Heading and COG

[Albro359]

I know the difference.
My issue is this :
the boat heading is always about 30 degrees less than the COG.
My data input is from my emtrak AIS
The data received is two sentences !AIVDO and $GNRMC ...
Neither of these have heading information (as far as i can see), so where is the boat heading coming from?
The heading is patently incorrect as it's always the same , compared to COG.
How can i get rid of it?
Thanks
Cheers
Alan

[stevead]

COG and True Heading are present in a VDO Message. For some reason your RMC sentences don't appear to contain COG nor Magnetic Variation values.

Unfortunately, like many others, you've merely taken a screenshot of the debug window which is of little use to anyone.

More useful is to use the "copy" button and include the text of the debug window in the post, so that it can be verified using any number of tools. No one is going to manually type the VDO message or parse it through an OCR tool to enable it to be decoded.

You'll probably find that magnetic variation would normally account for the difference between true & magnetic bearings.

[PippaB]

I'll agree with stevead, pictures are not very useful, copy the text into a code block in the forum to make getting help, much, much easier.

But, I'm bored waiting for someone to show up, so....

As stevead says, the information is not in your RMC, nor is it in your VDO. The VDO shows heading of "511" which is the default for "not available", so the snip does not show us anything other than COG of due N coming from the VDO. Variation/declination at that location is about 18°.

If you are talking the heading predictor line (as opposed to a data display), Options, Ships, Own Ship and set the length to zero.

[Albro359]

Pardon my technical ignorance.....that's why I'm asking questions....
I have the heading line set to zero.
Here's a copy of the NMEA screen

<GREEN>15:20:18 (Serial:COM10) $GNRMC,022017.00,A,3515.34170,S,17412.07750,E,0.05 0,,031224,,,A,V*0B<0x0D><0x0A>
<GREEN>15:20:18 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP00;7FpSru>GOQ3w`Ql000,0*54<0x0 D><0x0A>
<GREEN>15:20:18 (Serial:COM10) $AIALR,,,,,*7B<0x0D><0x0A>
<GREEN>15:20:19 (Serial:COM10) $GNRMC,022018.00,A,3515.34172,S,17412.07754,E,0.14 2,,031224,,,A,V*00<0x0D><0x0A>
<GREEN>15:20:19 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP00K7FpSru>GOQ3wa1l000,0*45<0x0 D><0x0A>
<GREEN>15:20:20 (Serial:COM10) $GNRMC,022019.00,A,3515.34171,S,17412.07758,E,0.14 9,,031224,,,A,V*05<0x0D><0x0A>
<GREEN>15:20:20 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP00K7FpSru>GOQ3waQl000,0*25<0x0 D><0x0A>
<GREEN>15:20:21 (Serial:COM10) $GNRMC,022020.00,A,3515.34171,S,17412.07763,E,0.15 7,,031224,,,A,V*08<0x0D><0x0A>
<GREEN>15:20:21 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP00c7FpTJu>GOQ3wb1l000,0*51<0x0 D><0x0A>
<GREEN>15:20:22 (Serial:COM10) $GNRMC,022021.00,A,3515.34172,S,17412.07766,E,0.01 4,,031224,,,A,V*09<0x0D><0x0A>
<GREEN>15:20:22 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP00;7FpTru>GOQ3wbQl000,0*51<0x0 D><0x0A>
<GREEN>15:20:23 (Serial:COM10) $GNRMC,022022.00,A,3515.34174,S,17412.07767,E,0.02 7,,031224,,,A,V*0D<0x0D><0x0A>
<GREEN>15:20:23 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP00;7FpTru>GOQ3wc1l000,0*30<0x0 D><0x0A>
<GREEN>15:20:24 (Serial:COM10) $GNRMC,022023.00,A,3515.34176,S,17412.07774,E,0.07 2,,031224,,,A,V*0C<0x0D><0x0A>
<GREEN>15:20:24 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP00K7FpTru>GKQ3wcQl000,0*24<0x0 D><0x0A>
<GREEN>15:20:25 (Serial:COM10) $GNRMC,022024.00,A,3515.34177,S,17412.07778,E,0.04 6,,031224,,,A,V*01<0x0D><0x0A>
<GREEN>15:20:25 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP00;7FpUJu>GKQ3wd1l000,0*0A<0x0 D><0x0A>
<GREEN>15:20:26 (Serial:COM10) $GNRMC,022025.00,A,3515.34178,S,17412.07782,E,0.07 0,,031224,,,A,V*0F<0x0D><0x0A>
<GREEN>15:20:26 (Serial:COM10) !AIVDM,1,1,,A,B7`BbJh00;7HpmJuTdmBLF<QjDWb,0*34<0x 0D><0x0A>
<GREEN>15:20:26 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP00K7FpUJu>GKQ3wdQl000,0*1A<0x0 D><0x0A>
<GREEN>15:20:27 (Serial:COM10) $GNRMC,022026.00,A,3515.34179,S,17412.07784,E,0.03 2,,031224,,,A,V*0D<0x0D><0x0A>
<GREEN>15:20:27 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP00;7FpUJu>GKQ3we1l000,0*0B<0x0 D><0x0A>
<GREEN>15:20:28 (Serial:COM10) $GNRMC,022027.00,A,3515.34179,S,17412.07788,E,0.11 0,,031224,,,A,V*01<0x0D><0x0A>
<GREEN>15:20:28 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP00K7FpUru>GKQ3weQl000,0*23<0x0 D><0x0A>
<GREEN>15:20:28 (Serial:COM10) !AIVDM,1,1,,A,B7`BFL@0Us7HTPJuPmJkGweUoP06,0*4C<0x 0D><0x0A>
<GREEN>15:20:29 (Serial:COM10) $GNRMC,022028.00,A,3515.34177,S,17412.07794,E,0.17 2,,031224,,,A,V*09<0x0D><0x0A>
<GREEN>15:20:29 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP00c7FpUru>GKQ3wf1l000,0*68<0x0 D><0x0A>
<GREEN>15:20:30 (Serial:COM10) $GNRMC,022029.00,A,3515.34176,S,17412.07798,E,0.10 1,,031224,,,A,V*01<0x0D><0x0A>


