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03-12-2024, 13:26
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#16
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Registered User
Join Date: Jun 2009
Location: Elyse is in New Zealand
Boat: Amel Super Maramu 2000
Posts: 605
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Re: Heading and COG
And the log from proAIS.........
#
# proAIS2 Started log file at 09:24:52 2024/12/04
#
!AIVDO,1,1,,,B3M@pMP01;7F`AJu9FCQ1lnQl000,0*72
$GNRMC,202446.00,A,3515.85565,S,17411.86886,E,0.29 5,,031224,,,A,V*04
!AIVDO,1,1,,,B3M@pMP00s7F`@ru9F?Q1lo1l000,0*1F
$GNRMC,202447.00,A,3515.85567,S,17411.86873,E,0.39 5,,031224,,,A,V*0C
!AIVDO,1,1,,,B3M@pMP01;7F`?ru9F?Q1loQl000,0*49
$GNRMC,202448.00,A,3515.85570,S,17411.86860,E,0.34 2,,031224,,,A,V*0D
!AIVDO,1,1,,,B3M@pMP00s7F`?Ju9F?Q1lp1l000,0*47
$GNRMC,202449.00,A,3515.85574,S,17411.86848,E,0.31 4,,031224,,,A,V*01
!AIVDO,1,1,,,B3M@pMP00s7F`>ru9F?Q1lpQl000,0*1E
$GNRMC,202450.00,A,3515.85574,S,17411.86834,E,0.37 9,,031224,,,A,V*09
!AIVDO,1,1,,,B3M@pMP01;7F`=ru9F?Q1lq1l000,0*35
$GNRMC,202451.00,A,3515.85575,S,17411.86821,E,0.34 7,,031224,,,A,V*00
!AIVDM,1,1,,A,14`Uwq@wh1<M0fKclaI02J;V0HNH,0*6E
!AIVDO,1,1,,,B3M@pMP00s7F`=Ju9F;Q1lqQl000,0*20
$GNRMC,202452.00,A,3515.85577,S,17411.86810,E,0.32 5,,031224,,,A,V*07
!AIVDO,1,1,,,B3M@pMP00s7F`<ru9F;Q1lr1l000,0*7A
$GNRMC,202453.00,A,3515.85578,S,17411.86799,E,0.29 1,,031224,,,A,V*09
!AIVDO,1,1,,,B3M@pMP00s7F`<Ju9F;Q1lJQl000,0*1A
$GNRMC,202454.00,A,3515.85579,S,17411.86789,E,0.32 5,,031224,,,A,V*00
!AIVDM,1,1,,B,B7`NA`03wk7?8nru07cQ3ws1jCwJ,0*3E
#
# proAIS2 Stopped log file at 09:24:54 2024/12/04
#
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03-12-2024, 14:01
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#17
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Registered User
Join Date: Sep 2023
Location: Cruising
Posts: 571
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Re: Heading and COG
still with wrong checksum, "$GNRMC,202446.00,A,3515.85565,S,17411.86886,E,0.2 9 5,,031224,,,A,V*04"
should be *24
signalk decodes the correct checksum to >
[
{
"updates": [
{
"source": {
"sentence": "RMC",
"talker": "GN",
"type": "NMEA0183"
},
"timestamp": "2024-12-03T20:24:46.000Z",
"values": [
{
"path": "navigation.position",
"value": {
"longitude": 174.19781433333333,
"latitude": -35.26426083333333
}
},
{
"path": "navigation.courseOverGroundTrue",
"value": null
},
{
"path": "navigation.speedOverGround",
"value": null
},
{
"path": "navigation.magneticVariation",
"value": null
},
{
"path": "navigation.magneticVariationAgeOfService",
"value": 1733257486
},
{
"path": "navigation.datetime",
"value": "2024-12-03T20:24:46.000Z"
}
]
}
],
"context": "vessels.self"
}
]
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03-12-2024, 14:48
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#18
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Registered User
Join Date: Mar 2011
Posts: 856
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Re: Heading and COG
The heading is coming from your AIS in the VDO messages and it does differ from COG by about 30 degrees. I have no idea how your AIS generates true heading.
As we've all commented previously, for some reason your AIS generates an invalid checksum for RMC sentences which ordinarily would be rejected by OpenCPN. Also as we have observed, the RMC sentence does not contain a value for COG. And some of the VDM sentences, though not all, also have an invalid checksum.
Also important to note is that some of your VDO sentences occasionally report heading unavailable and sometimes COG unavailable.
I believe you have a problem with your AIS which needs to be fixed before you can confidently use it as a reliable source for navigation data.
Perhaps send a copy of the log output from ProAIS to Emtrak
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03-12-2024, 14:56
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#19
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Registered User
Join Date: Nov 2012
Location: Orust Sweden
Boat: Najad 34
Posts: 4,443
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Re: Heading and COG
Alan..
Yes, better data. See my picture when I play it.
Since the GNRMC don't contain a heading it comes from the AIVDO => own ship data.
I would claim this is bad data from the AIS transceiver. If it where equal to COG then no big harm but like this it's a mess.
