signalk-derived-data

signalk-derived-data

Signal K server plugin that emits deltas for values derived from other Signal K values — wind angles computed from AWA + heading, CPA/TCPA from AIS tracks, moon illumination from time + position, tank volumes from calibrated levels, and so on.

Installation

Through the Signal K server admin UI — App Store → search for signalk-derived-data → install. Or from the command line in your ~/.signalk dir:

npm install signalk-derived-data

Requires signalk-server with Node >=22.

What it calculates

Every calculator here is optional and enabled individually in the plugin's config screen. Each has a derivedFrom list of Signal K paths and emits on one or more output paths.

Environment

• Air density from humidity, temperature, and pressure
• Air dew point from humidity and temperature
• Wind chill from wind speed and outside temperature
• Heat index from outside temperature and humidity
• Moon illumination, phase, rise, and set from time + position
• Sun mode (dawn / sunrise / day / sunset / dusk / night) from time + position
• Sun times — sunrise, sunriseEnd, goldenHourEnd, solarNoon, goldenHour, sunsetStart, sunset, dusk, nauticalDusk, night, nadir, nightEnd, nauticalDawn, dawn

Navigation

• Distance to go from courseGreatCircle.nextPoint.position
• Set and drift from heading, course over ground, speed through water, speed over ground, and magnetic variation
• True course over ground from magnetic COG + magnetic variation
• Magnetic course over ground from true COG + magnetic variation
• ETA to the active waypoint

Depth

• Depth below keel from depth.belowSurface + design.draft.maximum
• Depth below keel from depth.belowTransducer + depth.transducerToKeel
• Depth below surface from depth.belowKeel + design.draft.maximum

Wind

• True wind angle / direction / speed from speed-through-water, apparent wind angle, and apparent wind speed
• True wind direction from AWA and true heading
• Ground wind angle / speed from SOG, AWA, and AWS
• Magnetic wind direction (two variants: from AWA + magnetic heading, or from true wind direction + magnetic variation)
• Wind shift (experimental)

Performance

• VMG to course from the next-point true bearing, true heading, and SOG
• VMG to wind from true wind direction and SOG
• Propeller slip (requires per-engine pitch + gear ratio in defaults.json)
• Fuel economy from SOG and fuel rate

Boat state

• Battery power from battery voltage × current
• Tank volume from current level, using calibration pairs (>= 2 pairs for parallel sides, >= 3 for a wedge, >= 4 for more complex shapes)

AIS

• CPA / TCPA (closest-point-of-approach / time-to-CPA) for nearby vessels, with configurable notification zones

Writing a new calculator

Drop a file into src/calcs/. Each calculator is a default-exported factory function that receives the server app + plugin instance and returns a Calculation descriptor:

import type { Calculation, CalculationFactory } from '../types'

const factory: CalculationFactory = function (_app): Calculation {
  return {
    group: 'course data',
    optionKey: 'vmg_Wind',
    title: 'Velocity Made Good to wind',
    derivedFrom: [
      'environment.wind.angleTrueWater',
      'navigation.speedOverGround'
    ],
    debounceDelay: 200,
    calculator: function (trueWindAngle: number, speedOverGround: number) {
      const vmg_wind = Math.cos(trueWindAngle) * speedOverGround
      return [{ path: 'performance.velocityMadeGood', value: vmg_wind }]
    }
  }
}

module.exports = factory

derivedFrom lists the Signal K paths the calculator subscribes to; the plugin feeds their latest values into calculator in the same order. Return a { path, value } list (or [] / undefined to skip emission this tick).

Contributing

Issues and pull requests welcome at SignalK/signalk-derived-data. npm test runs the full mocha suite; npm run typecheck + npm run build guard the TypeScript surface; npm run mutation runs Stryker against the calcs.

License

Apache-2.0. See LICENSE.