SmartBeaconing problem when used on ocean-going vessels

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Patrick Bryant:
When I first enabled SmartBeaconing on my vessel's APRS beacon, I was baffled by the inappropriately high number of position-report beacons I was sending.

The cause is the nautical motion known as sway.

Heave
is the linear vertical (up/down) motion

Sway
is the linear lateral (side-to-side) motion

Surge
is the linear longitudinal (front/back) motion

GPS speed (velocity) and direction are derived by comparing the present position with the most recent previous position. Sway can be substantial, causing a boat to move sideways 20 feet or more on each crest and trough several times per minute (depending on the wave height and periodicity) at velocities greater than the boat's forward motion through the water.

When a boat sways in ocean swells, the GPS-derived course-over-ground is distorted in the direction of the sway. A boat sways twice for each swell: once for each crest and trough, with the greatest effect occurring when the swells are coming from abeam (to either side of the boat). With a swell period of 10 seconds, the boat will appear to turn 12 times per minute, even though its average course-over-ground ("true course") remains unchanged. The result: the boat appears to change direction (yaw) by a large factor - I've seen instantaneous apparent changes in direction of over 60 degrees. This apparent change in direction causes SmartBeaconing to falsely "pin" turns that actually did not occur.

I resolved this problem by setting the minimum turn angle for pinning corners at 90 degrees. Since I'm operating a sailboat, tacks nearly always involve turns greater than 90 degrees.

Jeremy C. Allen:
I can see you having that problem for sure.  When I am /MM, I tend to set my beacons for a set time to give a more accurate record of my CMG/SMG.  Are you using 2M only, or do you work with 30M as well?

73
Jeremy N1ZZZ

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