signalk-fallback

SignalK plugin that monitors a path/source and publishes a fallback value when no update has been received within a configurable timeout.

Features

• Monitors any SignalK path, optionally filtered by source ($source identifier)
• Activates fallback when no update is received within the configured timeout
• Three fallback modes:
  • Fixed value — publish a constant (number, boolean, or string)
  • Last known value — republish the last received value
  • Other path — use the current value of another SignalK path
• Multiple independent rules can run simultaneously
• Automatically deactivates fallback when the source resumes

How it works

The plugin acts as a transparent relay: it republishes every incoming update from the monitored path under its own source identifier (signalk-fallback). This means the signalk-fallback source is always up-to-date during normal operation, and keeps publishing (with the fallback value) when the real source goes silent. The transition is seamless — there is no gap between live and fallback mode.

As a consequence, the Signal K data model will contain two sources for each monitored path: the original source and signalk-fallback. To ensure downstream consumers see the plugin's values, configure Signal K's source priorities to prefer signalk-fallback for those paths.

Startup behaviour: if the plugin starts before a monitored path has received any update, the fallback activates immediately (elapsed time is treated as infinite). This is by design — the plugin cannot distinguish between a slow sensor and an absent one at startup.

Use cases

Instruments turned off at the dock

When moored, you may shut down some instruments (log, depth sounder…) while keeping the navigation software running. Without a speed-through-water signal, any plugin that computes true wind speed will stop working or produce invalid results, because the formula requires both wind and boat speed.

Configure the plugin to publish navigation.speedThroughWater = 0 as soon as the log goes silent. True wind calculations keep running correctly, since a stationary boat does indeed have a speed of zero through the water.

COG fallback to heading at low speed

Most GPS receivers stop publishing a reliable Course Over Ground (navigation.courseOverGroundTrue) when the boat is stationary or moving very slowly, because COG is meaningless at zero SOG. However, some derived calculations (routing, autopilot, wind correction) expect a continuous COG value.

Configure the plugin to substitute navigation.headingTrue for COG whenever the GPS goes silent on that path. When the boat is not moving, heading is the best available approximation of intended course.

Keeping derived calculations alive at anchor

On anchor watch, some instruments may be switched off to save power (log, wind, depth). Plugins that compute VMG, set & drift, or leeway need continuous input values to function. Using the last known value mode keeps those paths populated with the last valid reading, so dashboards and alarm systems remain operational.

Sensor failure watchdog

If a sensor unexpectedly stops transmitting (hardware fault, NMEA bus issue), the plugin acts as a watchdog: it detects the silence, logs it via SignalK debug output, and substitutes a safe default value so that downstream calculations degrade gracefully rather than crashing.

Installation

From the SignalK admin UI → Plugin Store, search for signalk-fallback.

Or manually:

npm install signalk-fallback

Configuration

Rules are defined as an array. Each rule accepts the following parameters:

| Parameter | Default | Description | |---|---|---| | watchedPath | — | SignalK path to monitor | | watchedSource | — | (Optional) Only consider updates from this $source | | timeout | 30 s | Duration without update before fallback activates | | interval | 10 s | How often to publish the fallback value while inactive | | fallbackType | lastKnown | fixed, lastKnown, or otherPath | | fixedValue | — | Value to publish when type is fixed. Enter a number (0), boolean (true/false), or JSON string ("moored"). Parsed as JSON. | | fallbackPath | — | Source path when type is otherPath |

Example

{
  "rules": [
    {
      "watchedPath": "navigation.speedThroughWater",
      "timeout": 30,
      "interval": 10,
      "fallbackType": "fixed",
      "fixedValue": 0
    },
    {
      "watchedPath": "navigation.speedOverGround",
      "watchedSource": "gps.primary",
      "timeout": 15,
      "interval": 5,
      "fallbackType": "otherPath",
      "fallbackPath": "navigation.speedThroughWater"
    }
  ]
}

Changelog

See CHANGELOG.md.

License

MIT