Strange AIS data

[Conachair]

I just watched a target go up the Thames at 40Kn.

Backwards!

Target moved upstream but on opencpn the COG arrow stayed at about 83deg, speed varying from zero up the 40kn.

Whats that all about?

Mmsi found this in google

http://www.shiptracking.eu/ais/#/getvesseldetails?mmsi=235080605

http://www.shiptracking.eu/ais/#/gotovessel?mmsi=235080605

Wierd. Any idea how or why?

Nothing on Ch16 or PLA on 14

 

[Hoolie]

Your links say it's an RNLI lifeboat ... ... ... with a knackered GPS linked to their AIS?

 

[capnsensible]

Trainee Intergalactic Starship Commander practising 'alien overboard' with warp drive failure.

Again. humph

 

[Conachair]

Your links say it's an RNLI lifeboat ... ... ... with a knackered GPS linked to their AIS?

Wierd. I thought the sog & cog were calculated from positions and time between the positions. So how could it go backwards?
V strange.

 

[yesod]

do you mean heading? heading is fed from instruments so can go awry (if its missing it should be reported as '511' but that confuses some plotters even more). cog and sog are derived from the gps attached (used for sotdma timing) and are more likely to be correct.

 

[BlueSkyNick]

The Lymington - Yarmouth ferries often appear to be going backwards on AIS, when their heading and COG are opposite to each other. This is because they are double ended and although the crew take the ensign down at one end and hoist it at the other, and at night the nav lights are switched over, the heading information into the AIS is left alone.

 

[prv]

Wierd. I thought the sog & cog were calculated from positions and time between the positions.

Presumably the site linked takes the "heading" data and labels it "course", not appreciating the difference between the two. Heading is put into an AIS transmitter from a heading sensor such as a gyro compass, not calculated from anything else. There's a special value in the AIS protocol to indicate "not available" if you don't have such a sensor. Presumably theirs is meant to be connected, but not working.

Pete

 

[Conachair]

do you mean heading? heading is fed from instruments so can go awry (if its missing it should be reported as '511' but that confuses some plotters even more). cog and sog are derived from the gps attached (used for sotdma timing) and are more likely to be correct.

Opencpn only shows cog on the chart, not sure if you can look at heading. Though it recognizes the NMEA sentence.

Presumably on here it´s cog data which is "course". It shows the boat going backwards.

So maybe their unit is transmitting dodgy heading data as a cog sentence.

 

[Conachair]

Presumably the site linked takes the "heading" data and labels it "course", not appreciating the difference between the two. Heading is put into an AIS transmitter from a heading sensor such as a gyro compass, not calculated from anything else. There's a special value in the AIS protocol to indicate "not available" if you don't have such a sensor. Presumably theirs is meant to be connected, but not working.

Pete

It was going backwards live on opencpn as well. Which I´m pretty sure plots cog not heading.

 

[yesod]

Opencpn only shows cog on the chart, not sure if you can look at heading. Though it recognizes the NMEA sentence.

Presumably on here it´s cog data which is "course". It shows the boat going backwards.

So maybe their unit is transmitting dodgy heading data as a cog sentence.

i'd say its more unlikely to be an incorrect cog just because the whole point of deriving it from the gps which has to be attached was to ensure it was accurate (unlike heading, next port etc all of which are wrong far too often). i'd need to see the actual transmitted data to convince me that was the fault.

how was opencpn getting the deta? from an ais receiver or over the internet?

 

[Conachair]

how was opencpn getting the deta? from an ais receiver or over the internet?

Nasa ais engine.


There seem to be a few 511degs in the marinetraffic "course" data, so maybe that site does use the ais heading data, but I´m close to 100% certain that opencpn uses the cog sentence.
And the live plot definitely showed the boat going backwards.

 

[yesod]

capture some of the nmea stream and we can have a look at it...

 

[Conachair]

capture some of the nmea stream and we can have a look at it...

just need to figure out a way to do that, opencpn will stream the gps data but not the ais.

Plan b..

 

[Conachair]

í'¨ÇÛÛþ7BCúí'¨ÇÛÛýCBCúí'¨ÇÛÛýCBCúí'¨ÇÛÛýCBCúí'(ÇÛûþcÂÆí'(ÆJ ŽA„ùí'(Ç#ó￾ûjg[í'(ÇA¯H`½Æüí'(ÇÛ¿￾gåí'(ÇKÓmjŸ￾süí'(Æ
Ž¼j)«í'(Ç@7…coæèí'(Ç»ß}WÆüí'(Ç ÓMöëÆèí'(ÇÓ{J:óÂÇí'(Ç
gvCæüí'(ÇÛÛþcgÊí'(Æ
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ÓÖ'Ccúí'(Æ
￾5jÊâþí'(ÇÓ{j?wBÇ

That´s what´s coming in the com port

Enough for now, wish I´d taken a screenshot. Nasa manual says it does all class A & B ais messages.
Wondering now if i should email rnli and tell them some odd might be going on.

 

[yesod]

That´s what´s coming in the com port

thats line noise, not ais. did you have the right speed?

 

[Conachair]

thats line noise, not ais. did you have the right speed?

Think so. Using this http://www.eltima.com/products/serial-port-monitor/

File got bigger each time the recieve led blinked.
Anyway, might not be much to be learned, was only that one target behaving odd, it´s parked up at waterloo now not moving. Every other target is travelling in the direction of their cog leader as expected.


edit not that, this - http://www.eltima.com/products/rs232-data-logger/

 

[RobbieW]

I think this is to do with some of the static AIS data like position of sensor etc. I'll see what I can find....

 

[RobbieW]

Best guess at the moment is a badly installed electronic heading input to the AIS. I found another YBW thread that starts off by covering the same sort question.

My first thought was based on there being some sort of orientation parameter in the targets AIS setup which had been incorrectly done but can find no evidence for that.

 

[Conachair]

Best guess at the moment is a badly installed electronic heading input to the AIS. I found another YBW thread that starts off by covering the same sort question.

My first thought was based on there being some sort of orientation parameter in the targets AIS setup which had been incorrectly done but can find no evidence for that.

Cheers, I´ve left a post on the opencpn forum just to check that it´s definitely cog which is displayed by the prediction arrow. Pretty sure it is though.

 

[Conachair]

Well still no idea what was going on with that boat but getting more into opencpn.

It displays both heading and cog.

http://www.cruisersforum.com/forums/f134/target-heading-and-cog-80560.html#post936225

With heading info in the nmea stream both COG and heading are displayed, if they don't coincide, in which case COG takes precedence.

And there´s a nifty plugin which records all the nmea data and can play it back afterwards.
Here´s some (what I think is) ais data recorded just now.

!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

And a handy online decoder..
http://rl.se/aivdm

And found the boat in question.

!AIVDM,1,1,,B,13P<;WOP00wwM`2MNF:k=gw6R6it,0*51

MMSI : 235080605 Navigational status: 15 (N/A) Since navstat is 15, no further data can be shown for this message.

Now what