Current warnings make the risk a reality
If the chartplotter suddenly jumps, the app shows the boat to be off course, or the depth reading displays absurd figures, it does not necessarily mean that the on-board electronics are faulty. In the Baltic Sea region, reports of disrupted satellite navigation have been increasing for some time. For motorboat operators, this is above all a reason to review their own navigation routines.
The Finnish Transport and Communications Agency, Traficom, reports ongoing disruptions to satellite navigation and mobile communications. According to the agency, GNSS observations have increased in the spring of 2026. With regard to shipping, Traficom identifies the eastern Gulf of Finland in particular as a relevant area and advises boaters not to rely solely on their chartplotters and to carry a paper nautical chart.
In Sweden, the Swedish Maritime Administration had also warned of GPS interference in the Baltic Sea. BOOTE has already reported on the Warning from the Swedish Maritime Administration regarding widespread GPS disruptions. Among other things, it recommends alternative methods of position fixing, up-to-date paper nautical charts, radar navigation and monitoring navigation warnings.
In practical terms, the cause is of secondary importance at first glance. Whether it is widespread GNSS interference, a faulty antenna, a software issue, a power cut, an overheated tablet or a flat battery: what matters is whether the crew knows what to do when the electronic position is missing or no longer reliable.
With motorboats, there is an additional factor: speed reduces reaction time. Anyone travelling at 20 or 25 knots covers a considerable distance in just a few minutes. An incorrect position is then not just a technical nuisance, but can quickly become critical in narrow fairways, when approaching harbours, in archipelago waters, on inland waterways or in poor visibility.
When the plotter is running but isn't working properly
A GPS problem doesn’t always start with a black screen. That’s precisely what makes it dangerous. The chartplotter may continue to run whilst displaying incorrect or implausible data. The boat then jumps around on the electronic chart, appearing to be next to the fairway or even on land. The ground speed fluctuates wildly, even though the engine is running smoothly. The track zigzags, the estimated time of arrival changes erratically, or several devices suddenly show different positions.
Checking the satellite display can help. If only a few satellites are being used, or if the accuracy shown fluctuates significantly, the crew should scrutinise the position data. Even more important is checking against reality: does the position match the buoy ahead? Does the heading towards the fairway match the compass reading? Does the water depth correspond to the chart? Is a landmark located where it should be according to the electronic chart?
Even AIS is no substitute for your own observations in such situations. The positions of other vessels are also based on satellite data. If GNSS data is disrupted or tampered with, AIS targets may also become implausible. BOOTE explains in the article Navigation: keeping track with AIS, why range, timeliness and device data must always be taken into account with AIS.
The most important rule is: don’t wait until things have completely broken down. If you carry out regular checks during normal operation, you’ll spot any glitches in the electronics sooner. Navigation doesn’t just start when the plotter fails.
Preparation begins before setting sail
The best aid in the event of a breakdown isn’t tucked away in a storage locker, but within easy reach at the helm or chart table. An up-to-date paper nautical chart of the area, a pencil, an eraser, a compass, a protractor or navigation ruler, and a working compass are all part of the basic equipment. A hand-bearing compass is particularly useful when you need to determine your position without GPS.
Before setting sail, you should do more than just enter the route into the chartplotter. It is a good idea to draw up a brief, traditional navigation plan: where are the narrow channels, shoals, restricted areas, bridges, locks or traffic separation zones? Which buoys, beacons, jetties, headlands, church towers or distinctive buildings are suitable as landmarks? Where are there safe alternative harbours, waiting areas, anchorages or open water in case navigation becomes more difficult?
Electronic nautical charts remain a valuable tool. However, it is important to know which chart data is available on which device and whether it works even without a data connection. BOOTE shows in the Comparative test of electronic nautical charts for chartplotters...just how varied chart systems, data formats and plotter compatibility can be.
Smartphones and tablets can also be a useful backup. BOOTE provides an overview Boating apps: the 33 best apps for skippers as well as in the latest post The best apps for navigation and safety in 2026 various digital aids. However, apps alone are not sufficient as a backup if the problem lies with the satellite signal or if the mobile device does not use an independent positioning source.
