (Frequency Structures and Power Levels of Jamming Signal)
All of the jammers used some type of sweeping tone to generate broadband interference.
Most of the jammers used linear chirp signals.
All of the jammers affected the L1 band, six jammed the L2 band, and none of them jammed the L5 band.
The sweeping signal period lasted 9 microseconds on average, sweeping across a range of no more than 20MHz.
Test 2: (Estimate of Effective Jamming Ranges)
The weakest jammer affected tracking at around 300 meters distance and acquisition at around 600 meters.
The strongest jammer affected tracking at a range of approximately 6000 meters distance and acquisition at around 8500 meters.
Geolocating GPS Jammers
While GPS jammers can be hard to locate, it is possible, and many companies have made it part of their mission to improve geolocation capabilities to enhance public safety and national security.
There are two primary ways to locate the position of a GPS jammer.
GPS Ground Station Networks
As you might assume, a ground station network is typically in a fixed location, so geolocating the source of a jammed signal is only possible within a certain range of the ground station. Ground stations can not be used to search for interference sources across broad spatial ranges, so the application for GPS jammer geolocation with a ground station is limited.
Satellites in Low Earth Orbit
Satellites operating in LEO offer a much more robust capability of GPS jammer identification and geolocation. Since satellites can be deployed across a range of orbits, it makes it possible to search a wide range of terrestrial space or even jammers in motion. Some services, like those from Spire Global, can even identify the type of jammer being used, which can help mitigate risk in many cases.
You can read more about geolocating GPS jammers from space in our article here.