Mapping SOG to Speed Thru Water

[Techstyle]

That solution was not working well / repeatable.  Also, the mulitple-sow-sources plugin would send out a ton of deltas.

Instead, I was able to cobble together my first Node-Red flow and thought I would share in case it may be of use to others.  Maybe not for the exact same purpose, but this example shows how to combine several Signal K messages into one new NMEA0183 sentence.  In this case, I combined magneticVariation (from GPS), headingMagnetic (from autopilot), and speedOverGround (from GPS) and form the VHW sentence (headingTrue, headingMagnetic, speedThroughWater).  I did not make a new Signal K delta for this, but instead just send the NMEA0183 formatted sentence, along with the nmea0183out event.  This would make this sentence available both to OpenCPN (via the local port 10110 connection) as well as my data connection to the multiplexer, which in turn sent it on the Raymarine SeaTalk1 network, such that my Tridata now reads SOG and Wind display can calculate TWS/TWA.

Code:

[{"id":"01c84fd2a95aa7a6","type":"tab","label":"SOG -> STW","disabled":false,"info":"This flow copies SOG into STW with the VHW sentence.\n\nMy paddlewheel that measures Speed through\nwater is currently broken, leaving the\nRaymarine Tridata display without Speed and\nthe Wind display without True Wind.\nAn (incorrect) approximation of using GPS\nSOG as STW has now brought life back into those\ndisplays, at least until I can get the paddlewheel\nfixed.\n\nThis flow uses headingMagnetic (from autopilot),\nalong with magneticVariation and speedOverGround\nfrom the GPS, and forms a full VHW NMEA0183\nsentence (headingTrue, headingMagnetic, speedThroughWater)\nand then sends this NMEA0183 sentence.  It does\nnot populate the Signal K key with speedThroughWater,\nalthough it could be added.  I just needed STW\ninto OpenCPN (local, port 10110) and also sent\nout my NMEA data connection to the multilexer\nsuch that the SeaTalk instruments could see \nthe data.","env":[]},{"id":"ad2b0fe6d3916011","type":"signalk-subscribe","z":"01c84fd2a95aa7a6","name":"navigation.speedOverGround","mode":"sendAll","flatten":true,"context":"vessels.self","path":"navigation.speedOverGround","source":"","period":"500","x":140,"y":160,"wires":[["9b78262d4f212efc"]]},{"id":"7a08028bb1ba9354","type":"signalk-subscribe","z":"01c84fd2a95aa7a6","name":"navigation.headingMagnetic","mode":"sendAll","flatten":true,"context":"vessels.self","path":"navigation.headingMagnetic","source":"WLN10.YD","period":"500","x":140,"y":100,"wires":[["9b78262d4f212efc"]]},{"id":"12ea71b9c4081683","type":"function","z":"01c84fd2a95aa7a6","name":"VHW","func":"var maghead,truehead,sent;\nvar sog,sogn,sogk;\nvar checksum = 0;\nmaghead = msg.payload[\"navigation.headingMagnetic\"]\ntruehead = (maghead + msg.payload[\"navigation.magneticVariation\"]) * (180.0 / 3.141592653589793238463);\nmaghead = maghead * (180.0 / 3.141592653589793238463);\nsog = msg.payload[\"navigation.speedOverGround\"];\nsogn = sog * 1.94384449;    // to knots\nsogk = sog * 3.6;           // to km/hr\nsent = \"IIVHW,\"+truehead.toFixed(1)+\",T,\" + maghead.toFixed(1) + \",M,\" + sogn.toFixed(2) + \",N,\" + sogk.toFixed(2) +\",K\";\n\nfor (var i=0;i<sent.length;i++){\n    checksum = checksum ^ sent.charCodeAt(i);\n}\nchecksum = Number(checksum).toString(16).toUpperCase();\nmsg.payload = \"$\" + sent + \"*\" + checksum;\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":210,"y":280,"wires":[["2bca9df90dcad788","e481a40b7cc184ea","9cf369789481f61e","ad7c5f35bc01f6c9"]]},{"id":"9b78262d4f212efc","type":"join","z":"01c84fd2a95aa7a6","name":"","mode":"custom","build":"object","property":"payload","propertyType":"msg","key":"topic","joiner":"\\n","joinerType":"str","accumulate":false,"timeout":"","count":"3","reduceRight":false,"reduceExp":"","reduceInit":"","reduceInitType":"","reduceFixup":"","x":350,"y":100,"wires":[["12ea71b9c4081683"]]},{"id":"917934523f2b0340","type":"signalk-subscribe","z":"01c84fd2a95aa7a6","name":"navigation.magneticVariation","mode":"sendAll","flatten":true,"context":"vessels.self","path":"navigation.magneticVariation","source":"","period":"500","x":140,"y":40,"wires":[["9b78262d4f212efc"]]},{"id":"b20c985349adeffb","type":"signalk-send-nmea0183","z":"01c84fd2a95aa7a6","name":"","nmea0183Event":"nmea0183out","x":570,"y":360,"wires":[]},{"id":"2bca9df90dcad788","type":"delay","z":"01c84fd2a95aa7a6","name":"","pauseType":"rate","timeout":"5","timeoutUnits":"seconds","rate":"2","nbRateUnits":"1","rateUnits":"second","randomFirst":"1","randomLast":"5","randomUnits":"seconds","drop":true,"allowrate":false,"outputs":1,"x":370,"y":360,"wires":[["b20c985349adeffb"]]},{"id":"e481a40b7cc184ea","type":"debug","z":"01c84fd2a95aa7a6","d":true,"name":"debug 1","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":540,"y":180,"wires":[]},{"id":"9cf369789481f61e","type":"tcp out","z":"01c84fd2a95aa7a6","d":true,"name":"","host":"","port":"1456","beserver":"server","base64":false,"end":false,"tls":"","x":560,"y":240,"wires":[]},{"id":"ad7c5f35bc01f6c9","type":"tcp out","z":"01c84fd2a95aa7a6","d":true,"name":"","host":"","port":"10110","beserver":"server","base64":false,"end":false,"tls":"","x":580,"y":300,"wires":[]}]

