IN 1730 – GNSS jamming and spoofing: Operational impacts and mitigation for dynamic positioning (DP)

This Information Note provides an updated overview of global navigation satellite system (GNSS) intentional interference, as known as jamming and spoofing, and its impact on marine operations, particularly dynamic positioning (DP).

Since the original publication of IMCA guidance on this topic, GNSS disruption has become more prevalent and, in certain regions, routine. Recent IMCA maritime security updates highlight that interference, jamming and spoofing are now being observed across multiple operating areas, including the Baltic, Black Sea, Eastern Mediterranean, Red Sea and Arabian Gulf.

These events are no longer isolated or short in duration and may affect multiple vessels simultaneously. As GNSS is widely used across navigation, timing and DP systems, disruption represents a credible operational risk that requires both technical and procedural controls.

• Interference: Any intentional or unintentional disruption, degradation, or masking of GNSS signals that reduces positioning, navigation, or timing performance
  
  
• Jamming: Intentional transmission of radio signals on or near GNSS frequencies to deny or degrade GNSS reception by reducing the receiver’s signal‑to‑noise ratio (that is, overpowering authentic signals)
  
  
• Spoofing: Intentional transmission of false GNSS signals to mislead a receiver into calculating an incorrect position or time; unlike jamming, spoofing may present as valid but incorrect data

GNSS disruption is now a recognised feature of the maritime operating environment in certain regions.

Jamming equipment is widely available and capable of denying or degrading GNSS signals over localised areas. More advanced systems, including those used in state-level operations, can affect wider regions and persist for extended periods.

Spoofing, previously considered rare, is now documented in multiple maritime theatres. These events may result in vessels reporting consistent but incorrect positions, sometimes affecting multiple vessels in the same area.

Operators should assume that GNSS degradation may occur without warning and may not be limited to a single vessel.

The ability to distinguish between jamming and spoofing remains important, although symptoms may overlap.

Indicators may include:

• loss or degradation of GNSS signal quality
  
  
• sudden or non-random position shifts
  
  
• stable but incorrect position data
  
  
• inconsistencies between GNSS and other onboard systems such as radar, ECDIS or visual observations
  
  
• multiple vessels reporting similar or identical positions.

Spoofing may present as a valid signal with incorrect position data, making it more difficult to detect than jamming.

Integrated or blended systems may mask underlying GNSS issues, and operators should not assume that continuity of position output indicates data integrity.

GNSS is widely used as a primary input to DP position reference systems (PRS), either directly or as part of augmented or blended solutions.

Disruption of GNSS signals can result in:

• degradation or loss of multiple PRSs simultaneously
  
  
• reduced confidence in position reference weighting
  
  
• incorrect but apparently valid position inputs
  
  
• increased risk of common-mode failure across redundant systems.

In spoofing scenarios, erroneous position data may be accepted as valid by the DP system, potentially leading to inappropriate system responses if not identified.

Operators should recognise that multiple GNSS-based PRSs do not provide true redundancy where the underlying signal source is compromised.

Effective mitigation requires a combination of system design, operational awareness and procedural response.

6.1   System design and equipment

• Use multi-constellation, multi-frequency GNSS receivers where practicable to improve resilience and increase robustness against signal degradation or disruption, alongside other principles.
  
  
• Integrate GNSS with inertial navigation systems (INS) to support continuity and detect anomalies alongside other principles.
  
  
• Consider the use of NTRIP corrections technology, as appropriate.
  
  
• Ensure robust installation, including antenna placement, shielding and cable integrity.
  
  
• Consider the use of anti-jamming and anti-spoofing technologies where appropriate, including the deployment of active and passive anti-jamming antennas.

6.2   Position reference strategy

• Ensure diversity and equal weighting of PRSs, including non-GNSS-based systems where practicable.
  
  
• Consider the use of INS-coupled, non-DGNSS PRSs when diversifying PRSs.
  
  
• Recognise that GNSS-based PRSs may share common vulnerabilities.
  
  
• Validate PRS independence and failure modes within FMEA and assurance documentation; in addition, review the independence and failures modes and incorporate that as part of operational planning.

6.3   Detection and monitoring

• Compare and cross-check GNSS-derived position in the following ways:
  
  
  • Between multiple GNSS receivers/antennas (ideally different models and physically separated).
    
    
  • Between different GNSS modes from the same receiver (for example: GPS only vs. multi-constellation, PPP vs RTK/DGNSS).
    
    
  • Against independent relative positioning solutions (for example, Long Baseline Acoustic Positioning).
    
    
• Monitor for abnormal or inconsistent behaviour across PRSs such as position/time jumps or unrealistic dynamics:
  
  
  • Compare against INS/IMU dead-reckoning.
    
    
  • Compare against vessel sensors and ECDIS plausibility (such as COG/SOG, rate of turn, acceleration).
    
