I have noticed that the Doppler speed in jibes is almost always a lot lower than the non-doppler speed (using a GT-31 and GPS Action Replay). The tighter the jibe, the bigger the difference. Minimum jibes speeds are often 1/3 slower with Doppler, sometimes more than 50% slower. That's true even with good GPS reception (8 satellites) and no obvious outliers in the positional track points.

The Doppler speeds also seem time-shifted relative to the positional speeds by 2-3 seconds (minimum Doppler speed is later). Anyone have a clue why?

The 1 Hz rate on the GT-31 is a bit slow for jibe analysis, anyway. Has anyone looked at the accuracy of newer 10 Hz units recently? It seems that the MTK chipset used in some of these can save Doppler data.