signalk-gps-heading

A Signal K plugin that computes vessel true heading and midpoint position from two GPS antennas mounted port and starboard.

How it works

The plugin subscribes to navigation.position updates from two configurable GPS sources. On each update it:

1. Computes the geodetic bearing between the port and starboard antennas.
2. Subtracts the known vessel-frame angle of the antenna baseline (derived from your mount dimensions) to recover true heading.
3. Applies an optional constant trim to absorb residual mounting error.
4. Smooths the result with a circular EWMA filter to reduce GPS noise.
5. Publishes the antenna midpoint as navigation.position and the smoothed heading as navigation.headingTrue.

Installation

Install via the Signal K app store or manually:

npm install signalk-gps-heading

Then restart the Signal K server and enable the plugin under Server → Plugin Config.

Configuration

| Field | Description | Default | |---|---|---| | Port antenna $source | $source string for the port GPS receiver | GPS-Primary.GN | | Starboard antenna $source | $source string for the starboard GPS receiver | GPS-Secondary.GN | | Port / stbd antenna positions | Distance aft of bow and athwartship offset for each antenna, in inches, feet, or metres | — | | Calibration trim (°) | Constant offset added after geometric correction | 0 | | EWMA filter alpha | Smoothing factor 0–1 (lower = smoother, higher = more responsive) | 0.15 |

Find the correct $source string for each GPS by expanding navigation.position in the Signal K Data Browser and reading the source label next to each value.

Calibration

1. Moor the vessel at a berth with a known true bearing.
2. Note the heading the plugin reports.
3. Set Calibration trim to (true bearing) − (reported heading).

License

MIT