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Old 23-03-2015, 19:33   #1
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Possible issue with AIS and VDR playback???

A guy on Sailnet posted this:
Yesterday we were sailing near Angel Island, when I looked at the plotter screen, and saw an AIS target moving fast, close, and on collision course (plotter shows a dotted line of projected course for AIS targets, the length of which relates to speed).

I looked in the direction of the target, and there was nothing there. No boats at all, not for miles. The target on the screen got closer and closer, until it passed straight through us - then vanished.

If this had been a foggy day, this would have quite freaked me out. Thankfully it was a nice sunny day.

Has this glitch been heard of before, or have we witnessed the first AIS ghost ship?

I wish I had had the presence of mind to select the target and see what the name of the vessel was.
I responded with this. Is this a possible vulnerability with OpenCPN's VDR plugin?
What OP describes could be all-too-easy to do, either on purpose or as an innocent mistake.

For instance, OpenCPN has a plugin that records and plays back NMEA instrument data. It can also share its NMEA data (both real-time and recorded) with other devices over a TCP/UDP network or Bluetooth connection. So if a guy records his track, then goes back to his marina or anchorage and replays it with his AIS transponder connected to the network, everyone within receiving range is going to see his previous track played out in real time.

I hadn't thought of this before - I may need to report it to the OpenCPN developers, since it's a potential liability if their program is abused in that way (intentionally or by accident).
My AIS is receive only, so I've never actually tried this. But I have taken VDR playback and shared it over a WiFi network with a second computer, so I see no reason that it couldn't be shared with an AIS transponder, creating a dangerous phantom vessel.

Have you guys thought of this issue before?

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Old 23-03-2015, 20:40   #2
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Re: Possible issue with AIS and VDR playback???

AIS tranceivers do not transmit NMEA stream they receive, they construct the transmission from their own, dedicated GPS. It does not mean that a VHF device transmitting total AIS crap can't be constructed, but it definitely can't be any normal AIS device and OpenCPN with VDR plugin running.


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Old 23-03-2015, 23:39   #3
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Re: Possible issue with AIS and VDR playback???

Consider that an AIS unit with a broken GPS (antenna or unit itself) can generate erratic data.

And we have all seen commercial class A AIS targets where the heading informations are just garbage. Completely incongruent with the real COG.

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Old 24-03-2015, 00:27   #4
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Re: Possible issue with AIS and VDR playback???

Could simply have been a GPS that was totally out of whack and therefore transmitting a false location. It does happen, but not usually to that extent. Was there any boats within visual range?
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Old 24-03-2015, 22:01   #5
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Re: Possible issue with AIS and VDR playback???

Actually, class B AIS transponders have a built-in GPS that is always used. I believe that class A transponders can be set up with external location input and hence could - in theory at least - use recorded tracks.

It could also be a fault in the transponder. Last summer I noticed a boat that seemed to go 60 knots and have a very erratic location. This happened several days over a couple of weeks and I finally called him and he restarted the AIS, which cleared the problem. The coast guard had also made note of the problem, so it was not just my installation.

On another occasion the GPS signal from my own AIS (an AMEC-101) sent out erratic data to OpenCPN causing the display of my own ship to be jumping all over the chart. I turned the AIS off and on and never saw the problem again.

Cheers, ....Erik.
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