Beta Test / Technical

[rgleason]

Originally Posted by nohal

It is not possible. (What you posted is a simple typo, the real reasons are completely different, but it is not possible)
Again, you don't have to compile anything yourself - in fraction of the time you will burn on getting it done, every single plugin may have automatic builds on Appveyor running.

Yes, a typo. What a nice Nohal software machine that will be!

[Tehani]

The OpenCPN RMB output statement shows incorrect data in field 12 after "Bearing to destination". This data is the VMG or effective speed towards the waypoint.
The version of OpenCPN is 4.8.4, and the data that appears Match with the SOG.
The correct information is the one that appears in the navigation window as VMG.

[Hakan]

Tehani..
What I can found out in the present code for OpenCPN Ver 4.99.1408 Beta, is the ECRMB transmit to for example an Autopilot during Navigation an active route the speed value is the SOG received from a GPS. (Or any incoming data from somewhere that has the shape of GPS data.)

Håkan

[Tehani]

Hakan,
Please read the NMEA0183 document: "Standard For Interfacing Marine Electronic Devices", version 3.01 of January 1, 2002. Field 12: Destination closing velocity, knots.
I interpret that this data must be the approach speed to the waypoint, that is, the VMG.
Therefore, the correct data is the calculated VMG = SOG * cos (COG - BRG) and not the one generated directly by the GPS receiver device (SOG).
Obviosuly, the SOG data can be obtained from RMC and VTC.

Regards, Tehani.

[bcn]

Tehani,

in case you have the NMEA document at hand, please contact Håkan how to make the content accessible for him. It is not in public domain. NMEA docs are closed source.

bcn

[transmitterdan]

The standard says “destination closing velocity knots”.

[Hakan]

It would be "Destination closing velocity in knots" should be interpreted to something like WMG. I'm not the English native here.
Why OpenCPN code is printing SOG I don't know. Some historical knowledge that can bring light to this?
One history can be old APs don't use this data to anything so than SOG can be good enough? How a modern AP take care of this data I'm not aware of. If it's important it would not be a big effort to change I suppose.
Håkan

[Tehani]

Hakan,
It is true, VMG / VMC are not useful data for autopilots, but they are very useful for "human sailors", and therefore, it is necessary to send them correctly to be visualized in the instrumentation, or in the apps that receive that data.

The information that OpenCPN delivers can be forwarded to other wired buses (NMEA0183 / NMEA2000) through gateways.
It is better to leave the field as null, than to introduce an erroneous data, because this prevents the subsequent calculation.

[Hakan]

Tehani...
I could be, but basically the output (EC)RMB from OpenCPN is meant for an AP and is only sent out while a route is activated for navigation.

If you need other data to any connected instruments or data bus the best would be to set up that output in the tools->connections dialog and select appropriate NMEA sentences to be forwarded by the inbound multiplexer.

But I may agree the ECRMB produced by OpenCPN would carry correct data according to the specification. So, again, anyone who knows the history for printing SOG instead of WMG are welcome to bring light to this.
Håkan

[transmitterdan]

I will do some research to see if there is a historical reason. I see no reason to send SOG in a field meaning "closing speed". It is ok that RMB is only sent when running a route. That's the only time closing speed has a meaning.

[transmitterdan]

So I had a little free time to look at the RMB velocity question. For sure O uses SOG in the RMB velocity field.

I also note that O sends COG in the RMC field intended for Track Made Good Degrees.

I suspect that there is no good reason for either of these. It's a simple matter to correct. I will create a PR and see what the main development team thinks.

[transmitterdan]

On further research it appears that the RMC field descriptors within the O source code are not accurate for the current NMEA standard. The RMC field for course is indeed supposed to be course over ground but in the sources it is called "TrackMadeGoodDegreesTrue".

[rgleason]

Dan, this is what we have in Nmea0183 from various sources
https://opencpn.org/wiki/dokuwiki/do...on_information

RMC -Recommended Minimum Navigation Information

1. Time (UTC)
2. Status, V = Navigation receiver warning
3. Latitude
4. N or S
5. Longitude
6. E or W
7. Speed over ground, knots
8. Track Made Good, degrees true
9. Date, ddmmyy
10. Magnetic variation, degrees
11. E or W

See
http://www.catb.org/gpsd/NMEA.html#_...on_information


Also sentences not recommended
https://www.nmea.org/Assets/100108_n...ew_designs.pdf



What is your source?

Later: Found http://www.gpsinformation.org/dale/nmea.htm#RMC
" 084.4 Track angle in degrees True"

Subtle but big difference, but the standards committee can't just change this year to year!



Another source has it as course made good (CMG)
http://aprs.gids.nl/nmea/#rmc

What does course made good in nmea string mean?

https://gis.stackexchange.com/questi...ea-string-mean

Originally Posted by transmitterdan

On further research it appears that the RMC field descriptors within the O source code are not accurate for the current NMEA standard. The RMC field for course is indeed supposed to be course over ground but in the sources it is called "TrackMadeGoodDegreesTrue".

[rgleason]

siRf starlite https://www.sparkfun.com/datasheets/...%20Manual1.pdf


I wonder what nmea 0183 v4.11 has.
https://www.nmea.org/Assets/nmea-upd...-nmea-0183.pdf

[rgleason]

Standard NMEA-0183 sentences description

RMC Recommended Minimum Navigation Information
. 1 2 3 4 5 6 7 8 9 10 11 12 | | | | | | | | | | | | $--RMC,hhmmss.ss,A,llll.ll,a,yyyyy.yy,a,x.x,x.x,xxxx, x.x,a*hh 1) Time (UTC) 2) Status, V = Navigation receiver warning 3) Latitude 4) N or S 5) Longitude 6) E or W 7) Speed over ground, knots 8) Track made good, degrees true 9) Date, ddmmyy 10) Magnetic Variation, degrees 11) E or W

GPS - NMEA sentence information
Shows all three versions: true course, track made good in degrees true, course made good true.


From

https://www.tronico.fi/OH6NT/docs/NMEA0183.pdf

RMCRecommended Minimum Navigation Information 12 1 2 3 4 5 6 7 8 9 10 11| | | | | | | | | | | | |$--RMC,hhmmss.ss,A,llll.ll,a,yyyyy.yy,a,x.x,x.x,xxxx, x.x,a*hh 1) Time (UTC) 2) Status, V = Navigation receiver warning 3) Latitude 4) N or S 5) Longitude 6) E or W 7) Speed over ground, knots 8) Track made good, degrees true 9) Date, ddmmyy10) Magnetic Variation, degrees11) E or W12) Checksum

Conclusion: I think it is

8) Track made good, degrees true


but this isn't a popularity contest. Does anyone have nmea 0183 v4.11? who can say we are right?