Hope this provides some clues.

Cheers

Alan

[PippaB]

Where are you seeing the heading that you want to get rid of? Or the data you would like to correct? In this case a screen shot might be helpful

[Hakan]

Alan..
The data example you have provided does not contain COG or Heading.
I've played it with VDR plugin. See my pict.
The RMC in your copy holds a position and a small SOG but no COG.
Example:

$GNRMC,022022.00,A,3515.34174,S,17412.07767,E,0.027,,031224,,,A,V*0D<0x0D><0x0A>

(Look here for details where the COG, field 8, should have been)
The !AIVDO could contain a heading. I've also seen some odd AIS class B that produce a (false)HDG equal to its COG. That could be the case for your AIS but not seen in the extract you have provided.

A tip:
Be sure to have plugin WMM activated in your system. The variation at the position is about 20°.
Another tip:
If heading from the AIS is still false you could test to filter out the AIVDO sentence from your AIS data source?

[smac999]

A ais can not provide heading. It has no compass.

A class a ais can receive heading. And send that data out, But that would require a compass sending heading data to it.

[Hakan]

Originally Posted by smac999

A ais can not provide heading. It has no compass.

True, but some transceivers produced by "Onwa" does actually transmit a heading equal to COG. That's my "(false)" indication above.

[PippaB]

Originally Posted by Albro359

...Neither of these have heading information (as far as i can see), so where is the boat heading coming from?...

I think we have all, in digging through NMEA, missed the OP's real question. He agrees there is no heading data, but OpenCPN is displaying a heading, why?

[barcoMeCasa]

your GNRMC sentences seem to have the wrong checksum

[stevead]

your GNRMC sentences seem to have the wrong checksum

Agreed, now that we can copy-n-paste the NMEA 0183 data!

but OpenCPN is displaying a heading, why?

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"

[PippaB]

Interestingly, the first character in every RMC checksum is a zero, and in every case it should be a two. And the second character is correct for each checksum. Can't figure out how to calculate it to make that happen, but probably means an e-mail to the vendor of the equipment.

[stevead]

Yes an interesting observation that the checksum for the RMC sentences all begin with a zero.

For example:
$GNRMC,022026.00,A,3515.34179,S,17412.07784,E,0.03 2,,031224,,,A,V*0D should have a checksum of 2D

and $GNRMC,022021.00,A,3515.34172,S,17412.07766,E,0.01 4,,031224,,,A,V*09 should have a checksum of 29.

Doesn't make much sense that the AIS Transceiver can calculate the correct checksum for VDM and VDO sentences, but not for RMC sentences.

Perhaps a faulty AIS Transceiver or a missing firmware upgrade. Possibly worth the OP logging the data directly from the AIS Transceiver using its configuration software, presumably ProAIS ?

