TrueWind

see also

• https://coaps.fsu.edu/woce/truewind/paper/index.html
• http://www.catb.org/gpsd/NMEA.html
• 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

• https://opencpn.org/wiki/dokuwiki/doku.php?id=opencpn:opencpn_user_manual:terminology
• http://signalk.org/specification/1.0.0/doc/vesselsBranch.html
• https://en.wikipedia.org/wiki/Euclidean_vector

vessel's Movement and Orientation

• 'OG' movement over ground
  • SOG Speed Over Ground (magnitude) navigation.speedOverGround
  • COG Course Over Ground (direction) [0-360°]
    • COGt Course Over Ground (relative to true north) navigation.courseOverGroundTrue
    • COGm Course Over Ground (relative to magnetic north) navigation.courseOverGroundMagnetic
• 'SH'
  • STW Speed Trough Water (magnitude) navigation.speedThroughWater
  • HDG Heading (direction) [0-360°]
    • HDT Heading (relative to true north) navigation.headingTrue
    • HDM Heading (relative to magnetic north) navigation.headingMagnetic
• Course (CRS) – Compass direction that the boat is moving through the water (HDG + Leeway). Note that the difference between CRS and COG is Current. CRS does not include the effect of current.
• Leeway – Leeway is the angle between the HDG and the CRS and results from the lateral movement experienced by the boat as she moves forward through the water

Wind

• https://github.com/SignalK/specification/issues/520

AW Apparent Wind (relative to vessel)

• AWS Apparent Wind Speed (magnitude) environment.wind.speedApparent
• AWD Apparent Wind Direction (direction) [0-2π rad][0-360°]
  • Apparent Wind Direction (relative to true north) environment.wind.directionTrue directionApparantTrue
  • Apparent Wind Direction (relative to magnetic north) environment.wind.directionMagnetic directionApparantMagnetic
• AWA Apparent Wind Angle (relative to HDG) negative to port [-π - +π rad][-180° - +180°] environment.wind.angleApparent

TW True Wind (relative to surface of the water)

• TWS True Wind Speed (magnitude) environment.wind.speedTrue speedWater
• TWD True Wind Direction (direction) [0-2π rad][0-360°]
  • True Wind Direction (relative to true north) add: environment.wind.directionWaterTrue
  • True Wind Direction (relative to magnetic north) add: environment.wind.directionWaterMagnetic
• TWA True Wind Angle (relative to HDG) negative to port [-π - +π rad][-180° - +180°] environment.wind.angleTrueWater angleWater

GW Ground Wind (relative to ground)

• GWS Ground Wind Speed (magnitude) environment.wind.speedOverGround
• GWD Ground Wind Direction (direction) [0-2π rad][0-360°]
  • Ground Wind Direction (relative to true north) add: environment.wind.directionOverGroundTrue
  • Ground Wind Direction (relative to magnetic north) add: environment.wind.directionOverGroundMagnetic
• GWA Ground Wind Angle (relative to HDG) negative to port [-π - +π rad][-180° - +180°] environment.wind.angleTrueGround angleOverGround

use

• AW one can measure onboard a vessel
• TW derived from AW + SH
  • historic as OG only became available by GPS
  • of interest at performance sailing
• GW derived from AW + OG
  • used in weather forecasts and reports
  • useful to plan a route
  • note weather changes

other

• 'CUrrent' movement of water by current, tide, river flows etc. (relative to ground)
  • DFT Drift (speed, magnitude) environment.current.drift
  • SETt Set (direction)
    • SETt Set (direction relative to true north) environment.current.setTrue
    • SETm Set (direction relative to magnetic north) environment.current.setMagnetic

calculate Ground Wind from Apparent Wind

• http://davidburchnavigation.blogspot.com/2013/04/true-true-wind-from-apparent-wind.html
• AWA Apparent Wind Angle (relative to HDG)
• HDT Heading (relative to true north)
  • AWD Apparent Wind Direction (relative to true north)
  • AWD = MOD(H+AWA;360)

• AWS Apparent Wind Speed
• COG Course Over Ground
• SOG Speed Over Ground
  • GWS Ground Wind Speed
  • 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
• SOG Speed Over Ground
• HDT Heading (relative to true north)
• STW Speed Trough Water in direction of HDG
  • DFT Drift (current speed relative to ground)
  • 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)
  • 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)