I understand the PPM offset is the pants per million difference in frequency from other feeds that I am synchronised, with but what causes Sync Error (measured in micro seconds)? My two feeds are from the same antenna, that eliminates a lot of variables, so is it the delay in processing / decoding the messages differing between the two receivers? I'm just curious.
OK If I look at the timing at my two feeds from a feed with a high sync count and a low PPM error, then the difference is more consistent between them. In fact, if I look at the feed with the highest sync count and ignore PPM error it is probably just rounding errors due to the lack of decimals.
I think it's a statistical estimate which will go down when the receivers share more sync message. This is some of what i understand is going on: https://github.com/wiedehopf/adsb-wiki/wiki/MLAT
