I've been looking a bit at the ardupilot firmware and as best as I can tell, no where does the code compute an actual airspeed.
It appears that a pressure reading is taken when the autopilot is engaged, and then the system attempts to maintain that pressure value (without ever knowing what the actual airspeed of the aircraft is.)
Am I reading the code correctly? Has anyone done any work to come up with a formula to compute indicated airpspeed from the raw ADC value? Anyone have any tricks for calibrating the airspeed against some other truth source?
I have a couple things I can do, but my ideas so far are kind of unsophisticated. Maybe someone's already been down this road and has come up with a clever solution or clever observation? (like 1 kt airspeed == x units of ADC change???) Is this even a linear relationship?