Knowledge / Navigation and collision

GNSS interference: misleading position, shared dependencies and independent checks

Distinguish jamming from spoofing, displays sharing GNSS data, alternative position evidence and dead-reckoning uncertainty.

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GNSS failure does not always appear as loss of position. A vessel following a smooth track on a display is not proof that the track is correct. Reliable navigation requires understanding how position is produced, which other systems consume the same information and whether independent observations support the solution.

Missing data and misleading data differ

Jamming can degrade or prevent reception of genuine satellite signals. Spoofing can cause a receiver to produce plausible but false position or time information. The operational distinction matters: an explicit loss alarm may be easier to recognise than a convincing but incorrect number.

The IMO, ICAO and ITU statement of 25 March 2025 formally highlights jamming and spoofing affecting satellite navigation. A position jump on a particular vessel, however, does not by itself establish deliberate attack or identify an actor. Antenna, cable, receiver, interface and source-selection faults remain possible explanations to investigate.

Why two displays are not two independent checks

ECDIS, a radar's ground-referenced presentation, AIS and track control can consume the same GNSS output. Apparent agreement between displays may therefore be duplication of one source. A second receiver sharing the same interference environment does not provide complete independence merely by adding hardware. Independence depends on the physical information and failure causes that are shared.

MCA's MGN 719, published 1 October 2026, explains dependencies extending beyond navigation into timing and shore systems, and recommends independent verification and alternative navigation competence. It is UK guidance and does not replace another flag State's specific procedures.

Look for independence in the measurement itself

Radar range to an identified fixed coastal object can provide physical information different from satellite position. The same radar's geographically placed chart overlay, however, may carry the shared position error. Raw measurement and processed presentation must be distinguished by their data sources. Correct target identification, radar settings and chart reference remain necessary. Nevertheless, comparing independent radar coastline or target returns with the ENC can reveal misregistration, provided heading, chart datum and target identity are checked. Agreement between ship-position symbols drawn from the same source remains dependent evidence.

A visual bearing, another fixed object, a depth pattern or suitable celestial observation can add evidence; none is a universally decisive solution. A displaced buoy, misidentified light or ambiguous shoreline can create false reassurance. MGN 379 Amendment 1 is an official reference on electronic-aid limitations and the risk of incorrect position reaching chart systems.

Dead-reckoning uncertainty grows with time

Assume constant speed through water of 12 kn and constant heading for 15 minutes after the last reliable position. Distance through water is 12 × 15/60 = 3.0 nautical miles. An unknown error of 0.5 kn in the cross-current component produces, from that effect alone, 0.125 nautical miles or 231.5 m of cross-track position difference. One nautical mile is taken as 1,852 m.

Separately, a constant heading error of 2° produces a cross-track component of approximately 3 sin 2° = 0.105 nautical miles, or 194 m, over that 3.0-nautical-mile through-water path. These effects combine according to direction; they are not always aligned. The example establishes neither total position uncertainty nor a safety radius. Initial position, speed measurement, leeway and time-varying current need separate consideration.

A calculation using GNSS-derived speed over ground does not independently verify that same GNSS source. Advancing a known position along the steered course using through-water speed and elapsed time gives a dead-reckoning position (DR). Applying estimates of current and leeway gives an estimated position (EP), a distinction defined in Bowditch. Repeating a calculation on another display may add little assurance unless its genuinely independent inputs are identified.

Establish a reliable navigation picture when information conflicts

When a discrepancy appears, retain the source, time and symptom record. Distinguish whether position, speed, heading or time information is affected. Determine whether the apparently reliable device distributes information to other systems. The team must assess uncertainty together with available sea room, traffic and manoeuvring options.

The suitability of measures such as reducing speed, adding a lookout, using alternative fixes or changing control mode depends on circumstances and vessel procedures. Manually moving a ship symbol on the chart does not repair its underlying data source. Restarting every device simultaneously before preserving evidence can also erase diagnostic clues; investigation must not take priority over safe operation.

Signal restoration is not automatically restored confidence

When position output returns, assess its continuity and agreement with the last reliable information and independent observations. A few plausible points do not establish that every connected system has returned to a correct source. Other systems using timestamps or data-quality flags may also have been affected.

The incident record should distinguish the time interval, confidence in position, affected equipment, observed symptoms and actions taken. A suspect position should not be recorded as the definite incident location. Required reporting follows vessel, company, coastal-authority and manufacturer channels. Preparation is about knowing in advance when different information sources remain usable, rather than assuming one device is infallible.

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