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Old 21-04-2012, 14:58   #1
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Target heading & COG

I saw a strange thing today, a target travelling in a roughly west direction at up to 40kn (turned out to be a life boat from the mmsi no) , but the ais COG arrow predictor was pointing at 84deg, almost backwards.
I suspect a fault on the targets system, but could someone confirm....

  • Is the COG arrow onscreen derived from the targetīs COG transmitted data?

  • And is it possible the display the targets heading data in opencpn?

Hereīs the boat - Shiptracking.eu

Interestingly, looking at a few other vessils tracks there were quite a few 511deg tracks, I wonder if the likes of marine traffic use hdg instead of cog. Bit odd if they did - anyone know?

Ta V much
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Old 21-04-2012, 15:44   #2
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Re: Target heading & COG

Click image for larger version

Name:	hdg-cog.png
Views:	81
Size:	16.2 KB
ID:	40227

With heading info in the nmea stream both COG and heading are displayed, if they don't coincide, in which case COG takes precedence.
In the pic above COG ends with the square and HDG ends with an open circle.
I'm not quite sure how the HDG predictor would behave if going 40 kts backward, especially the length. Of course this can be tested by "composing" relevant nmea sentences. This is a case that OpenCPN should handle in reasonable way as, for example some ferries, could possibly display this behavior on a regular basis.

It would be very interesting if you could catch the nmea stream with the VDR(voyage data recorder) plugin, if you get another chance.

Thomas
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Old 21-04-2012, 16:19   #3
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Re: Target heading & COG

Thanks, didnīt know of that plug in. Been trying to get to the ais data for a while Justto see what it looks like.

Looks like this..

Not sure if there is any heading data in there, off to google we go

Quote:
!AIVDM,1,1,,B,63P:gGt0SJPB>P7mU:=TbqttVh,4*3E

!AIVDM,1,1,,B,13P:V@hP10OwOcLMNGG:b?wh28Q2,0*28

!AIVDM,1,1,,B,13P:gGhP1sOwcDDMN9Gsr?wj2@QP,0*61

!AIVDM,1,1,,B,63P8a2H0000206P020,4*01
!AIVDM,1,1,,B,13P:`4hP00OwbODMN8ppgOv026hP,0*19

!AIVDM,1,1,,B,63P:gGh0SJP@>P7mU:=TbqttVh,4*20

!AIVDM,1,1,,B,63P:`4h0SJP@>P4hP9S3G=Vvl0,4*76

!AIVDM,1,1,,B,63MCPbt0SJP@>P6WKGkGMl1KD0,4*55

!AIVDM,1,1,,B,13P:gFPP4bOwn7rMN@asTwv82D0q,0*22

!AIVDM,1,1,,B,13P:gGhP1tOwc90MN9wKr?v82D0g,0*66

!AIVDM,1,1,,B,63P8a2H0000206P020,4*01

Edit.
http://rl.se/aivdm
Isnīt the web just wonderful
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Old 22-04-2012, 02:36   #4
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Re: Target heading & COG

What I said in my last post applies to "own ship". For AIS targets, the info is there, but it's not displayed on the screen directly.

Click image for larger version

Name:	hdg-cog1.png
Views:	56
Size:	38.3 KB
ID:	40255

In the AIS target query box both heading and course are displayed, and above, they are not the same.
OpenCPN gives one indication of this, by showing the direction of the turn, but then the rate of turn must be above a minimum value. It looks like this:



"conachair", if you copy your ais sentences into a text file and point VDR to it, you will see some action on the Thames!

Thomas
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Old 22-04-2012, 02:45   #5
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Re: Target heading & COG

Quote:
Originally Posted by cagney View Post
What I said in my last post applies to "own ship". For AIS targets, the info is there, but it's not displayed on the screen directly.



In the AIS target query box both heading and course are displayed, and above, they are not the same.
OpenCPN gives one indication of this, by showing the direction of the turn, but then the rate of turn must be above a minimum value. It looks like this:



"conachair", if you copy your ais sentences into a text file and point VDR to it, you will see some action on the Thames!

Thomas
Thanks for that, seems that no boats on the river are transmitting heading data, which is why I couldnīt find it in the ais target query. I had a look by decoding a few of the sentences online (they came out of VDR in the first place) , but none had heading data, or rather it was 511deg. Which i believe is the same thing.
The boat which was behaving strange is still berthed up river, the ais query doesnīt show head but just course, wonder if it could have been transmitting heading data as a course data sentence?
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