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.

If there were a conversion error I would expect it to be consistently wrong. The values recorded into your flight log are taken every 5 seconds, and represent what your GPS is saying the speed is at that instant. I wouldn't expect a fluctuating value if you're not seeing one onscreen.

Hi chicken, thanks for this very understandable explaination of how speed is determined by GPS. I suppose that heading is determined the same way, but this is (apparently) more stable in the output rather than the speed, how come?
Any way, I will look for a better way to record my flight parameters to make the flight vids more enjoyable

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.

I would not expect iOS to do any raw GPS signal or data processing. This is usually left to the GPS chipset installed which provides a clean and hassle free data interface to the operating system (usually sending over NMEA sentences). The operating system usually knows nothing about the nuts and bolts stuff going on behind the scenes.

Greetings, -- Chris.