Satellite navigation is not always reliable everywhere. The UK’s Maritime and Coastguard Agency (MCA) is therefore urging the shipping industry to be prepared for such outages and disruptions.
In the report published on 1 October 2026, Guideline MGN 719 The authority lists, amongst others, the Baltic Sea, the Black Sea, the Gulf region and parts of East Asia as the areas particularly affected. Disruptions have also been reported in the eastern Mediterranean. The Joint Maritime Information Centre reported in a bulletin published via UKMTO Situation report dated 17 May 2026 Recurring electronic and navigational disruptions off the coast of Cyprus and off the Levantine coast. The warning therefore also applies to recreational boaters in this area.
The guideline also highlights a risk that could prove life-threatening in an emergency: Electronic distress devices also rely on reliable position data. If the signal is tampered with, AIS-MOB transmitters or modern EPIRBs may transmit incorrect coordinates. The search could then begin in the wrong location. A 406 MHz beacon for satellite distress alerts can provide additional safety. It can be located independently of the GNSS position of its transmitter.
GPS is only part of the problem
GPS is the US satellite navigation system. The technical term for this group of systems is GNSS, short for ‘Global Navigation Satellite System’. In addition to GPS, this includes the European Galileo, the Russian GLONASS and the Chinese BeiDou. The satellites transmit signals that enable receivers to determine their position and the exact time.
On board, a GNSS receiver often transmits this data to several devices. As a result, incorrect information may be fed simultaneously into the chartplotter, AIS, radio and autopilot. According to the MCA, radars with GNSS inputs, emergency radio systems and equipment used to coordinate rescue operations may also be affected.
Jamming is noticeable, whilst spoofing is not necessarily so
In the event of jamming, an interfering signal masks the extremely weak signals from the navigation satellites. The receiver loses its fix or reports poor reception quality. This is dangerous, but can often be detected.
Spoofing is more insidious. It involves transmitting artificial satellite signals, from which the receiver calculates an incorrect position or time. This data may appear plausible. The vessel may then appear to be a few nautical miles off course at a credible position.
The Royal Institute of Navigation (RIN) describes the technical implications in its report published in January 2026 Report on GNSS disruptions in shipping. The clear warning contained therein is that connected devices – and distress systems in particular – may provide incorrect position and time data in the event of a fault.
For the crew, sudden jumps in position or implausible changes in course and speed over ground may serve as warning signs. Equally suspicious are AIS targets in obviously incorrect locations or discrepancies between the radar image and the electronic nautical chart. A satellite display that initially appears normal does not rule out spoofing. A smartphone or handheld GPS can also receive the same deceptive signals and is therefore not, on its own, a reliable cross-check.
From a navigation error to a distress situation
The potential unreliability of satellite-based electronic navigation can put a crew in difficulty. However, traditional chart navigation using a compass and bearings, as well as independent radar positioning, can help to verify one’s position and continue the journey safely. Conversely, the situation can become critical or even life-threatening if emergency beacons report an incorrect position. The crew cannot correct the position of these beacons’ built-in GNSS receivers by taking a bearing.
The warning also applies to the GMDSS, the global maritime distress and safety radio system. One example is the Digital Selective Calling (DSC) system: when a distress call is triggered on a VHF radio, an available position is automatically transmitted. If this position originates from a tampered GNSS receiver, even this initial distress message may give an incorrect location for the incident.
This does not mean that all maritime distress communications fail in the event of GPS disruptions. VHF radio does not require a satellite position for the actual radio transmission. The crew can still describe the emergency and point out any questionable coordinates. However, with automatic emergency transmitters, this option for immediate geolocation is not available.
When MOB transmitters and emergency beacons report incorrect positions
An AIS MOB transmitter determines its position using a built-in GNSS receiver and then transmits the coordinates via AIS on VHF to vessels within range. GPS therefore provides the basis for the position calculation. The device then transmits the coordinates determined in this way directly by radio to the yacht carrying out the search.
Consequently, the accuracy of the transmitted position depends on the GNSS receiver: if it accepts tampered satellite signals, it will calculate incorrect coordinates, which the AIS-MOB transmitter will then relay. The crew and other vessels might then search in the wrong location. In the event of jamming, the current position may be completely missing. Some AIS-MOB devices can also trigger a DSC alarm. However, this does not alter the fact that any position transmitted also originates from the GNSS receiver.
An AIS-SART (AIS Search and Rescue Transmitter), which can also be used, for example, to locate a life raft, likewise uses GNSS for positioning and AIS for transmission. An AIS-SART does not automatically include an additional DSC function. It should not be confused with a conventional radar SART: the latter responds to signals from a search radar and does not require a GNSS position to do so.
Even modern EPIRBs and PLBs can be affected by incorrect satellite positions. An EPIRB (Emergency Position-Indicating Radio Beacon) is a distress beacon registered to the vessel. A PLB (Personal Locator Beacon) is carried by an individual. Unlike a dedicated AIS MOB transmitter, both alert a rescue coordination centre via the international Cospas-Sarsat satellite system.
Here, too, position determination and transmission are two separate processes. A built-in GNSS receiver calculates the coordinates. The beacon incorporates these into its digital distress signal and transmits them at 406 megahertz to the search and rescue satellites. If the receiver accepts a tampered position as correct, both EPIRBs and PLBs can therefore transmit incorrect coordinates.
