esp32-nmea2000 icon indicating copy to clipboard operation
esp32-nmea2000 copied to clipboard
verified publisher icon wellenvogel

True Wind Calculation

Open quantenschaum opened this issue 1 year ago • 8 comments

First, thanks for this project! :tophat:

related to https://github.com/wellenvogel/avnav/issues/367 and https://github.com/kdschmidt1/Sail_Instrument

I noticed that the gateway computes true wind from COG/SOG and AWA/AWS, but I suspect the calculations not to be correct.

Testing

I fed the gateway with the following test data (as return by the GW in PCDIN) and got this output. The test data values were generated using CourseData.

Wind data is sent to the GW as PGN130306 N2kWind_Apparent.

1. No Current

  • SET/DFT = 0 and thus
    • COG/SOG = HDT/STW and
    • true wind = ground wind
print(CourseData(SET=0,DFT=0,HDT=90,STW=5,AWA=-50,AWS=15,angles360=1))

AWA=-50
AWS=15
COG=90.0
DFT=0
GWD=21.99692847308269
GWS=12.392814795155259
HDT=90
SET=90
SOG=5.0
STW=5
TWA=291.9969284730827
TWD=21.996928473082676
TWS=12.39281479515526

GW output

$PCDIN,01F802,0000EF27,14,FFFC5C3D0101FFFF*2A
$GPVTG,90.0,T,,M,5.0,N,9.3,K*56
$PCDIN,01FD02,0000EF28,14,FF040359D302FFFF*23
$GPMWV,310.0,R,7.7,M,A*2B
$GPMWV,292.0,T,6.4,M,A*24
$GPMWD,292.0,T,292.0,M,12.4,N,6.4,M*66
  • VTG COG/SOG = 90°/5kn :heavy_check_mark:
  • MWV-R AWA/AWS = 310°/7.7m/s :heavy_check_mark:
  • MWV-T TWA/TWS = 292°/6.4m/s :heavy_check_mark:
  • MWD TWD/TWS 292°/12.4kn :x: same as MWV-T but should be true wind direction not angle (need to add HDT) Forwarding this to AvNav results in a wrong TWD.

2. With Current

  • SET/DFT != 0 and thus
    • COG/SOG != HDT/STW and
    • true wind != ground wind
print(CourseData(SET=30,DFT=2,HDT=90,STW=5,AWA=-50,AWS=15,angles360=1))

AWA=-50
AWS=15
COG=73.89788624801398
DFT=2
GWD=20.46505232502011
GWS=10.416016194605621
HDT=90
SET=30
SOG=6.244997998398398
STW=5
TWA=291.9969284730827
TWD=21.996928473082676
TWS=12.39281479515526

GW output

$PCDIN,01F802,0019FB96,14,FFFC62324101FFFF*2F
$GPVTG,73.9,T,,M,6.2,N,11.6,K*6F
$PCDIN,01F503,0019FB98,14,FF0101FFFF00FFFF*29
$GPVHW,90.0,T,,M,5.0,N,9.3,K*5A
$PCDIN,01F112,0019FB9E,14,FF5C3DFF7FFF7FFC*54
$GPHDT,90.0,T*0C
$PCDIN,01FD02,0019FBA9,14,FF040359D302FFFF*5E
$GPMWV,310.0,R,7.7,M,A*2B
$GPMWV,286.5,T,6.2,M,A*22
$GPMWD,286.5,T,286.5,M,12.0,N,6.2,M*64
  • VTG COG/SOG = 73.9°/6.2kn :heavy_check_mark:
  • VHW HDT/STW = 90°/5kn :heavy_check_mark:
  • HDT 90° :heavy_check_mark:
  • MWV-R AWA/AWS = 310°/7.7m/s :heavy_check_mark:
  • MWV-T TWA/TWS = 286.5°/6.2m/s :x: should be 292° 12.4kn (6.4m/s) because true wind is relative to water and must be calculated from HDT/STW, here COG/SOG is used and yields ground wind instead
  • MWD :x: same problem as in 1. (angle instead of direction)
  • SET/DFT are not calculated or converted (PGN 130577)

Workaround

Blacklisting MWD, but MWV-T is still wrong.

Suggestions

  • The calculation should be fixed like it is done in the Sail_Instrument.
    • Leeway estimation makes this more complicated.
  • Set and drift should be calculated and/or converted as well.
  • There should be a config option to turn off all calculations, so only conversion is performed by the gateway and the calculations are done by downstream software. (It is not possible to just blacklist calculated sentences, MWV is sent in two flavors which cannot be blocked separately.)

quantenschaum avatar Aug 20 '24 14:08 quantenschaum

I agree... Mainly old code there. And switching of computations is for sure a good idea.

wellenvogel avatar Aug 20 '24 17:08 wellenvogel

OK

The conversions also seem to be wrong.