    
• Monitor signal quality indicators (such as signal-to-noise ratio, residuals, RTK/PPP status, automatic gain control flags).
  
  
• If available, cross check timing by comparing GNSS time (for example, 1PPS/UTC) against an onboard holdover clock (stable oscillator).
  
  
• Be aware that stable position output does not necessarily indicate valid data.
  
  
• Prepare for jamming or spoofing when military vessels are in the area.
  
  
• Consider the use of geo-fenced operational bounds checks with an alarm signal to indicate if the vessel has left a permitted area, crossed safety zones, or shows implausible proximity changes.

6.4   Operational response

• Define clear actions within Activity Specific Operating Guidelines (ASOG) for GNSS degradation or loss.
  
  
• Establish thresholds for reducing DP capability or suspending operations.
  
  
• Train DP operators to recognise and respond to GNSS-related anomalies, including spoofing scenarios; this can be leveraged through DP drills.
  
  
• Report incidences of jamming and spoofing in order to build a data base of location, events and actions.

6.5   Planning and awareness

• Consider regional GNSS interference risks during planning and risk assessment.
  
  
• Monitor available advisories and warnings from relevant authorities such as the International Maritime Organization, US Maritime Administration and constellation providers' websites and news.
  
  
• Share lessons learned and operational experience within the organisation and industry.

7.1   Member feedback

Recent feedback from IMCA Members highlights several practical considerations when operating in areas affected by GNSS interference, jamming and spoofing:

• Increased risk during project start-up: GNSS disruption can be particularly critical during early project phases, including calibration of sensors, beacon deployment and target setup, where reliance on GNSS is often highest.
  
  
• GNSS heading degradation as an early indicator: Where GNSS is used to provide heading (for example, baseline RTK solutions), loss or instability of heading may be observed before position degradation, providing an early indication of interference or spoofing.
  
  
• Impact across the wider marine spread: GNSS disruption may affect multiple vessels and systems within the same field, including DP vessels, walk-to-work units, CTVs and PSVs. This should be considered in planning and risk assessments.
  
  
• Use of mission equipment as position references: In certain operations, mission equipment such as gangway sensors, pile gripper systems or subsea acoustic references may provide additional or alternative position reference inputs.
  
  
• Use of doppler velocity logs (DVL): DVL systems may be used to support or augment position reference solutions where GNSS reliability is degraded. Operators should consult with relevant SMEs when considering the use of DVLs.
  
  
• Acoustic positioning systems (LBL): Long baseline (LBL) systems may provide an alternative to GNSS-based positioning. However, operators should be aware of potential compatibility limitations between different equipment manufacturers and the requirement for pre-installation and calibration before being operational. Operators should consult with relevant SMEs when considering the use of LBL.

These observations reinforce the need for system diversity, operational awareness and robust procedures when operating in GNSS-degraded environments.

7.2   Risk associated with automatic fallback or follow-target functions

Where DP or control systems utilise automatic fallback or follow-target modes based on GNSS inputs, erroneous or drifting position data may be propagated if not actively monitored. This may result in inappropriate system responses unless cross-checking with independent references is maintained.

7.3   Military vessels in the area

Increased DGNSS degradation has been reported when military vessels are operating in the region.

GNSS jamming and spoofing are now routine risks in some regions and may affect multiple vessels at the same time. For DP operations, this is not just a navigation issue. It can degrade or invalidate several position reference systems simultaneously, creating a real common mode failure risk.

Operators should not treat GNSS-based PRSs as independent where the underlying signal is compromised. Spoofing in particular may present stable but incorrect data unless it is actively cross-checked.

Industry experience shows the practical impact. In affected regions, operations have required multiple anti jamming systems, with outputs shared between survey and DP to maintain position. This reinforces that GNSS resilience is not theoretical, it is operational.

Effective management relies on PRS diversity including non-GNSS inputs, clear ASOG responses, awareness of regional risks, and training to recognise anomalies. GNSS disruption should be explicitly addressed in DP risk assessments, FMEA and operational planning.

• IN 1712 – Maritime security update – August 2025
  
  
• IN 1663 – Enhanced GNSS differential corrections for dynamic positioning (DP) operations
  
  
• IN 1634 – Increasing solar activity and the effect on GNSS positioning
  
  
• IN 1525 – GPS systems – Warning of interference
  
  
• IMCA S015 – Guidelines for GNSS positioning in the oil and gas industry
  
  
• IMCA M242 – Guidance on satellite-based positioning systems for offshore applications
  
  
• IMCA M273 – A guide to conducting DP drills and ensuring preparedness for DP failures
  
  
• IMCA M235 – Guidelines on the shared use of sensors for survey and positioning purposes

Note: Where GNSS-derived data or correction signals are shared between systems, operators should ensure that position reference inputs remain independent and segregated, and that sharing does not introduce common-mode vulnerabilities or hidden dependencies.

For more information about this, please contact 
[email protected] and [email protected].