Compass Heading Error Correction using GPS

Compass Heading Error Correction using GPS

(or GPS/Magnetic Compass Fusion)

Magnetic compass calibration is constant problem, but if we assume the GPS Course of Ground (COG) represents the actual heading of the vessel, then we can establish a difference between the two sensors, and apply a correction to the compass.
Of course, we can only assume the GPS COG is correct if we assume there is no movement of the water. That is, no tidal flow or currents.

The GPS COG is only valid when the GPS is moving. The COG provided by a stationary GPS device tends to point in all directions is has little meaning.
Hence, the COG can only be used to establish a Compass heading error if the GPS Speed of the Ground (SOG) is above a reasonable level.

The procedure below is currently in use, and forms Compass error value based on the GPS COG, when the SOG is greater than 0.2 m/s.
The resulting error is passed through a low pass filter to dampen its movement and then applied to the measured Compass Heading.


void NavigationUpdate_FastData(void)
{
 // Calculate True Heading From Magnetic Heading.
 // This is expected to be called from a fast loop .
 // about 50ms
 // V1.0 31/10/2016 John Semmens
 // V1.1 6/4/2019 added CompassOffsetAngle for handling mounting orientation
 // V1.2 11/7/2019 added Heading Error calculation and correction.

NavData.HDG_Mag = int(wrap_360(myIMU.heading + Configuration.MagnetVariation + Configuration.CompassOffsetAngle));

 // calculate and error value from the GPS and dampen it using low pass filter
 int RawHeadingError;
 if (NavData.SOG_mps > 0.2) // if the SOG is reasonable then assume the
 {
  RawHeadingError = wrap_180(NavData.HDG_Mag - NavData.COG);
 }
 else
 {
  RawHeadingError = 0;
 }

 // apply a low pass filter
 NavData.HDG_Err = HeadingErrorFilter.Filter(RawHeadingError);

 NavData.HDG = int(wrap_360(NavData.HDG_Mag - NavData.HDG_Err));

}



The GPS based compass correction has made a noticeable difference on the water.
The image below depicts the longest mission to date for the vessel. The wind is from the NNW.
The image clearly shows that the vessel has steered accurately to the waypoints on the downwind legs.

The longest course so far, incorporating GPS Based Compass Correction

The image below shows a similar, but shorter course, that was completed prior to the GPS based compass compensation.
The image shows that the vessel is steering significantly to the left as it approaches each waypoint that it can sail to without tacking.

A previous course successfully completed, without GPS compensation for the Compass 

And of course a photo from today's sailing...

Note: This is part of the ongoing development of a low cost autonomous oceangoing sailing drones, utilising a self-trimming wingsail. This is the Voyager series of sailing drones.