Chad,

Another solution is to add the derived data plugin and then add a custom calculation.  you basically can make a text file that takes the SOG and outputs a STW.  The code is very simple and there is already a TWS, TWA calculator that you could just copy, rename and then remap the 'navigation.speedThroughWater' to 'navigation.speedOverGround' or just use the same format and just map  'navigation.speedOverGround' to 'navigation.speedThroughWater'.  Once you have the new file, put it in the calc directory and the plugin comes with a config screen in SignalK to turn on what ever calc you want to enable.  Here is the TWS, TWA calc code:

Code:

module.exports = function (app) {
  return {
    group: 'wind',
    optionKey: 'trueWind',
    title:
      'True Wind Angle, Direction and Speed (based on speed through water, AWA, AWS, headingTrue)',
    derivedFrom: [
      'navigation.headingTrue',
      'navigation.speedThroughWater',
      'environment.wind.speedApparent',
      'environment.wind.angleApparent'
    ],
    calculator: function (headTrue, speed, aws, awa) {
      var apparentX = Math.cos(awa) * aws
      var apparentY = Math.sin(awa) * aws
      var angle = Math.atan2(apparentY, -speed + apparentX)
      var speed = Math.sqrt(
        Math.pow(apparentY, 2) + Math.pow(-speed + apparentX, 2)
      )

      var dir = headTrue + angle

      if (dir > Math.PI * 2) {
        dir = dir - Math.PI * 2
      } else if (dir < 0) {
        dir = dir + Math.PI * 2
      }

      return [
        { path: 'environment.wind.directionTrue', value: dir },
        { path: 'environment.wind.angleTrueWater', value: angle },
        { path: 'environment.wind.speedTrue', value: speed }
      ]
    }
  }
}