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Unsafe AIS oversight

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I've found a specific combination of Class B AIS transceiver and vessel speeds which do not show a vessel's icon on some MFDs.

The AIS unit I discovered the issue with is a B&G NAIS500. I'm sure all Navico units would be the same. As far as I can tell, the actual manufacturer for Navico units is Alltech Marine Electronics Corp (AMEC). I would also presume all AMEC units regardless of whose name is on it would present the same issue. The MFDs I'm aware of that play an integral part in this issue are Standard Horizon (model CP300I) and Garmin (model 740S).

The issue occurs when the suspect AIS measures a low or zero SOG, whereupon it sets COG as invalid. The suspect MFDs do not display an icon without a valid COG. However, the AIS vessel is listed in the AIS List. So the MFD knows the vessel's location but does not display the icon on the chart. When the AIS vessel's SOG increases, the COG becomes valid and the icon is then displayed on the chart.

Looking further into this issue I found other AIS units set COG as valid when SOG is low or zero. So these AIS units' icon will be displayed on the suspected MFDs. Also, other MFDs (verified on B&G Zeus Touch and Raymarine A series) will display an icon even when COG is invalid.

For my setup, I also found that if Heading is sent to the NAIS500 it will transmit this parameter in AIS Message 18 and the suspected MFDs will then display an icon. This lead me to conclude that the suspected MFDs need at minimum either a valid COG or valid Heading to display the icon. If both are invalid then the icon is not displayed. Heading can be sent via NMEA0183 or NMEA2000, I tested both with success.

If you have a suspected AIS and want to be seen on all MFDs but do not want to wait for firmware updates, then look into sending Heading to your AIS.

Has anyone else noticed this issue? Was a particular vessel's icon display sometimes and not at other times? Could this difference be attributed to being underway versus anchored or moored? Do you have a Navico, McMurdo, or AMEC AIS and someone didn't see you on their MFD? Do you have a Garmin, Standard Horizon, or other MFD and didn't see a known AIS vessel on your MFD? If known, who produced the AIS that wasn't displayed?

7 Replies

  • Thanks, Phil. This looks like carefully done research well explained, and hopefully some boaters with the relevant equipment can check it out.

    Incidentally, what does invalid COG look like? Is it expressed as dashes or 360? (I've seen both from Heading sensors that could not sense Heading.)

  • In message 18 (Class B position report) the actual value sent is 360. But, 360 is defined to be "Undefined (Default)". Values between 0 and 359 are "Defined" values. The receiver, or rather the display of the received message, can show anything the programmer chooses. I've seen dashes on a few displays, but I don't believe there is a standard that all programmers adhere to when it comes to displaying invalid, undefined, default, unknown, etc. values.

  • How do I know if I have this issue? I have a Digital Yacht AIS. When I am not moving, the course as reported on Marine Traffic is 0. I have OpenCPN and a CP180 that both get the AIS data feed. Should I look for anchored targets that don't appear on the CP180?

  • Hi Coquina. Yes, it would be good if you looked for stopped AIS targets that don't appear on your Standard Horizon CP180, but this odd little issue does seem to involve a combination of certain AIS transceivers made by Alltech and certain MFDs. And I've got some fresh information on it I'll put in next comment.

  • Phil, Navico has confirmed the bug you found! Here's what they've found so far:

    "Regarding the possible new issue with NAIS-500, here is a quick overview of our findings;

    Situation occurs like this;.
    - Boat A has an NAIS-500
    - Boat B has a Garmin 740s or 7612xsv with any AIS receiver.

    If Boat A is not moving (SOG = 0) then COG is unknown so will be transmitted as an invalid number (375°). This is standard practise.

    If Boat B has a 740s then Boat A will not appear on the chart screen. You can still see Boat A if you go into the vessels tab.

    If Boat B has a 7612xsv then Boat A appears fine.

    The issue only appeared on the 740s. I believe the issue is in the Garmin software and the issue is that the device gets confused by the invalid COG value and so doesn’t display anything.

    Interestingly, if you know where the vessel is meant to be and you tap the chart screen there it pops up a dialog box with the vessel name (see attached photo).

    When the 740s was running an older SW version (2.90) the issue was not present and everything worked fine.

    The issue only appeared when the device was updated to the latest SW, 4.30."

  • Question: Does Marine Traffic translate bad COGs to 0 degrees or does my AIS transceiver set 0 degrees when not moving?
    I think I will call Digital Yacht and ask. If I understand this issue, other people will see just fine with a 0 degree heading.

  • Ben,
    Good to hear Navico is able to replicate this issue. Now to wait for a s/w update.
    Regarding Navico's comment stating "This is standard practise". Only for Navico & Allteck. Most (or all) other AIS units set COG=0 when SOG=0.

    From what I've seen so far on Marine Traffic, you can not use it to see if you have the problem. I've looked at two stationary vessels that I can see on my AIS as well as on Marine Traffic. On OpenCPN one shows a valid COG and the other does not. On Marine Traffic they both show 0 deg Course when I hover the mouse over the vessel. But, when viewing vessel details and looking under latest position, both show a dash for course.

    One way to determine if your AIS has this problem is to find a friend with OpenCPN and have him/her look at your vessel's COG while you are stationary. If he/she sees a dash then you have the problem, if he/she sees a number then you do not have the problem.

    Yes, all MFDs will show your icon on the chart if you are sending 0 degree COG or Heading.