TrueWind

see also

• http://davidburchnavigation.blogspot.com/2013/04/true-true-wind-from-apparent-wind.html
• https://coaps.fsu.edu/woce/truewind/paper/index.html
• http://signalk.org/specification/1.0.0/doc/otherBranches.html
• http://www.catb.org/gpsd/NMEA.html
• https://opencpn.org/wiki/dokuwiki/doku.php?id=opencpn:opencpn_user_manual:terminology
• http://www.ybw.com/forums/showthread.php?474209-Raymarine-True-amp-Apparent-Wind-calculation
• https://www.sailingworld.com/what-are-my-electronics-telling-me-about-boatspeed-and-heading
• https://en.wikipedia.org/wiki/Apparent_wind
• https://github.com/cape-io/true-wind/blob/master/true-wind.coffee

Terms

vessel's Movement and Orientation

• SOG Speed Over Ground navigation.speedOverGround
• COGt Course Over Ground (relative to true north) navigation.courseOverGroundTrue
• COGm Course Over Ground (relative to magnetic north) navigation.courseOverGroundMagnetic
• STW Speed Trough Water navigation.speedThroughWater
• HDG Heading
  • HDT Heading (relative to true north) navigation.headingTrue
  • HDM Heading (relative to magnetic north) navigation.headingMagnetic

Wind

• AW Apparent Wind (relative to vessel)
  • AWS Apparent Wind Speed environment.wind.speedApparent
  • AWDt Apparent Wind Direction (relative to true north) environment.wind.directionTrue directionApparantTrue
  • AWDm Apparent Wind Direction (relative to magnetic north) environment.wind.directionMagnetic directionApparantMagnetic
  • AWA Apparent Wind Angle (relative to HDG) environment.wind.angleApparent

• TW True Wind (relative to surface of the water)
  • TWS True Wind Speed environment.wind.speedTrue speedWater
  • TWDt True Wind Direction (relative to true north) add: environment.wind.directionWaterTrue
  • TWDm True Wind Direction (relative to magnetic north) add: environment.wind.directionWaterMagnetic
  • TWA True Wind Angle (relative to HDG) environment.wind.angleTrueWater angleWater

• GW Ground Wind (relative to ground) used in weather forecasts and reports
  • GWS Ground Wind Speed environment.wind.speedOverGround
  • GWDt Ground Wind Direction (relative to true north) add: environment.wind.directionOverGroundTrue
  • GWDm Ground Wind Direction (relative to magnetic north) add: environment.wind.directionOverGroundMagnetic
  • GWA Ground Wind Angle (relative to HDG) environment.wind.angleTrueGround angleOverGround

Current

• DFT Current Drift (speed relative to ground) environment.current.drift
• SETt Current Set (direction relative to true north) environment.current.setTrue
• SETm Current Set (direction relative to magnetic north) environment.current.setMagnetic

calculate Ground Wind from Apparent Wind

• AWA Apparent Wind Angle (relative to HDG) environment.wind.angleApparent
• HDT Heading (relative to true north) navigation.headingTrue
  • AWD Apparent Wind Direction (relative to true north) environment.wind.directionTrue
  • AWD = MOD(H+AWA;360)

• AWS Apparent Wind Speed environment.wind.speedApparent
• COG Course Over Ground navigation.courseOverGroundTrue
• SOG Speed Over Ground navigation.speedOverGround
  • GWS Ground Wind Speed environment.wind.speedOverGround
  • GWx = OGx - AWx = SOG * Sin (COG) - AWS * Sin (AWD)
  • GWy = OGy - AWy = SOG * Cos (COG) - AWS * Cos (AWD)
  • GWS= SQRT (GWx*GWx + GWy*GWy)
  • GWD Ground Wind Direction (relative to true north) ???
  • GWD = ATAN ( GWx / GWy )

(calculate current)

• COG Course Over Ground navigation.courseOverGroundTrue
• SOG Speed Over Ground navigation.speedOverGround
• HDT Heading (relative to true north) navigation.headingTrue
• STW Speed Trough Water in direction of HDG
  • DFT Drift (current speed relative to ground) environment.current.drift
  • DFTx = OGx - SHx = SOG * Sin (COG) - STW * Sin (HDT)
  • DFTy = OGy - SHy = SOG * Cos (COG) - STW * Cos (HDT)
  • DFT= SQRT (DFTx*DFTx + DFTy*DFTy)
  • SET Set (current direction relative to true north) environment.current.setTrue
  • SET = ATAN ( DFTx / DFTy )

signalk-derived-data groundWind.js

• https://github.com/SignalK/signalk-derived-data/blob/master/calcs/groundWind.js
• SOG Speed Over Ground navigation.speedOverGround
• AWA Apparent Wind Angle (relative to HDG) environment.wind.angleApparent
• AWS Apparent Wind Speed environment.wind.speedApparent
  • AWx = AWS * Cos (AWA)
  • AWy = AWS * Sin (AWA)
  • GWA Ground Wind Angle (relative to HDG) environment.wind.angleTrueGround
  • GWA = ATAN2(AWy, -SOG + AWx)
  • GWS Ground Wind Speed (relative to ground) environment.wind.speedOverGround
  • GWS = SQRT (POWER(AWy,2) POWER(-SOG + AWx,2))

signalk-derived-data windDirection.js

• https://github.com/SignalK/signalk-derived-data/blob/master/calcs/windDirection.js
• HDG Heading
  • HDT Heading (relative to true north) navigation.headingTrue
  • HDM Heading (relative to magnetic north) navigation.headingMagnetic
• AWA Apparent Wind Angle (relative to HDG) environment.wind.angleApparent
  • AWD Apparent Wind Direction
    • environment.wind.directionTrue
    • environment.wind.directionMagnetic
  • AWD = HDG + AWA
if (AWD > Math.PI * 2) AWD -= Math.PI * 2
else if (AWD < 0) AWD += Math.PI * 2

Sensors

Heading

• see also http://www.navlab.net/Publications/The_Seven_Ways_to_Find_Heading.pdf
• magnetic compass
  • H true = H magnetic + Magnetic declination
• Fluxgate compass
  • H true = H magnetic + Magnetic declination
• Gyrocompass
  • H true
• moving baseline Real-time kinematic (GPS)
  • H true

SOG and COG

• GPS receiver

STW

• paddle-wheel

AWS and AWA

• wind-vane (AWA)
• anemometer
  • cup (AWS)
  • hand held propeller (AWS)
  • ultrasonic (AWS and AWA)