The different roles and alerting procedures of AIS-MOB, EPIRB and PLB are crucial for selecting and assessing one’s own emergency equipment. Not every product labelled as a MOB system operates on the same principle; some combined devices link several radio channels.
Why EPIRBs and PLBs can still be located
A faulty GNSS receiver does not automatically silence a 406-megahertz beacon. The reception of navigation signals and the transmission of the distress call take place on different frequencies. An interference source in the GNSS frequency band therefore does not necessarily prevent an alarm from being triggered via Cospas-Sarsat.
The system can also locate a beacon regardless of its own GNSS position. The Technical explanations of the NOAA SARSAT programme describe the satellite systems used for this purpose: In the case of LEOSAR, the Doppler shift of the beacon signal is analysed. MEOSAR enables independent positioning based on differences in transit time and frequency of the transmission received via several satellites. Geostationary GEOSAR satellites can quickly relay an alarm, but cannot determine their own position without the coordinates transmitted alongside the signal.
In the event of spoofing, the emergency control centre may therefore have conflicting information regarding the accident location: the GNSS coordinates reported by the beacon and a position determined independently from the 406-megahertz transmission. The latter can therefore offer additional security, but is no guarantee that an incorrect position will be detected immediately and that any delay will be avoided. A pure AIS-MOB transmitter does not have this independent positioning via Cospas-Sarsat.
Where beacons are suitably equipped, a direction-finding signal at 121.5 megahertz is also transmitted. Rescue teams with the appropriate equipment can use this to determine the direction of the transmitter without having to rely on its GNSS coordinates. This signal assists with navigation to the scene.
The rescuers, too, must arrive without GPS
If the rescuers are in the same area of interference, their own GNSS receivers may also fail or provide incorrect data. In that case, they must verify their route using independent methods and, if necessary, continue without reliable satellite navigation.
This has a practical implication for a yacht’s crew. When making an emergency call via VHF radio, clearly identified landmarks, bearings, water depths and the time of the last reliable position can supplement the description of the situation. If there is any doubt about the electronic coordinates, this should be explicitly stated. A suspected GNSS jammer is no reason to refrain from making a necessary emergency call or activating an emergency beacon.
The North Sea and the Baltic Sea: different locations
There is one officially recorded incident in the northern North Sea: in November 2025, the British authorities reported GPS jamming during an operation in the waters north of Scotland whilst en route to the Faroe Islands. According to the Royal Navy and civilian vessels. In the Baltic Sea, however, the situation is quite different: Warnings are in place across a wide area there. All countries bordering the Baltic Sea are issuing ongoing warnings about disruptions to GNSS, AIS and radar. These affect large parts of the Baltic Sea, including the Gulf of Finland, the Gulf of Riga and the Sea of Åland. Previous GPS disruptions in the Baltic Sea have already demonstrated just how extensive the affected areas can be. Crews in these areas must expect satellite positions to be temporarily unavailable or incorrect.
Which equipment offers greater safety?
Compared with a standard AIS MOB transmitter, a 406-megahertz beacon provides an additional safety net in the event of GNSS interference: EPIRBs and PLBs can not only alert the relevant rescue coordination centre, but can also be located independently of the GNSS coordinates they transmit. This is the crucial advantage in relation to the risk described here. However, this does not mean that a PLB replaces the AIS MOB transmitter in every situation. The two systems fulfil different functions.
If someone falls overboard, the vessel’s own crew are best placed to help. AIS helps them locate the person on the chart plotter, provided that position determination is working. An additional 406 MHz distress alert calls in the organised maritime rescue services and provides an independent means of location if the crew cannot find the person or the GNSS position is unusable. For personal safety, therefore, a combination of AIS-MOB and a 406 MHz PLB makes more sense than relying on AIS alone. Both functions can be provided by separate devices or a suitable combined unit.
In the event of an emergency involving the entire yacht, a registered 406-megahertz EPIRB with a 121.5-megahertz homing transmitter is a sensible basic requirement. An additional AIS function can make it easier for nearby vessels to locate the yacht. However, an EPIRB that remains on board is no substitute for a transmitter worn on the body: if a person drifts away from the yacht, the distress signal must pinpoint their location, not that of the boat. The specific choice therefore also depends on whether the vessel, a life raft or an individual person is to be safeguarded.
The beacon operating at 121.5 megahertz provides an additional means of locating the vessel. Rescue vessels or aircraft equipped with the appropriate technology can determine the direction to the beacon without needing to retrieve its GNSS coordinates. To do so, they must be able to receive the signal and have suitable direction-finding equipment. It is not a display that appears on every yacht plotter. The Combination of 406 megahertz, AIS and bearing signal is now also available in personal emergency beacons.
However, this does not automatically mean that a combined device is protected against spoofing. Its coordinates, transmitted via AIS and satellite, may originate from the same spoofed GNSS receiver. The added security lies in the various alerting and positioning methods: direct notification to nearby vessels, alerting the rescue coordination centre, independent satellite positioning and radio direction-finding.
A different fallback option is still required for navigation: Compass, nautical chart, bearings and, where applicable, radar.
What GPS disruptions have you experienced so far, and what impact did they have on your on-board equipment?
Ursula Meer
Redakteurin Panorama und Reise