This article has drawn attention to the growing vulnerability of maritime navigation to GPS interference, particularly in geopolitical conflict zones.
GPS disruptions are caused by a mixture of factors: natural events like solar flares, equipment problems such as receiver or antenna malfunctions, and, increasingly, deliberate interference.
Deliberate interference has become a feature of modern conflict and geopolitical tensions. In areas experiencing conflict, strategic rivalry, or heightened tensions, States are actively using GPS interference.
GPS jamming vs. spoofing
Two terms often used interchangeably but with distinct meanings are GPS jamming and GPS spoofing. GPS jamming is the act of blocking or interfering with legitimate GPS signals by overwhelming them with stronger, unauthorized radio signals.
GPS spoofing is the act of transmitting false GPS signals designed to deceive a receiver into calculating an incorrect position, velocity, or time. Instead of blocking the signal, a spoofer imitates a legitimate GPS signal, making the vessel’s receiver believe it’s real. The GPS display will show a position, but it will be inaccurate, potentially by a significant margin. Derived speed and course information will also be incorrect.
Recognizing the signs of GPS disruption
Mariners must maintain heightened attention and awareness for signs of GPS disruption, as numerous onboard systems — including ECDIS, Radar/ARPA, Gyro compass, course recorder, and the autopilot — are heavily reliant on the GPS feed and will likely be impacted by any disruption.
Depending on whether the disruption is caused by jamming or spoofing, the tell-tale signs can vary from clear audible or visual alarms to no alarms at all.
When GPS jamming occurs, mariners may observe unusually high HDOP values, e.g., greater than ‘4’ indicating unreliable accuracy, RAIM alerts entering caution or unsafe modes, or elevated Signal-to-Noise Ratio (SNR) values.
On ECDIS, jamming can trigger sensor failure alarms, potentially leading to a switch to backup sensors or dead reckoning and may even freeze the chart display if no secondary source is defined. Conversely, spoofing presents a more deceptive threat as the GPS receiver might report an erroneous but seemingly valid position, often without RAIM detection.
In such spoofing scenarios, ECDIS can display incorrect positions, and radar/ARPA systems, when GPS-fed, will show erroneous data, while gyro compasses may enter an alarm state if relying on GPS for drift stabilization.
Alarm fatigue and sensory overload
A significant challenge during GPS signal loss is alarm fatigue. The disruption or loss of GPS signal often triggers numerous simultaneous alarms across the bridge, leading to a sensory overload that can be both disconcerting and distracting for the crew.
Effectively managing these alarms and prioritizing critical information is essential to maintain situational awareness and ensure safe navigation.
Covert GPS failures
There are also situations where no alarms are triggered, making detection much harder. For example, the Australian Transport Safety Bureau reviewed a near-grounding incident involving a vessel navigating the Great Barrier Reef. In this case, a malfunctioning GPS unit (due to an antenna fault) fed incorrect positional data to the ECDIS, radars, and other bridge equipment.
Because the ship’s position wasn’t being monitored through other means and no alarms were activated, the inaccurate GPS data and the vessel’s deviation from its planned course went unnoticed by the crew, pilot, and even Vessel Traffic Services (VTS).
While not linked to jamming or spoofing, such cases underscore the dangers of unaddressed GPS anomalies, whether from technical faults or external interference. They highlight the inherent risks of relying solely on a single source of navigational data, even when it appears functional, and emphasize the importance of crew training in recognizing and responding to these events.
Key strategies such as switching to a secondary receiver different from GPS (if available), employing parallel indexing, utilizing RADAR overlay on ECDIS, and manual position plotting on ECDIS.
It’s crucial to emphasize that once a GPS interference alarm is triggered, mariners must identify its root cause instead of simply silencing or deactivating it. Regarding manual position plotting, the varying levels of user-friendliness of ECDIS remains a significant concern.
Gard Club highlights that beyond the technical measures, vital operational decisions become critical, such as:
- reducing speed, which not only allows more time for assessment but also significantly lessens potential damage during an incident like grounding,
- increasing bridge manning, and
- making informed decisions on whether to proceed with the voyage.
This critical go/no-go decision should be guided by a comprehensive set of considerations, ideally integrated into the vessel’s GPS disruption response plan. Such factors include:
- the complexity of the passage,
- room to manoeuver,
- the availability and capability of pilots or local tugs for assistance,
- the reliability of buoys and fairway markings,
- the presence of safe anchoring points along the route,
- the density of traffic,
- effectiveness of Vessel Traffic Service (VTS) management, visibility, and the geographic extent of the GPS disruption.