It is equally important to be aware of the interdependencies on board. Chartplotters, tablets, AIS, autopilots, instrument displays and apps may all use the same GNSS source or process the same data from the on-board network. A second screen is therefore not automatically an independent backup. If several devices adopt the same incorrect position value, a false sense of security can easily arise.
One simple habit can significantly reduce the risk: regularly noting down your last known position. If you record your position, time, course and route on a map or in a logbook whilst underway, you won’t be left with a blank page should your equipment fail. You can then continue from this last known position.
You should keep this to hand
up-to-date paper nautical chart of the area
- Pencil and eraser
- Compass
- Compass or protractor
- a functioning steering compass
- Hand-held compass
- Logbook
- easy-to-read fish finder
- Plan for alternative ports or safe waiting areas
In an emergency, ease off the accelerator first
If the chart plotter or app fails or provides implausible data, the crew should not rush to adjust the settings. The situation on deck is what matters most. Ease off the throttle, increase the number of lookouts, check for other vessels, and, if possible, increase the distance from shoals, buoys, the shore and the edges of the fairway.
This reaction is particularly important when on a motorboat. When planing, there are often only a few moments between a potential hazard and danger. The faster the boat is travelling, the sooner you should slow down. This buys you time, reduces stress at the helm and makes it easier to check for buoys, landmarks and the traffic situation.
In narrow waters, it may be safer to briefly leave the fairway – provided the draught allows – and head for a safe waiting position or steer into deeper, clearer water. In poor visibility, heavy traffic, cross-currents or in the approach to locks and bridges, the pressure to act increases. In such cases, a decision should be made early on as to whether to continue the journey, wait it out or head for a safe spot.
The next step is to note the last reliable position of the vessel. If the electronic position was still plausible a few minutes ago, it should be immediately plotted on the paper chart, along with the time. If the last position is uncertain, visual observations, water depth, compass heading and bearings can help to narrow down the likely area.
This is where dead reckoning begins. Course, speed and elapsed time are plotted on the chart. The position calculated in this way is no substitute for GPS accuracy, but it keeps the crew able to act. It is important to allow for a generous margin of error. Currents, wind, steering inaccuracies, turning, planing and changing speed all cause the boat to drift. The longer one sails without a confirmed position, the greater the uncertainty becomes.
Navigating using a chart, bearing, plumb line and radar
When navigating on land, terrestrial navigation is the most useful aid. A bearing taken from a clearly identifiable object provides a line of position. Two bearings taken from different objects provide an observed position at their intersection. Suitable landmarks include lighthouses, fixed navigation marks, piers, headlands, church towers, distinctive buildings, bridges or other objects that are clearly marked on the map.
The angle of the bearings is important. If the lines of position intersect at a very shallow angle, the position will be inaccurate. It is better to have bearings that differ significantly. Additional checks, such as water depth, compass bearing and sightings of buoys, the shore or the coastline, increase accuracy.
The depth sounder is more than just a depth indicator. It can help to verify the presumed location. If the map shows a depth of twelve metres at the presumed location, but the sounder consistently reads three metres, something is amiss. In areas with distinct depth contours, on rivers, lakes or in coastal waters, depth can be a key factor in verifying your position.
Radar, if available and properly used, is a powerful tool. Distances to coastlines, islands, buoys, bridges or harbour jetties can be checked against the chart. Radar can be particularly helpful in poor visibility for narrowing down an uncertain position. BOOTE explains in its test Radar antennas: sharp pulses, how modern radar systems work and what is important when it comes to displaying data on the chartplotter.
Without a view of land, dead reckoning remains the primary navigation method. From the last known position, the course, speed and time are recorded. As soon as reference points, buoys, beacons, bridges, a distinct shoreline or radar echoes become visible again, the position is checked and corrected.
The plotter is an excellent tool. However, it does not replace the skipper’s responsibility to check the information it provides. Anyone who prepares a chart, compass and bearing methods, thinks ahead regularly and notes the last safe location will not lose their bearings in the event of GPS malfunctions or technical problems. The journey may then be slower and more cautious. But it remains manageable.
Lars Bolle
Chief Editor Digital
