GNSS jamming

29 Sep 2025 The Navigator

GNSS such as GPS, GLONASS, Galileo and BeiDou are key elements of positioning, navigation and timing. Their signals, however, can be affected not only by unintentional interference but also by intentional jamming. The growing reliance of ships on GNSS means that seafarers need to be aware of how GNSS jamming happens and how to detect it

What every mariner needs to know about GNSS jamming

by Rear Admiral Tim Henry obe afni

GNSS satellites orbit Earth at an altitude of approximately 20,000 km. They transmit very faint signals orders of magnitude below background noise levels. GNSS receivers can detect these faint signals through complex processing, but this also means that the signals can be overwhelmed by any stronger radio signal being emitted at the same frequency.

In addition to the distance that GNSS signals need to travel, they must also pass through the Earth’s atmosphere (including the ionosphere and troposphere). This further dilutes and distorts the signal, leaving it susceptible to natural interference. This can be from solar storms, ionospheric scintillation (turbulence in the Earth’s atmosphere) or heavy rainfall. However, deliberate interference by jammers is the most serious threat, because it can suppress signals instantly and mislead those trying to read them.

Jamming is when a device deliberately transmits on or near GNSS frequencies with the intention of obstructing or dominating authentic satellite signals. This is easily done because GNSS uses fixed and well-known frequencies and a simple jammer can prevent GNSS receivers from operating over a large area. A jammer putting out only a few milliwatts of power can cover hundreds of metres or more to deny GNSS service.

How does GNSS jamming work?

  • Broadband noise jamming: A jammer emits very high noise that is distributed across a wide GNSS frequency range, lifting the noise floor and interrupting the receivers’ ability to track satellite signals. Since GNSS signals are weak, this requires little power. Broadband jammers can have a simultaneous impact on multiple GNSS constellations; not just GPS, but also Galileo, GLONASS and BeiDou, since they all have intersecting frequency bands.
  • Continuous Wave (CW) Interference: Interference is sent as a calibrated, narrow CW tone on or near a GNSS carrier frequency. It prevents receivers from correlating or comparing signals. It is power-efficient and, if the frequency is correct, can impact receivers at long range.
  • Swept-tone or chirp jamming: A swept-tone jammer quickly moves a narrow signal back and forth across GNSS bands, striking multiple frequencies with relatively low power. Chirp jammers are also standard in low-cost devices, such as ‘privacy jammers,’ which are used to interfere with GNSS across a wide band.
  • Pulsed jamming: A pulse jammer transmits short bursts of noise or tones. Even if these do not entirely block the reception of GNSS signals, they may decrease the receiver’s ability to lock or generate reliable position data. Pulses synchronised with the receivers’ acquisition windows are particularly disruptive.

Maritime navigation disruptor types

  • Personal privacy jammers (PPJs): These small gadgets, sold online as ‘GPS blockers,’ are designed to conceal the location of their users or a vehicle. Not only can they jam a ship’s GNSS receivers but also ECDIS, AIS time sync and dynamic positioning. These devices are typically made to be plugged into a vehicle cigarette lighter or USB port and they emit just enough power to jam GNSS signals within tens or hundreds of metres. Due to their common use and low cost, PPJs are a frequent source of interference.
  • Shipborne jammers: In order to conceal illegal activities, evade detection or even confuse coastal authorities, certain vessels deploy more powerful jammers. These devices can interfere with the GNSS of other ships over several kilometres, especially in crowded or confined waters.
  • Shore-based jammers: Jammers placed on land (eg by criminal and state actors) in proximity to ports, straits or coastlines can generate GNSS-denied zones. Even low-power shore jammers can create significant problems for vessels manoeuvring in port or transiting through confined waters.

How can you tell if your GNSS is being jammed?

If you notice one or more of the following, jamming might be taking place:

  • Loss of signal or position: The receiver displays ‘NO FIX’ or equivalent messages.
  • Inconsistent positioning: The vessel’s reported position jumps or the speed and course over ground (SOG and COG) change unrealistically.
  • Time sync problems: Equipment using GNSS time (eg AIS, GMDSS, ECDIS) shows timing errors or alarms.
  • Multiple receivers being affected at once: If all receivers are losing lock at the same time, it is likely to be something external causing it, such as jamming.
  • Receiver interference alerts: Some GNSS receivers are equipped with internal interference detection, which warns when high power signal levels are occurring and/or there is a sudden drop in the signal-to-noise ratio.

Another sign might be an abrupt loss of satellite-based augmentation systems (SBAS) differential corrections (eg SouthPAN or ESMAS), followed by a degradation in position accuracy before signal is lost completely.

Where is jamming is most likely?

  • Near conflict zones: Military forces often employ GNSS jamming to protect their operations. This can spill over into nearby shipping lanes, affecting commercial vessels.
  • High-risk ports: Ports in areas with a history of GNSS problems, such as parts of the Black Sea or the eastern Mediterranean, regularly experience outages reported by mariners.
  • Regions with smuggling or illegal fishing: Ships in areas with high criminal activity may use jammers to hide their movements from authorities.
  • Onboard sources: Crew or passengers with PPJs can jam their own ship’s navigation systems without realising it.

Disrupting GNSS is illegal under international law. The International Telecommunication Union (ITU) explicitly bans intentional radio interference, and most countries enforce severe penalties for owning or using jammers. Despite this, GNSS jamming happens every day, posing a threat to safety at sea. By understanding how jamming occurs, identifying its signs, and learning to be able to quickly make use of alternative means of navigation, mariners can continue to operate safely in the absence of GNSS. An essential part of keeping your ship safe when satellite navigation fails is to stay alert and always expect the unexpected.

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