So, there is a confusion of (see https://github.com/wellenvogel/avnav/issues/367)

  • true wind and ground wind and
  • direction and angle

quantenschaum avatar Aug 21 '24 11:08 quantenschaum

N2K-0183-Conversion

For NMEA0183 sentences and PGNs see https://github.com/wellenvogel/avnav/issues/367.

AFAIK there are only 2 possible direct conversions between N2K and 0183.

  • PGN130306-Apparent <-> MWV-R (AWA/AWS) (as well as VWR)
  • PGN130306-True_water <-> MWV-T (TWA/TWS) :exclamation: ANGLE (not direction)

There is no PGN containing TWD, it must be calculated as TWD=TWA+HDT to be able to send MWD (analogous for reception).

There are no NMEA0183 sentences for ground wind.

I suspect that many of the (expensive) devices out there do not exactly follow these specs and send/receive wrong wind data or process them incorrectly. It's a mess. And then you wonder why your $$$ instruments display funny numbers.

quantenschaum avatar Aug 21 '24 11:08 quantenschaum

To make the converter work correctly with systems that send wrong wind data (i.e. true wind in 130306-True_North [ground wind]) it would make sense to have a config option to select how the data in PGN130306 is interpreted. Like 2 pairs of dropdown boxes N2K <-> 0183 with the following options

N2K 0183
0 true north ground dir AWA/S
1 magnetic ground dir TWA/S
2 apparent angle TWD/S
3 ground angle GWA/S
4 true angle GWD/S

Then you could select something like

  • 2 apparent angle <--> AWA/S
  • 0 true north ground dir <--> TWD/S

which tell the GW how to translate the data.

This way you can tell the GW to do the conversion right even if your devices on the bus send the data with the wrong reference flags.

quantenschaum avatar Aug 21 '24 20:08 quantenschaum

I started to implement the proposed mappings for the N2K wind reference. As far as I see we currently can only use 3 values at the NMEA0183 side: AWA/S,TWA/S,TWD/S. For the others we would need to implement more computations - similar to Sail_Instrument. Finally I'm not really sure if this would make sense in the gateway... @quantenschaum: could you imagine to write a short summary for the wind definitions and issues that we could directly add to the documentation? Maybe just a wind.md in the doc directory? Currently the information is a bit distributed and potentially hard to follow for the normal user.

wellenvogel avatar Oct 07 '24 17:10 wellenvogel

That sounds good!

could you imagine to write a short summary for the wind definitions Currently the information is a bit distributed and potentially hard to follow

That's true. Sure, I could wrap this up. I would merge the information from

  • https://github.com/quantenschaum/Sail_Instrument/blob/master/docs/3-Calculated-Data.md#wind
  • https://github.com/wellenvogel/avnav/issues/367
  • https://github.com/SignalK/specification/issues/520#issuecomment-2304288433
  • https://github.com/wellenvogel/avnav/issues/320#issuecomment-1931770688
  • https://github.com/SignalK/signalk-derived-data/issues/116
  • https://github.com/SignalK/signalk-derived-data/issues/75
  • https://github.com/SignalK/signalk-derived-data/issues/5

Finally I'm not really sure if this would make sense in the gateway

Performing calculations in the gateway can make sense in certain cases when the devices receiving the data need these fields but are not capable of calculating them on their own. But it should be optional. Performing the calculations downstream in the receiving devices/software might be more flexible.

quantenschaum avatar Oct 08 '24 07:10 quantenschaum

Performing calculations in the gateway can make sense in certain cases when the devices receiving the data need these fields but are not capable of calculating them on their own. But it should be optional. Performing the calculations downstream in the receiving devices/software might be more flexible.

Completely agree.

wellenvogel avatar Oct 08 '24 09:10 wellenvogel

first steps with https://github.com/wellenvogel/esp32-nmea2000/releases/tag/20241114

wellenvogel avatar Nov 14 '24 19:11 wellenvogel

Hi, what's the current status of the true wind calculation? Any way to use it with 20250305? The release log for 20241114 mentions remove invalid true wind calc, allow to configure some mapping - partly #75 but no further details. I looked into https://github.com/wellenvogel/esp32-nmea2000/pull/76/commits/54d6d51519a65c8cab6af187a8286d85b5ded2a1 and tried to understand what has changed and it seems that commenting out lines 506 ff. essentially disabled TWS/TWA calculation.

I am currently using the gateway for NMEA2K to WiFi only. My instruments are Simrad IS12, so running on SimNet, which I have physically converted to NMEA2K. The wind vane outputs AWS+AWA directly to the network and the IS12 Wind instrument then calculates and displays TWS+TWD. Unfortunately it does not feed the calculated values back into the network though, which is why the option to have the calculations done in the gateway would come in very handy.

skrue avatar Jul 04 '25 07:07 skrue

Currently not on the priority list... Just waiting for someone who is willing to create a PR.

wellenvogel avatar Jul 04 '25 19:07 wellenvogel