Erroneous GPS coordinates from AIS Messages possibly with "null" values

[Bucket1616]

While tracking down an issue in SignalK/n2k-server, I found that if an ATON sends out a location update that looks like this:

{
  "pgn": 129041,
  "prio": 4,
  "src": 20,
  "dst": 255,
  "timestamp": "2026-02-03T00:54:37.244Z",
  "description": "AIS Aids to Navigation (AtoN) Report",
  "fields": {
    "messageId": "ATON report",
    "repeatIndicator": "First retransmission",
    "userId": null,
    "longitude": -111.8481066,
    "latitude": -111.8481066,
    "positionAccuracy": "Low",
    "raim": "not in use",
    "timeStamp": "0",
    "lengthDiameter": 2,
    "beamDiameter": 2,
    "positionReferenceFromStarboardEdge": 1,
    "positionReferenceFromTrueNorthFacingEdge": 1,
    "atonType": "Default: Type of AtoN not specified",
    "offPositionIndicator": "No",
    "virtualAtonFlag": "Yes",
    "assignedModeFlag": "Autonomous and continuous",
    "spare": 0,
    "positionFixingDeviceType": "Surveyed",
    "reserved19": null,
    "atonStatus": 0,
    "aisTransceiverInformation": "Channel A VDL reception",
    "reserved22": null,
    "atonName": "ROXK@@@@@@@@@@@@@@@@"
  },
  "id": "aisAidsToNavigationAtonReport"
}

The userId being null causes SignalK to assume the message has a context of "Self". This has been found and addressed in SignalK/n2k-server#2308, and prevents a mis-behavaing AIS device from causing the "Self" position to move to something like "-111.8481066,-111.8481066", as is seen in the above example. (This caused continuous false alarm problems with the Anchor Alert plugin.)

I tried to track down where these position numbers are coming from in code with the help of Google Gemini, but could not for the life of me find it. It's possible that this is what is actually being sent out by the ATON. Ideally, if the value is null, which Gemini seemed to think this was a number that could results from a 7FFFFFFF value being mis-converted, the value should create a Delta with a position of (null, null), not some wrong, but possible coordinates.

[Bucket1616]

I just checked to see hos this erroneous ATON is showing up on my B&G Chartplotter and the answer is that it does not. This is likely not worth fixing, since I'm guessing most devices will just ignore it. Please feel free to close if you agree.

[sbender9]

Do you have the raw N2K data for this? Should be able to get it using visual-analyzer.

[sbender9]

Assuming the userId is really null in the n2k message, then it is best handled in n2k-signalk.

[Bucket1616]

Hi Scott,The userId is definitely null in the raw data as per the visual analyzer. I have already identified and posted a fix (which was merged) to N2K-signal.The position was coming in as -111.3……., -111.3……. (don’t remember the exact values) in that same message.   Since the userId was addressed, I wanted to pass on the issue with the odd gps cords in case those should be addressed too. My gut says it doesn’t matter, but I decided to still post the issue for you to evaluate. If the values shown in the signal analyzer is exactly how the packet comes across the bus, then noting can be done and it’s fine. I just don’t know enough about the backend of the system to know that for sure. MikeOn Feb 5, 2026, at 09:02, Scott Bender ***@***.***> wrote:sbender9 left a comment (canboat/canboatjs#393) Assuming the userId is really null in the n2k message, then it is best handled in n2k-signalk. —Reply to this email directly, view it on GitHub, or unsubscribe.You are receiving this because you authored the thread.Message ID: ***@***.***>

[sbender9]

Can you get me the raw data so I can do some testing?

[Bucket1616]

I have a filter set up to catch it, but it's been a while and no luck. It was a mis-behaving ATON near Panama City, Panama. Not sure if it just died or what, but it is not transmitting AIS anymore. I was able to go back in my logs and find a debug message I created in the Anchor Alarm Plugin: 2026-02-03T00:06:37.299Z signalk-anchoralarm-plugin FULL UPDATE: [{"source":{"label":"N2K","type":"NMEA2000","pgn":129041,"src":"20","deviceInstance":0},"$source":"N2K.20","timestamp":"2026-02-03T00:06:37.165Z","values":[{"path":"navigation.position","value":{"longitude":-111.8481066,"latitude":-111.8481066}}]}] Below is a capture I just got from a working ATON: { "pgn": 129041, "prio": 4, "src": 20, "dst": 255, "timestamp": "2026-02-05T18:48:10.728Z", "description": "AIS Aids to Navigation (AtoN) Report", "fields": { "messageId": "ATON report", "repeatIndicator": "Initial", "userId": 993511045, "longitude": -79.51943, "latitude": 8.8929216, "positionAccuracy": "High", "raim": "in use", "timeStamp": "10", "lengthDiameter": 2, "beamDiameter": 2, "positionReferenceFromStarboardEdge": 1, "positionReferenceFromTrueNorthFacingEdge": 1, "atonType": "Floating AtoN: special mark", "offPositionIndicator": "No", "virtualAtonFlag": "No", "assignedModeFlag": "Autonomous and continuous", "spare": 0, "positionFixingDeviceType": "GPS", "reserved19": null, "atonStatus": 0, "aisTransceiverInformation": "Channel B VDL reception", "reserved22": null, "atonName": "FLAMENCO BUOY@@@@@@@" }, "id": "aisAidsToNavigationAtonReport" } Some of the values are obviously different from what I saw with the failing one(atonName, positionaccuracry, etc), however, what I know with 100% certainty is that the following fields had these values, exactly as you see them below: "userID": null, "longitiude": -111.8481066, "Latitude": -111.8481066, Mike

…

------ Original Message ------ From: "Scott Bender" ***@***.***> To: "canboat/canboatjs" ***@***.***> Cc: "Michael Pail" ***@***.***>; "Author" ***@***.***> Sent: 2/5/2026 1:26:44 PM Subject: Re: [canboat/canboatjs] Erroneous GPS coordinates from AIS Messages possibly with "null" values (Issue #393)

sbender9 left a comment (canboat/canboatjs#393) <#393 (comment)> Can you get me the raw data so I can do some testing? — Reply to this email directly, view it on GitHub <#393 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/BCH7VWS7YNX6ZT4G7SJ3SF34KODOJAVCNFSM6AAAAACTYEJL6OVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTQNJUHE2TANBYGU>. You are receiving this because you authored the thread.