What Happens to a Ship When GPS Is Jammed

When GPS is jammed, the receiver loses its fix and the position alarm sounds. The ECDIS, AIS, radar overlay, track control and DSC radio usually take their position from that receiver, so they run out together, and a second GPS does not help because it listens to the same jammed frequencies. Radar echoes, the gyro, the log and the echo sounder carry on, and the watch navigates on those.

This now happens to a lot of ships. The maritime analytics firm Windward counted 124,235 distinct vessels affected by GPS jamming worldwide in the third quarter of 2026, up 11.7% on the quarter before, and it puts 30% of the jamming it observes in the Black Sea. In January, fourteen European coastal states warned the shipping industry that "all vessels are at risk".

What stops working on the bridge?

Anything that takes its position or time from the GNSS receiver. The receiver reacts first: the IMO performance standard for shipborne GPS gives it five seconds to flag that no new position is being calculated, after which it should output the last known position, marked as such.

ECDIS loses own-ship position, and with it route monitoring and the cross-track, safety-contour and area alarms. Many sets carry on in dead reckoning and say so on screen.

AIS keeps transmitting, but its position source sits under the same jamming, so its reports can go out with the position marked as unavailable.

Radar still paints every echo, which is why it becomes the main tool, but ground stabilisation and the chart overlay go, along with any speed over ground that came from GNSS.

Track control has nothing left to steer by and should raise an alarm, leaving the officer on heading control or in hand steering.

The DSC radio can only send the position it was last given. A distress alert an hour later carries an hour-old position unless somebody has updated it by hand.

How can you tell it is jamming?

Signal strength falls across all satellites at once, often on more than one frequency, and every receiver on board drops at the same moment, including the one inside the AIS. A failed antenna or cable only takes out what is connected to it. Weak jamming is less tidy: the fix may come and go, or wander a little before it drops.

What should the watch do?

Note the time and the last good position, then take the autopilot out of track mode. Put the ECDIS on dead reckoning, plot radar ranges and visual bearings as often as the waters demand, and switch the radar to log speed. Enter the DSC position by hand and report the event according to the ship's procedures. When the fix returns, check it against radar before relying on it again, because jamming often travels with spoofing, which is much harder to see.

Can a receiver hold its position through jamming?

Some can, and the way to find out is to put one under real transmitters. At Jammertest, the open-air campaign held at Bleik on Andøya in Norway, we ran two installations on one car roof in September 2026: a regular GPS receiver, and our own system backed by a CRPA anti-jam antenna, which turns nulls towards interference sources low on the horizon.

In one 34-minute parked stretch of the first day's jamming, each installation logged 2,041 seconds: the regular receiver had a valid position in 1,173 of them, the anti-jam installation in 1,966. On the second evening, across 2,135 scored seconds of another jamming test, the anti-jam installation gave a valid position 94.2% of the time and never strayed more than 7.03 m from its reference, while the ordinary GNSS stream inside the same installation was valid in two of those seconds.

It was one campaign on land, with no surveyed ground truth and a recording gap of about half an hour that we have left as a gap.

Download the Jammertest 2026 field bulletin (PDF)

Frequently asked questions

Can a ship navigate without GPS?

Yes. Radar fixes, visual bearings, soundings and dead reckoning from gyro and log all still work. It takes more people and more attention, and it gets harder out of radar range of land.

Does an anti-jam antenna stop all jamming?

An array can only null a limited number of directions, so several strong sources at once can still overcome it. It works best alongside interference monitoring and integrity alerts in the receiver.

SEALINK builds marine GNSS and bridge equipment. GuardianGPS, our MED approved navigator, was the anti-jam installation in these tests. It combines multi-frequency reception with interference detection and automatic switchover to a protected position source.

Get the test-by-test data: download the Jammertest 2026 field bulletin (PDF)