You could filter the AIVDO from your COM10 source to get rid of the misdirected own boat symbol but still you will transfer the same heading to other boats around by the AIVDM sent to other AIS.
I would have had the AIS transceiver checked.
Håkan
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03-12-2024, 19:29
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#20
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Registered User
Join Date: Jun 2009
Location: Elyse is in New Zealand
Boat: Amel Super Maramu 2000
Posts: 605
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Re: Heading and COG
Quote:
Originally Posted by stevead
Agreed, now that we can copy-n-paste the NMEA 0183 data!
Hakan's screenshot and my own testing do not show any heading information.
We need the OP to post a screenshot of their OpenCPN showing the heading.
Presumably with no heading or COG value, by default OpenCPN will orient the boat due north.
My conclusion, the OP's system is rather f....d.
RMC Sentences that contain a position, however don't contain a COG value and have an invalid checksum, AIS VDO sentences that also contain a position however with no data available for both COG and True Heading, (values of 360 and 511 respectively).
As the saying goes "Garbage in, garbage out"
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Despite all this, it works....the boat moves across the chart, the position, SOG and COG are displayed and they are correct. AIS targets are all there and move....
The only thing that is wrong is that the boat heading is always about 30 degrees less than the COG......
I've sent copy of the AIS log to emtrak to ask them what they think.....when i get a response, I'll post it.
Cheers
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04-12-2024, 02:12
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#21
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Registered User
Join Date: Sep 2023
Location: Cruising
Posts: 571
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Re: Heading and COG
Quote:
Originally Posted by Albro359
Despite all this, it works....the boat moves across the chart, the position, SOG and COG are displayed and they are correct. AIS targets are all there and move....
The only thing that is wrong is that the boat heading is always about 30 degrees less than the COG......
I've sent copy of the AIS log to emtrak to ask them what they think.....when i get a response, I'll post it.
Cheers
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Looks like you are reading your own position from AIS data in the VDO messages, which is your own AIS being broadcast. Opencpn can ignore that in Ships|MMSI properties.
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04-12-2024, 02:57
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#22
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Registered User
Join Date: Jun 2022
Location: Cantabria
Boat: Jeanneau - Merry Fisher 925
Posts: 657
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Re: Heading and COG
Quote:
Originally Posted by Albro359
Despite all this, it works....the boat moves across the chart, the position, SOG and COG are displayed and they are correct. AIS targets are all there and move....
The only thing that is wrong is that the boat heading is always about 30 degrees less than the COG......
I've sent copy of the AIS log to emtrak to ask them what they think.....when i get a response, I'll post it.
Cheers
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having a little technical background, apart from "A" system that a far more complicated.
we must consider the "B" transceiver are at least 3 systems in one,
from one side the VHF module, the one who receive/transmit the data stream from sources from/to air.
the GNSS module that receives the position
the processor/multiplexer which receives/send data from other instruments and process the data to be received/transmitted.
one of the modules could not be doing the proper function, and it seems if there is a checksum error it is in the processor, much better to have technical advice from manufacturer.
that you could have correct positioning? could be, but having such error you should not trust in the system until get rid of it, unless you could minimize in the meantime it, by rejecting the incorrect checksum data, but you'll lost quite a lot of data from the AIS transceiver.
__________________
Corsair
Roses don't bloom on the sailor's grave
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04-12-2024, 03:02
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#23
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Registered User
Join Date: Jun 2022
Location: Cantabria
Boat: Jeanneau - Merry Fisher 925
Posts: 657
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Re: Heading and COG
Quote:
Originally Posted by barcoMeCasa
Looks like you are reading your own position from AIS data in the VDO messages, which is your own AIS being broadcast. Opencpn can ignore that in Ships|MMSI properties.
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also, just in case, if you have your MMSI registered in the tab above mentioned, uncheck if you have convert VDM > VDO for this target
__________________
Corsair
Roses don't bloom on the sailor's grave
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04-12-2024, 21:42
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#24
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Registered User
Join Date: Jun 2009
Location: Elyse is in New Zealand
Boat: Amel Super Maramu 2000
Posts: 605
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Re: Heading and COG
OK, the heading info is definitely coming from the AIVDO string....
I have Timezero as well and looking at the port that the emtrak is connected to, it detects Position, COG, SOG, Heading and AIS. If I uncheck the heading box, the 30 degree thing goes away in Timezero.
I can't do that in OpenCPN. Even if i say for that port Ignore HDG, HDT...it doesn't do anything, because that's not how the information is coming in.
I tried the vessel ignore in the MMSI tab....no difference.
No matter what i do it seems I'm stuck with it.
emtrak's response was the usual 1st off the line stuff that doesn't help much.
AND I have to say that I had this issue when I had my old Camino AIS as well.....on a different PC with earlier versions of OpenCPN.
So, unless anyone has something positively constructive to point me to ....other than just to say my system is f...d, I'll leave it for now and just live with it 
Thanks for all your research.....
