Hi David
This is great data, I am sure this is you, and it has (just) bypassed the filter !
Basically your transponder encoder is disagreeing with the PilotAware alt encoder, here is the first message
$PFLAA,3,1500,,-31,1,FF0A60!C-0A60,,,,0.0,8*06
31 metres below, this equates to 101.7ft, the accuracy of a ModeC encoder is rounded to 100ft increments (PilotAware is 3ft increments) so this is just outside the scope of the filter - AARRGHGHH!
I have seen this a couple of times now, and I think we need to counter for the fact of this rounding issue in the ModeC Encoder. Interestingly, I can find nowhere where the Altitude encoder rounding mode is defined, it could be
- round up
- round down
- round to nearest
but not defined anywhere I have seen.
I think we can address this.
Other observation, previously I have seen this in ascent or descent, whereas in your case it looks like level flight. I wonder if there was suddenly a drop in external air pressure compared to internal air pressure perhaps ?
Thx
Lee