Generates deltas for values derived from signalk values

It currently calculates:

• Outside air density (based on humidity, temperature and pressure)
• Battery Power
• Depth Below Keel (based on depth.belowSurface and design.draft.maximum)
• Depth Below Keel (based on depth.belowTransducer and depth.transducerToKeel)
• Depth Below Surface (based on depth.belowKeel and design.draft.maximum)
• Distance To Go (based on courseGreatCircle.nextPoint.position)
• Outside air dew point (based on humidity and temperature)
• Fuel economy (based on speed over ground, fuel rate)
• Propeller Slip calculation (requires defaults.json to include propulsion.*.drive.propeller.pitch and propulsion.*.transmission.gearRatio)
• Sets environment.sun to dawn, sunrise, day, sunset, dusk or night (based on navigation.datetime or system time and navigation.position)
• Tank Volume (based on currentLevel (requires calibration pairs (>2 for parallell sides, >3 for straight wedge and >4 for more complex shapes))
• Velocity Made Good to Course (based on courseGreatCircle.nextPoint.bearingTrue heading true and speedOverGround)
• Velocity Made Good to wind (based on wind.directionTrue and speedOverGround)
• Set and Drift (based on headingMagnetic, courseOverGroundTrue, speedThroughWater, speedOverGround, magneticVariation)
• Outside air wind chill (based on wind speed and temperature)
• True Wind Angle, Direction and Speed (based on speed through water, AWA and AWS)
• True Wind Direction (based on AWA and headingTrue)
• Ground Wind Angle and Speed (based on SOG, AWA and AWS)
• Magnetic Wind Direction (based on AWA and headingMagnetic)
• Magnetic Wind Direction (based on wind.directionTrue and magneticVariation)
• Wind Shift (experimental)
• Moon illumination and times (based on time and navigation.position)
• Sunlight Times: sunrise, sunriseEnd, goldenHourEnd, solarNoon, goldenHour, sunsetStart, sunset, dusk, nauticalDusk, night, nadir, nightEnd, nauticalDawn, dawn (based on time and navigation.position)
• Outside Heat Index (based on temperature and humidity)
• True Course Over Goround (based on courseOverGroundMagnetic and magneticVariation)
• Magnetic Course Over Ground (based on courseOverGroundTrue and magneticVariation)

To add new calculations, just create a new file under the ./calcs directory.

For example. This is the VMG calculator.

module.exports = function(app) {
  return {
    group: 'vmg',
    optionKey: 'vmg',
    title: "Velocity Made Goog (based on courseGreatCircle.nextPoint.bearingTrue heading true and speedOverGround)",
    derivedFrom: [ "navigation.courseGreatCircle.nextPoint.bearingTrue",
                   "navigation.headingTrue",
                   "navigation.speedOverGround" ],
    calculator: function (bearingTrue, headingTrue, speedOverGround)
    {
      var angle = Math.abs(bearingTrue-headingTrue)
      return [{ path: "navigation.courseGreatCircle.nextPoint.velocityMadeGood",
                value: Math.cos(bearingTrue-headingTrue) * speedOverGround}]
    }
  };
}