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05-12-2024, 03:38
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#25
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Registered User
Join Date: Jun 2022
Location: Cantabria
Boat: Jeanneau - Merry Fisher 925
Posts: 657
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Re: Heading and COG
if you have GNSS different from AIS and a compass connected to O, make a rule to reject in the incoming AIS data the sentence VDO, you can source if heading is correct either is coming from another source.
__________________
Corsair
Roses don't bloom on the sailor's grave
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05-12-2024, 04:16
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#26
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Registered User
Join Date: Nov 2012
Location: Orust Sweden
Boat: Najad 34
Posts: 4,443
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Re: Heading and COG
Quote:
Originally Posted by Albro359
OK, the heading info is definitely coming from the AIVDO string....
......
I can't do that in OpenCPN. Even if i say for that port Ignore HDG, HDT...it doesn't do anything, because that's not how the information is coming in.
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No, there is no "HDG/HDT" sentence received. The heading is within the AIVDO sentence.
But, as mentioned, you can instead try to filter out AIVDO. You won't need that since you receive (GN)RMC.
But still: The incorrect heading will be seen by all other ships receiving your AIS data and show your ship symbol with a 30° wrong direction although the COG prediction arrow will be correct.
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06-12-2024, 14:51
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#27
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Registered User
Join Date: Mar 2011
Posts: 856
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Re: Heading and COG
This is puzzling so I've had another look.
First of all, as we've all agreed, the decoded AIS VDO message (own ship) does include a value for the heading field which is erroneous and differs from the COG field by about 30 degrees.
Other vessels receiving your AIS transmissions will receive this erroneous data and if displayed on their chartplotters, your vessel's orientation will be incorrect, although the position. COG & SOG will be valid.
At present OpenCPN is receiving the AIS VDO Message and using the erroneous heading value to orient your vessel on the screen. That is why you are seeing the COG & Heading predictor lines differ. Also don't confuse the heading value embedded within an AIS VDO message, with a HDG or HDT sentence. You can't reject heading values from a VDO message by adding a filter that excludes HDG or HDT sentences.
Secondly, the RMC sentences from either OpenCPN and ProAIS.
At first glance they appear to have an invalid checksum. But in looking at the sentence more closely there is a space inserted between the digits of the SOG field. It is impossible to determine if this is a genuine bug in the AIS Transceiver or a display anomaly when text is copied to a forum post. When the space is removed, the NMEA 0183 Checksum is valid. Could you examine either the OpenCPN or ProAIS logs and verify if there is or isn't a space inserted between the digits of the RMC SOG field.
Also, a few RMC sentences do include a COG field, so it is unclear why the AIS transceiver intermittently lacks the COG value in RMC sentences.
If the inserted space is merely an anomaly of the forum, and the lack of a COG value in the RMC sentence is an irregular/random event, then it may be possible to remove the erroneous heading display from OpenCPN by simply adding an input filter that rejects AIVDO sentences; OpenCPN will obtain the vessel's position, COG & SOG from the RMC sentence.
Finally it would be worthwhile to enable the output of GGA & GLL sentences from the AIS transceiver using the ProAIS configuration tool. Should the RMC sentences indeed have an invalid checksum, perhaps the GGA & GLL sentences may not and they can provide position data to OpenCPN.
Nonetheless, there is something weird with you emtrak AIS transceiver inserting an incorrect heading value in VDO messages. Perhaps there is a firmware update available?
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06-12-2024, 16:56
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#28
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Registered User
Join Date: Sep 2023
Location: Cruising
Posts: 571
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Re: Heading and COG
Well spotted that there was a space in the sentence, remove the space & it seems OK in signalk and recognized by opencpn, and survives cut n paste from opencpn debug intact.
GREEN>23:52:52 (TCP:10.42.0.1:10110) $GNRMC,202446.00,A,3515.85565,S,17411.86886,E,0.29 5,,031224,,,A,V*04<0x0D><0x0A>
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09-12-2024, 08:02
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#29
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Registered User
Join Date: Jun 2020
Location: South Africa
Boat: Leopard 40
Posts: 756
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Re: Heading and COG
Greetings All,
Please excuse my ignorance if this is a silly question or statement, or maybe I missed something. In order to get your ships heading would you not need a fluxgate compass or sat compass to provide that data?
Cheers
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09-12-2024, 08:23
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#30
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Registered User
Join Date: Jun 2022
Location: Cantabria
Boat: Jeanneau - Merry Fisher 925
Posts: 657
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Re: Heading and COG
Quote:
Originally Posted by aqfishing
Greetings All,
Please excuse my ignorance if this is a silly question or statement, or maybe I missed something. In order to get your ships heading would you not need a fluxgate compass or sat compass to provide that data?
Cheers 
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not silly question, very accurate.
to have available the course True/magnetic you need a device that will give such info e.g. fluxgate, sat compass, compass with reading device, etc.
otherwise the course available will be COG but this will be only realible (more or less) when underway, you won't have a trusty heading while stopped. because this is based in two consecutive GNSS positions.
__________________
Corsair
Roses don't bloom on the sailor's grave
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