The image below shows a similar, but shorter course, that was completed prior to the GPS based compass compensation. The longest course so far, incorporating GPS Based Compass Correction The image clearly shows that the vessel has steered accurately to the waypoints on the downwind legs. The image below depicts the longest mission to date for the vessel. The GPS based compass correction has made a noticeable difference on the water. NavData.HDG = int(wrap_360(NavData.HDG_Mag - NavData.HDG_Err)) NavData.HDG_Err = HeadingErrorFilter.Filter(RawHeadingError) RawHeadingError = wrap_180(NavData.HDG_Mag - NavData.COG) If (NavData.SOG_mps > 0.2) // if the SOG is reasonable then assume the calculate and error value from the GPS and dampen it using low pass filter NavData.HDG_Mag = int(wrap_360(myIMU.heading + Configuration.MagnetVariation + Configuration.CompassOffsetAngle)) V1.2 added Heading Error calculation and correction. V1.1 added CompassOffsetAngle for handling mounting orientation This is expected to be called from a fast loop. Calculate True Heading From Magnetic Heading.

The resulting error is passed through a low pass filter to dampen its movement and then applied to the measured Compass Heading. 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. 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 COG provided by a stationary GPS device tends to point in all directions is has little meaning. The GPS COG is only valid when the GPS is moving. Of course, we can only assume the GPS COG is correct if we assume there is no movement of the water. 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.

Compass Heading Error Correction using GPS

Voyager disk map