Troubleshooting Common AIS Interference Issues on Vessels
Troubleshooting Common AIS Interference Issues on Vessels (Real-World Case Studies)
AIS interference is one of those problems that sneaks up on you. The system looks fine on the bridge. The display shows targets. Nobody is raising alarms. Then a coast station calls and asks why your vessel disappeared from their screen for six hours. Or worse-another ship calls on VHF, angry, because your AIS is reporting a position that puts you right on top of them when you are actually two miles away.
We build AIS equipment. We have spent years watching how these systems fail in the real world, on real vessels, with real crews trying to figure out what went wrong. And the patterns are surprisingly consistent. The interference almost always comes from one of four places.
Case One: The VHF Radio That Blinds Your Own AIS
The most common complaint we hear goes something like this. The AIS works fine in port. Targets show up on the screen. Then the vessel gets underway, the watch officer picks up the VHF handset to call a pilot station or another vessel, and suddenly half the AIS targets disappear. When the transmission ends, the targets come back. It happens every single time.
The root cause is receiver desensitization. AIS receives on 161.975 and 162.025 MHz. VHF voice radios transmit on 156 to 162 MHz. Those bands are close enough that a 25-watt VHF transmission can overload the front end of a sensitive AIS receiver. The AIS does not stop working. It just goes deaf for the duration of the transmission. And if the VHF is keyed frequently-during a busy port approach, for example-the AIS misses a substantial portion of the update cycle.
The US Coast Guard issued a safety alert on this exact problem. Their analysis found that a VHF transmission on your own vessel, or even transmissions from a nearby vessel roughly 136 meters away, can block AIS reception while the radio is keyed. And this can affect all classes of AIS equipment regardless of how the antennas are installed.
The fix depends on the installation. IMO guidelines say the AIS VHF antenna should be mounted directly above or below the ship's primary VHF antenna, with no horizontal separation and a minimum of two meters vertical separation. If that is not possible, the distance between the AIS antenna and any other transmitting VHF antenna should be at least five meters and preferably ten. In one case we investigated, the antennas were mounted just two meters apart, violating the separation rule entirely.
Relocating the antennas is the permanent fix. If the mast geometry makes that impossible, a high-isolation VHF/AIS splitter with at least 60 dB of isolation can help. But the splitter is a compromise, not a solution. Physical separation is always better.
Case Two: The LED Lights That Only Cause Problems at Night
This one is stranger. The AIS works perfectly during the day. Targets show up at normal range. Then night falls, the navigation lights come on, and the AIS starts dropping targets. Distant contacts vanish first. Then mid-range targets start flickering. By midnight, the screen is showing only the closest vessels.
The culprit is LED lighting. Not all LED lights. Specific models with cheap driver circuits that emit broadband radio frequency noise. That noise leaks into the VHF band and raises the noise floor at the AIS receiver. The AIS is still receiving signals, but the weak ones-the distant targets-are buried under the noise.
The US Coast Guard has been tracking this problem since 2018. In one reported incident, a maritime rescue coordination center could not contact a ship involved in a traffic separation scheme incident by VHF radio. The ship also had very poor AIS reception. The interference was traced to its LED navigation lights.
Testing for LED interference is simple. Turn off the LED lights. Tune the VHF radio to a quiet channel-Channel 73 works well. Adjust the squelch until you hear background noise. Then turn the LED lights back on. If the radio starts crackling, the lights are the problem.
The fix is straightforward. Replace uncertified LED fixtures with models that comply with IEC 60945 and have low RFI emissions. If replacement is not immediately possible, add a π-type power filter to the LED supply line and ensure the light housing is properly grounded. Route the AIS antenna cable away from the LED power wiring. The general rule is at least 30 centimeters of separation, and never bundle them together.
Case Three: GNSS Jamming That Makes You Look Like You Are Somewhere Else
This case is the most alarming. The vessel's AIS reports show it jumping kilometers off course. Sometimes the position appears on land-on an airport runway, or a beach. Nearby vessels' AIS data shows impossible speeds or erratic tracks. The bridge crew has done nothing wrong. The equipment is functioning. But the position data feeding the AIS is corrupted.
