GNSS Disruption at Sea: What Bridge Teams Need to Know Modern maritime navigation relies heavily on Global Navigation Satellite Systems (GNSS), such as GPS, for positioning, navigation and timing. GNSS data feeds directly into ECDIS, AIS, radar overlays, autopilot systems and other critical bridge equipment. While these systems have transformed navigational safety and efficiency, recent experience has shown that satellite positioning cannot always be assumed to be reliable – particularly in areas affected by geopolitical tension. Maritime security advisories issued by US MARAD have repeatedly highlighted widespread GNSS disruption (including jamming and spoofing) in regions such as the Persian Gulf, Strait of Hormuz, Gulf of Oman, Red Sea, Black Sea, Baltic Sea and Eastern Mediterranean, reinforcing that this is no longer an isolated or theoretical risk. GNSS interference is now a credible operational risk that Masters and bridge teams must be prepared to manage.   Jamming and spoofing – Understanding the risk GNSS interference typically occurs in two forms: jamming and spoofing . Jamming involves radio-frequency interference that overwhelms GNSS signals, resulting in loss of position, degraded accuracy or integrity alarms. While disruptive, jamming is usually recognisable because systems fail or revert to dead reckoning. Spoofing is more hazardous. False GNSS signals are transmitted to deceive receivers into calculating an incorrect but plausible position, course or speed. Alarms may not activate, and bridge systems can appear to function normally. If not detected, spoofing can lead to serious navigational incidents, including grounding or collision. In simple terms, jamming causes loss of signal; spoofing creates misleading information. Such interference is often associated with geopolitical tensions and may affect multiple vessels simultaneously within a region.   How GNSS disruptions appear on the bridge GNSS interference rarely affects only one system