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

Terms

vessel's Movement and Orientation

• SOG Speed Over Ground navigation.speedOverGround
• COG Course Over Ground (relative to true north) navigation.courseOverGroundTrue
• COG 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
  • AWD Apparent Wind Direction (relative to true north) environment.wind.directionTrue directionApparantTrue
  • AWD 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
  • TWD True Wind Direction (relative to true north) missing environment.wind.directionWaterTrue
  • TWD True Wind Direction (relative to magnetic north) missing 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
  • GWD Ground Wind Direction (relative to true north) missing environment.wind.directionOverGroundTrue
  • GWD Ground Wind Direction (relative to magnetic north) missing 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
• SET Current Set (direction relative to true north) environment.current.setTrue
• SET Current Set (direction relative to magnetic north) environment.current.setMagnetic

calculate True Wind from Apparent Wind

• AWA Apparent Wind Angle (relative to H) environment.wind.angleApparent
• H 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
  • TWS True Wind Speed (relative to ground) environment.wind.speedOverGround
  • TWx = OGx - AWx = SOG * Sin (COG) - AWS * Sin (AWD)
  • TWy = OGy - AWy = SOG * Cos (COG) - AWS * Cos (AWD)
  • TWS= SQRT (TWx*TWx + TWy*TWy)
  • TWD True Wind Direction (relative to true north) ???
  • TWD = ATAN ( TWx / TWy )
• S Speed over ground in direction of H
  • DFT Drift (speed relative to ground) ???
  • DFTx = OGx - SHx = SOG * Sin (COG) - S * Sin (H)
  • DFTy = OGy - SHy = SOG * Cos (COG) - S * Cos (H)
  • DFT= SQRT (DFTx*DFTx + DFTy*DFTy)
  • SET Set (direction relative to true north) ???
  • SET = ATAN ( DFTx / DFTy )

signalk-derived-data (assuming COG=H)

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