[barnakiel]

Originally Posted by smac999

A ais can not provide heading. It has no compass.

A class a ais can receive heading. And send that data out, But that would require a compass sending heading data to it.

"...
Detailed description: Class B units

Class B transceivers are smaller, simpler and lower cost than Class A transceivers. Each consists of one VHF transmitter, two VHF Carrier Sense Time Division Multiple Access (CSTDMA) receivers, both alternating as the VHF Digital Selective Calling (DSC) receiver, and a GPS active antenna. Although the data output format supports heading information, in general units are not interfaced to a compass, so this data is seldom transmitted. Output is the standard AIS data stream at 38.400 kbit/s, as RS-232 and/or NMEA formats. To prevent overloading of the available bandwidth, transmission power is restricted to 2 W, giving a range of about 5–10 mi.
Four messages are defined for class B units:
Message 14Safety Related Message: This message is transmitted on request for the user – some transceivers have a button that enables it to be sent, or it can be sent through the software interface. It sends a pre-defined safety message.Message 18Standard Class B CS Position Report: This message is sent every 3 minutes where speed over ground (SOG) is less than 2 knots, or every 30 seconds for greater speeds. MMSI, time, SOG, COG, longitude, latitude, true headingMessage 19Extended Class B Equipment Position Report: This message was designed for the SOTDMA protocol, and is too long to be transmitted as CSTDMA. However a coast station can poll the transceiver for this message to be sent. MMSI, time, SOG, COG, longitude, latitude, true heading, ship type, dimensions...."


Imho, both Class B and A may contain HDM.


At least this is what I have assumed writing my AIS decoder.


b.

[Albro359]

OK,

Here's a screenshot....see the heading is about 30 degrees less than COG

Here's the NMEA from Open CPN

<GREEN>09:14:12 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP09K7Fcoru:TQiE<5Ql000,0*5B<0x0 D><0x0A>
<GREEN>09:14:12 (Serial:COM10) $GNRMC,201412.00,A,3515.73238,S,17411.91495,E,3.60 5,180.98,031224,,,A,V*10<0x0D><0x0A>
<GREEN>09:14:13 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP09;7FcoJu:SAi9<61l000,0*1B<0x0 D><0x0A>
<GREEN>09:14:13 (Serial:COM10) $GNRMC,201413.00,A,3515.73340,S,17411.91486,E,3.60 9,181.88,031224,,,A,V*11<0x0D><0x0A>
<GREEN>09:14:13 (Serial:COM10) !AIVDM,1,1,,B,B7`BAL@0cC7H0?ruaBDV;wV5kP06,0*7B<0x 0D><0x0A>
<GREEN>09:14:14 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP09;7FcoJu:SAie<6Ql000,0*27<0x0 D><0x0A>
<GREEN>09:14:14 (Serial:COM10) $GNRMC,201414.00,A,3515.73443,S,17411.91477,E,3.66 0,181.58,031224,,,A,V*1E<0x0D><0x0A>
<GREEN>09:14:15 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP09K7FcnJu:R1iQ<71l000,0*72<0x0 D><0x0A>
<GREEN>09:14:15 (Serial:COM10) $GNRMC,201415.00,A,3515.73544,S,17411.91475,E,3.64 0,180.41,031224,,,A,V*10<0x0D><0x0A>
<GREEN>09:14:16 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP09;7FcnJu:QIhi<7Ql000,0*20<0x0 D><0x0A>
<GREEN>09:14:16 (Serial:COM10) $GNRMC,201416.00,A,3515.73644,S,17411.91474,E,3.64 6,181.39,031224,,,A,V*19<0x0D><0x0A>
<GREEN>09:14:17 (Serial:COM10) !AIVDM,1,1,,B,B7`B6600>S7@r<Ju:MrSww`5CP06,0*0F<0x 0D><0x0A>
<GREEN>09:14:17 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP09;7Fcmru:PiiI<81l000,0*74<0x0 D><0x0A>
<GREEN>09:14:17 (Serial:COM10) $GNRMC,201417.00,A,3515.73744,S,17411.91474,E,3.53 2,180.61,031224,,,A,V*15<0x0D><0x0A>
<GREEN>09:14:18 (Serial:COM10) !AIVDO,1,1,,,B3M@pMP08s7Fcmru:P9hq<8Ql000,0*34<0x0 D><0x0A>
<GREEN>09:14:18 (Serial:COM10) $GNRMC,201418.00,A,3515.73845,S,17411.91481,E,3.65 5,179.18,031224,,,A,V*14<0x0D><0x0A>