The cause is GNSS jamming or spoofing. AIS relies entirely on GNSS for position. If the GNSS signal is interfered with, the AIS broadcasts whatever false position the GNSS receiver reports.
In November 2024, a port in Qinzhou, China experienced exactly this. Multiple vessels reported AIS positions that were severely offset from their actual locations. Speed and heading data were scrambled. AIS aid-to-navigation positions were abnormal. An investigation traced the interference to an anti-drone defense system installed by a local enterprise. The system's jamming signal was bleeding into the GPS band. When the enterprise reduced the system's power output, the interference disappeared and vessel positions returned to normal.
This is not an isolated incident. Anti-drone systems, port security devices, and intentional jammers in high-risk regions are all potential sources. The fix is not something a vessel can implement on its own. The response is operational. Cross-verify AIS positions against radar, ECDIS, and visual bearings. If the data does not match, trust the other sources. Switch to dead reckoning if the GNSS is compromised. Report the interference to the local coast guard and reroute around known interference hotspots when possible.
Case Four: When You Broadcast Two Identities at Once
This one is less common but more confusing. The vessel's AIS shows up correctly on its own display. But other vessels and shore stations report seeing two overlapping identities for the same ship. Port authorities flag a duplicate MMSI. The vessel's signal conflicts with another ship's signal.
The causes vary. Sometimes it is a misconfigured AIS unit that was cloned from another vessel's settings. Sometimes it is a crew error-an MMSI entered incorrectly, or two AIS units on the same vessel both transmitting. Occasionally it is something more deliberate: a vessel operating in an area with sanctions or regulatory restrictions using a false identity to avoid detection.
The troubleshooting starts with the basics. Check that the MMSI programmed into the AIS matches the vessel's official documentation. Verify that no other AIS unit on board is transmitting. If the vessel has both a Class A and a Class B unit for redundancy, ensure that only one is active at a time. If the duplicate identity persists and no onboard cause can be found, the issue may be external. Report it to the flag state and the local coast station.
What to Do When Interference Strikes
The troubleshooting process follows a predictable sequence. Start with the simplest checks. Verify power supply, antenna connections, and cable integrity. Restart the equipment. For most temporary interference issues, a restart clears the fault.
If the problem persists, isolate the interference source. Turn off suspected equipment one at a time-LED lights, DC motors, winches, hydraulic pumps-and observe whether the AIS behavior changes. This is the same basic method that radio surveyors use, and it works because most interference sources are intermittent and correlated with specific equipment being switched on or off.
If onboard isolation does not identify the source, the interference may be external. A spectrum analyzer connected to the AIS antenna can show exactly where the noise is coming from. If the interference is in the GNSS band, it is likely jamming or spoofing. If it is in the VHF band, it could be a nearby transmitter, a malfunctioning device on another vessel, or a shore-based source.
Record everything. The time, the location, the symptoms, the equipment that was running. This record is valuable not just for your own troubleshooting but for reporting to authorities. Interference events that affect navigation safety should be reported to the flag state and the local coast station.
Why This Matters
AIS is not a convenience. It is a collision avoidance tool. When it fails, the consequences are real. The US Coast Guard has documented a case where a vessel involved in a traffic separation scheme incident could not be contacted by VHF and had degraded AIS reception. That is a vessel that could not hear, could not be heard, and could not be seen on the electronic chart displays of nearby ships.
The interference problems we see are almost always preventable. Antennas mounted too close together. LED lights with cheap drivers. Cables routed alongside power lines. GNSS interference from shore-based systems that nobody coordinated with maritime authorities.
We build our AIS equipment with these problems in mind. But we also know that even the best equipment can be defeated by a bad installation or an external jammer. The difference between a vessel that navigates safely through interference and one that becomes a hazard is the crew's ability to recognize the problem and respond. That starts with knowing what the interference looks like.
