By Capt Aly Elsayed AFNI, Senior Technical Manager, The Nautical Institute; Lars Løge and Magnus Hov Lieng, Space Norway

GNSS interference is creating a navigation challenge in which everything on the bridge can appear to be working normally while the vessel’s displayed position is wrong. Drawing on early observations from The Nautical Institute’s STEER project and technical insight from Space Norway, this article examines how bridge teams can recognise the problem, respond safely and build greater resilience before an incident occurs.

It is hard to argue with a screen that has been right for years, especially when the display beside it agrees.

An officer who checks both can reasonably feel reassured, although neither may have provided an independent fix. The same false position can travel through several bridge systems and return looking like confirmation.

Nothing has to go blank, and there need be nothing wrong with the chart. The mistake is the little ship symbol moving across it.

At 12 knots over the ground, a ship covers a nautical mile in five minutes. Near a hazard, time spent trying to reconcile the instruments with what is happening outside can quickly become part of the danger.

When two screens are really one source

Jamming and spoofing put the navigator in different predicaments.

Jamming interferes with reception of signals from global navigation satellite systems (GNSS), potentially depriving equipment of accurate position and time. Systems relying on those inputs may begin to drift apart or provide conflicting information.

Spoofing can be harder to recognise because the receiver accepts false signals and supplies a position or time that appears usable. Equipment drawing on the same information may continue to agree with one another.

Waiting for an alarm is therefore no answer if the equipment itself has not recognised the deception.

Shipping places considerable faith in the instruction to cross-check. But looking at another screen is not necessarily the same as checking another source.

Fitting a second receiver does not automatically provide protection either if both receivers depend on the same underlying signal or are vulnerable to the same interference. A shipowner should therefore be able to ask a supplier where each system gets its position and time, and receive a clear explanation of what happens when those inputs are wrong. Leaving the watchkeeper to discover those connections during a difficult passage is a poor substitute for understanding them beforehand.

What seafarers are seeing

Nor can owners assume that recognising the problem will come naturally to every qualified officer.

The Nautical Institute’s STEER project is examining how technology works for the people who use it at sea. Most seafarers interviewed so far had experienced GNSS disruption. Experienced masters appear better able to manage it, while newer officers may need more support before they feel confident. These are early observations rather than findings about the entire fleet, but the accounts are revealing.

Some seafarers describe frequent drills. Others report masters or chief officers remaining on the bridge to support the watchkeeper in restricted waters, alongside clearer instructions about using traditional navigation methods. Companies taking these steps are giving less experienced officers the chance to learn from experienced colleagues before an incident forces the issue. There is an important distinction, however, between practising the loss of a signal and practising deception.

A drill that simply switches off the position feed makes the problem obvious. Leave the display running with a believable false position and the bridge team has a much more demanding task. Officers must recognise the discrepancy, decide which evidence deserves their trust and continue to navigate safely while doing so. That is worth rehearsing properly and discussing afterwards.

What first raised the doubt? Was it shared quickly enough? What information did the team trust, and why?

Those details can tell a master more about the team’s readiness than whether everyone remembered the checklist. The debrief also gives an inexperienced officer an opportunity to hear how a master worked through the same uncertainty, including which evidence ultimately persuaded them to act.

Training for a believable error

Much of the evidence needed to challenge a false position may already be available on the bridge.

In suitable visibility, bearings and transits on known landmarks can be compared with radar ranges and bearings to fixed objects. The vessel’s observed movement and the depth beneath it may tell a different story from the plotted position. Even here, judgement matters.

A buoy can move. A radar’s measured range to a headland is independent of the ship’s GNSS position, whereas a GNSS-derived symbol overlaid on the display is not.

Knowing the difference is part of knowing the instrument. The master belongs in that response too.

One officer should not be left to solve the discrepancy alone, and the contingency plan must allow the bridge team to keep the vessel safe while it investigates. Depending on the circumstances, that may mean reducing speed or stopping.

The question is not simply whether the bridge team can navigate without GNSS. It is whether the team can recognise when apparently functioning equipment should no longer be trusted.

Building an independent check

That challenge is also encouraging work on alternative ways of establishing or verifying position and time.

Space Norway is among those examining whether maritime radio and satellite communications could provide an additional, independent source of information when GNSS cannot be trusted.

The underlying principle is straightforward: if several bridge systems all depend on the same positioning source, agreement between them does not necessarily provide meaningful confirmation. A genuinely independent source could help the bridge team determine whether the electronic position is plausible.

Such a system does not need to replace GNSS to have value. Early technical work suggests that alternative radio-based ranging may initially provide much less precise positioning than conventional GNSS. That would make it unsuitable as a primary navigation source in confined waters.

But precision is not the only consideration. If an independent estimate places a vessel broadly in one area while GNSS shows it somewhere significantly different, that discrepancy could be enough to prompt the bridge team to question the apparently authoritative display.

In that sense, an alternative positioning source may be most useful as an independent check rather than as a direct replacement for satellite navigation. The usefulness of any such approach will ultimately depend on its accuracy, coverage, availability and independence from the systems it is intended to verify. Those capabilities need to be demonstrated in real operating conditions before bridge teams can rely on them.

A warning is not a position

There is also an important distinction between being told that GNSS may be unreliable and being given an independent means of determining where the vessel actually is. A warning about interference can alert the bridge team to exercise greater caution. It cannot, on its own, establish the vessel’s position.

That means future navigation resilience may require several complementary elements: better awareness of interference, independent sources of position and time, robust bridge procedures and officers who understand the limitations of each source.

No single technology removes the need for professional judgement.

Authentic does not always mean correct

Digital navigation also raises a broader question about trust in information.

Technical measures can help verify where a digital message came from and whether it has been altered. That is valuable, particularly as more maritime information is exchanged electronically. But authentication is not the same as accuracy.

A message can come from a legitimate source and still contain an error. It may also remain technically valid after the information it contains has become outdated.

The bridge team therefore still has to judge its age, relevance and reliability. The same principle applies to positioning information. The presence of a digital signal, a familiar symbol or a reassuring display does not remove the need to ask whether the information makes sense when compared with independent evidence.

Technology still needs judgement

Alternative positioning technologies may eventually give bridge teams more independent ways to challenge a false position, but resilience cannot wait for the next system to arrive.

Seafarers are already describing more practice, stronger support on the bridge and renewed attention to skills that allow them to test the electronic picture against independent evidence.

Experienced masters have knowledge worth passing on, and companies can make room for that in the working day.

The important question is not simply whether the bridge team can operate without GNSS. It is whether they can recognise when a system that appears to be working should no longer be trusted.

A watchkeeper who questions a convincing position should know what evidence to seek next, when to call the master and what action will keep the vessel safe while the discrepancy is resolved.

There may only be a few minutes in which to get it right.