“Ghost” AIS targets (that patently I was about to crash in to)

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Phil_boat

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Last week to the West of Arran I had an AIS target pop up twice briefly that my plotter thought we were about to crash in to.

It alarmed, marked the crash site and then disappeared.

My first thought was it was alarming against ourselves, but it was a different MMSI.

Nothing on marine traffic, no SAR helis around and no (obvious) submarines.

Any ideas? Never done it before or since and unfortunately because it was so brief I don’t have the MMSI to look up what it was.
 
Phil_boat said:
Last week to the West of Arran I had an AIS target pop up twice briefly that my plotter thought we were about to crash in to.

It alarmed, marked the crash site and then disappeared.

My first thought was it was alarming against ourselves, but it was a different MMSI.

Nothing on marine traffic, no SAR helis around and no (obvious) submarines.

Any ideas? Never done it before or since and unfortunately because it was so brief I don’t have the MMSI to look up what it was.
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What were the atmospheric conditions? they can play havoc with RF sometimes :unsure:
 
Phil_boat said:
Last week to the West of Arran I had an AIS target pop up twice briefly that my plotter thought we were about to crash in to.

It alarmed, marked the crash site and then disappeared.

My first thought was it was alarming against ourselves, but it was a different MMSI.

Nothing on marine traffic, no SAR helis around and no (obvious) submarines.

Any ideas? Never done it before or since and unfortunately because it was so brief I don’t have the MMSI to look up what it was.
Click to expand...
I think I've seen the sort of behaviour you are talking about. A vessel transmits its AIS location at some point in time, then for whatever reason we don't see it transmit again (we/it gone round a headland, moved out of range, turned off etc). We might have been miles away when the boat was at the last known position and so had little interest in it. By the time we get there a while later the boat has long since disappeared but we still have its last location stored so it shows up on the plotter. I think there are settings somewhere for how long to keep displaying vessels after you don't hear from them again. Our AIS antenna is mounted low down which makes it a bit more flaky about when it can / can't see stuff.
 
If it was a safety beacon you’d have the details in your log either on the plotter or AIS system.
French fishing boats transmit an AIS beacon (alongside the boat transmit) when they drop nets so you know where the net is so the transmitter may not be where the blip is on screen.
If it was brief I’d assume it was something starting up with a bad fix that then corrected itself. A boat in VHF range could do this unknowingly and from Arran there are plenty of places a boat could be with cold GPS stating up with a bad position after winter.
 
bikedaft said:
Watch out for submarines...

(It doesn't sound like that, but they are often around there...)
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While up there a few years ago we had several occasions when RAF jet pilots targeted: You'd see/hear one of them coming in fast and low, then while you were watching that one, a second would arrive going even faster and lower from another, direction; it scared the crap out of us. When we related this experience in a local yacht club, we were told 'Aye and the submarine boys will likely have torpedoed you a few dozen times too... But they sunk you more quietly'
 
lustyd said:
If it was a safety beacon you’d have the details in your log either on the plotter or AIS system.
French fishing boats transmit an AIS beacon (alongside the boat transmit) when they drop nets so you know where the net is so the transmitter may not be where the blip is on screen.
If it was brief I’d assume it was something starting up with a bad fix that then corrected itself. A boat in VHF range could do this unknowingly and from Arran there are plenty of places a boat could be with cold GPS stating up with a bad position after winter.
Click to expand...

Not sure if the plotter keeps a log but I’ll have a look next time I’m down.

The bad fix is probably the most likely, but least interesting, reason!
 
The log is only for beacons and messages, not standard AIS targets. We got a “boat message” on AIS the other day from Portsmouth naval base. Took me 20 minutes to find the log and then couldn’t work out what the message was but at least found the location 😂
 
I have had an AIS alarm or display occasionally that was detecting an own boat collision!

Not usual but worth checking.

It was easily overcome in the short term by putting own mmsi number in the search list.

It required a software update from China!
 
TSB240 said:
I have had an AIS alarm or display occasionally that was detecting an own boat collision!

Not usual but worth checking.

It was easily overcome in the short term by putting own mmsi number in the search list.

It required a software update from China!
Click to expand...

It’s not own boat as the MMSI displayed wasn’t mine. That was an issue with the old plotter though, it would display our boat permanently. It’s fine now it’s connected to the Garmin and ONWA have just released an update to switch off own boat output.
 
Phil_boat said:
Any ideas? Never done it before or since and unfortunately because it was so brief I don’t have the MMSI to look up what it was.
Click to expand...
It might have been one of those fishing net AIS transmitters (usually Chinese origin), their transmitting signal can be set up to several minutes intervals; if they are very near the water level, say on a pot pole, you might have received its transmission while inside its limited range, then sailed outside their range before it sent the following signal.
 
Alex_Blackwood said:
What were the atmospheric conditions? they can play havoc with RF sometimes :unsure:
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That would increase the number of targets seen, but the AIS message contains the Lat Long of the vessel so wouldn't place it in the incorrect place.

Does the OP's display have the ability to filter out targets not seen within x mins?
 
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