I sometimes record a flight with an actioncam, I synchronize the airdata (derived from the GPX log recorded by SD) with the video in Dashware.


The heading and altitude information are very accurate, but the speed is very unstable resulting in running up and down the speed tape on the left side of the videoscreen, see the analysis below:

I have recorded many log by means of different iPads and an Android Smartphone, but the result is always the same.
Is something going wrong during the conversion process from SD to GPX format? During the flight I do not notice a fluctuating airspeed on SD.

Actually, the speed reported by GPS is not solely calculated by difference in position over time but also by Doppler shift of carrier. Still, a 1 second speed update will suffer from large inaccuracies, as you pointed out. Taking a 5 seconds running boxcar average will go a long ways towards having it be more stable.

-- Chris.