GNSS Jamming: How Interference Blinds Receivers

Introduction

GNSS jamming is the deliberate transmission of radio frequency interference to prevent receivers from acquiring or tracking satellite signals. It is the simplest form of GNSS attack - requiring no knowledge of signal structure, no synchronisation with satellites, and minimal technical skill. Yet its consequences range from minor navigation inconvenience to airport approach disruptions and road freight positioning failures.

Key Concept: GNSS jamming works by raising the noise floor at the receiver above the level of the satellite signal, preventing the correlation process from extracting usable data. A 1-watt jammer at 1 km can deny service to every receiver within a radius of several hundred metres.

Personal Privacy Devices

The most common source of intentional GNSS jamming is the Personal Privacy Device (PPD) - colloquially called a GPS jammer. These devices, typically plugging into a vehicle 12V socket, were originally developed to defeat fleet tracking systems. A driver wishing to hide their location from an employer telematics system would activate the PPD, preventing the tracker from obtaining a GPS fix and reporting the vehicle position.

PPDs typically broadcast wideband noise or a swept carrier across the GPS L1 band (1575.42 MHz), sometimes also covering L2 (1227.60 MHz) and GLONASS frequencies. Output power is typically 10 to 100 milliwatts, sufficient to jam receivers within 100 to 500 metres of the device. More powerful models, occasionally encountered in freight vehicles, can affect receivers at distances of several kilometres.

Real-World Incidents

Documented PPD incidents illustrate how localised interference can have disproportionate consequences:

Newark Liberty Airport, 2009: Engineers at the FAA Wide Area Augmentation System (WAAS) ground station at Newark identified recurring daily interference events lasting several minutes, always during morning rush hour. The source was eventually traced to a truck driver commuting on the New Jersey Turnpike adjacent to the airport perimeter, using a PPD to defeat his employer fleet tracker. The interference disrupted the WAAS reference station measurements without affecting the airport ILS-based approach procedures - but it demonstrated how a single PPD could corrupt a safety-critical augmentation system.
UK M6 motorway: The UK Sentinel project, which monitored GNSS interference along major roads, detected hundreds of jamming events per year, overwhelmingly associated with freight vehicles. Clusters of events occurred predictably near motorway service areas where drivers stopped and activated PPDs before re-entering the road.

Receiver Response to Jamming

A GNSS receiver experiencing jamming typically exhibits a characteristic sequence:

Degraded signal quality: Carrier-to-noise ratio (C/N0) values decrease across all tracked satellites simultaneously - a key indicator of jamming versus localised blockage, which would affect only satellites in a specific direction.
Loss of lock: Tracking loops lose lock on individual satellites as signal quality drops below threshold.
Position unavailable: With fewer than four satellites tracked, the receiver can no longer compute a 3D position fix and outputs an error flag.
GNSS denied: At sufficient jamming power, the receiver loses all satellites simultaneously and may enter cold start mode, requiring minutes to re-acquire after the jammer is removed.

Note: Unlike spoofing, jamming is detectable. A simultaneous drop in C/N0 across all satellites is a reliable jamming indicator. Many professional receivers include Automatic Gain Control (AGC) monitoring specifically to detect this signature.

Legal Status and Enforcement

GNSS jammers are illegal in most jurisdictions. In the United States, the Communications Act prohibits the manufacture, importation, sale, and operation of jammers. The FCC has issued fines of up to $100,000 for PPD use. In the EU, jammers are prohibited under the Radio Equipment Directive. Despite this, millions of PPDs are in use globally, readily available online from jurisdictions where export controls are not enforced.

Unintentional Interference

Not all GNSS interference is malicious. Common sources of unintentional interference include:

Harmonics from digital electronics: Poorly shielded computing equipment, LED drivers, and switching power supplies can radiate harmonics that fall within GNSS bands.
Wideband transmitters: LTE and 5G transmissions in bands adjacent to GNSS frequencies occasionally cause desensitisation in receiver front-ends.
TV transmitters: The LightSquared controversy (2011 to 2012) demonstrated that high-power terrestrial transmitters in adjacent spectrum can overwhelm GNSS receiver front-ends even when frequency separation appears adequate.

Summary

GNSS jamming is widespread, largely driven by the proliferation of cheap PPDs in the commercial freight sector. It represents a denial-of-service attack on positioning and timing infrastructure that is easy to execute and difficult to prevent at the receiver. The following lesson examines spoofing - a more sophisticated and dangerous threat where the receiver is not denied service but given a false position instead.