Modern bridge teams depend heavily on satellite-based positioning for ECDIS, AIS and other integrated navigation systems. But GNSS interference is an increasingly important navigational concern, particularly in regions affected by jamming, spoofing or other disruption.
Loss of a GNSS position may be immediately obvious. Spoofing can be more difficult to recognise because navigation equipment may continue displaying a position that appears credible but is incorrect.
Safe navigation therefore depends not only on electronic positioning, but on the navigator's ability to recognise abnormal information, question what is being displayed and verify the vessel's position using independent means.
1. GNSS Jamming and Spoofing Are Not the Same
Jamming can degrade or prevent reception of genuine satellite-navigation signals, potentially causing loss of position or unreliable GNSS information.
Spoofing can cause equipment to calculate and display incorrect position, time, speed or other derived information.
Apparently normal electronic information should not automatically be assumed correct when independent navigational evidence disagrees with it.
2. Recognising That Something May Be Wrong
Warning signs can include unexpected position movement, radar or visual observations that disagree with the GNSS-derived position, abnormal course or speed over ground, AIS inconsistencies, ECDIS position disagreement with radar ranges and bearings, and GNSS or sensor alarms.
The bridge team's immediate priority is not to diagnose the technical cause but to recognise that the position information may be unreliable.
3. Never Depend on a Single Source of Position
When GNSS information becomes unreliable, the bridge team should cross-check the vessel's position using appropriate independent information available for the circumstances.
Where suitable, this may include radar ranges and bearings; visual bearings and terrestrial position fixes; parallel indexing; echo-sounder and depth comparison with charted information; gyro and magnetic compass information; vessel speed and log information; dead reckoning and estimated position; and celestial navigation, including sextant observations, where practicable and where the navigator is competent to use them.
No single method is universally appropriate. Navigators should select and compare suitable independent methods according to the vessel, visibility, sea area, available aids and circumstances.
4. Returning to Fundamental Navigation Skills
GNSS interference reinforces the continuing importance of traditional navigational competence. When reliable automatic positioning is unavailable, bridge teams should be capable of maintaining an independent navigational picture using appropriate charts and publications, radar, visual observations, compass information, vessel movement and manual plotting.
This includes maintaining a dead reckoning or estimated position and recording appropriate manual or independently obtained fixes at suitable intervals.
Traditional skills should remain practised and familiar before they are needed during an actual GNSS disruption.
5. Radar as an Independent Cross-Check
Correctly identified radar targets, radar ranges and bearings, and parallel indexing where appropriate, can provide a practical independent check on the vessel's position and track.
Radar information should be compared with charted features and other available navigational information. Correct target identification is essential, and radar itself must be interpreted properly rather than treated as infallible.
6. ECDIS Position Must Be Questioned When GNSS Is Unreliable
ECDIS remains an important navigational tool, but the ship symbol and automatically displayed position depend on the position sensor feeding the system.
If GNSS information is suspected to be jammed, spoofed or otherwise unreliable, bridge teams should not assume that a convincing-looking vessel position on ECDIS represents the ship's true position.
Navigators should use appropriate independent position-fixing methods and compare the resulting position with the electronic display. Where equipment and procedures permit, manually obtained or independently verified positions should be used to maintain awareness of the vessel's actual position.
Alarms, sensor status and differences between independent navigational information should be investigated rather than simply acknowledged and ignored.
7. AIS Information Also Requires Caution
AIS information should never replace radar, visual lookout or other appropriate means of assessing surrounding traffic.
GNSS interference can affect position-related information used by navigation systems, and information received from other vessels may also be inaccurate or inconsistent.
If an AIS target's reported position, COG or SOG does not agree with radar or visual observation, the bridge team should recognise the discrepancy and navigate using the overall independently assessed traffic picture.
AIS operation should remain in accordance with applicable regulations, Flag State requirements, company procedures and relevant official security guidance.
8. When Communications Are Also Degraded
GNSS interference and loss or degradation of satellite communications such as Sat-C are different problems, although they may occur during the same period or operating environment.
If normal satellite communications become unavailable, ships should follow their approved communication procedures and use appropriate available alternative communication methods in accordance with the vessel's equipment, GMDSS arrangements, company procedures and applicable requirements.
Bridge and shore teams should avoid assuming that loss of communications automatically means loss of navigation capability, or vice versa.
9. Bridge-Team Actions When GNSS Becomes Unreliable
The response should be organised and deliberate. The Master should be informed in accordance with standing orders and company procedures, and the bridge team should be made aware that GNSS-derived information is suspected unreliable.
Independent position fixing should be increased. Radar, visual bearings, compass, speed and log information, and depth should be cross-checked where appropriate. Dead reckoning and estimated position should be maintained, and manual plotting continued as appropriate.
Current navigational warnings should be reviewed. Significant alarms, discrepancies and actions taken should be recorded. Appropriate ship-shore communication should be maintained throughout.
Bridge Resource Management is important throughout. Everyone on the bridge should understand which information is considered reliable, which is questionable, and how the vessel's position is being verified.
10. Navigation in High-Risk or Interference-Prone Areas
Preparation should start before entering a known interference area. Current navigational warnings, company and Flag guidance, passage-plan contingencies and independent position-fixing methods should be reviewed in advance. Relevant charts, publications and bridge equipment should be ready.
If official warnings identify additional hazards, Masters and bridge teams should follow current official warnings, applicable recommended or promulgated routes and company, Flag and appropriate maritime-authority guidance.
WAN Insight
Electronic navigation has transformed the modern bridge, but it has not removed the navigator. When satellite-derived position becomes unreliable, the ability to question the display, recognise conflicting information, cross-check independent sources and maintain a safe navigational picture becomes critical.
Traditional skills including radar navigation, terrestrial fixing, DR/EP, manual plotting and celestial navigation remain valuable professional competencies when electronic positioning cannot be fully trusted.
Disclaimer: This article is for general maritime awareness only. Masters and bridge teams must follow applicable regulations, company SMS and standing orders, Flag State requirements, current navigational warnings and relevant official guidance.