Where does airspeed come from in SITL/X-Plane 11?

I’ve been doing some code-hacking on some improvements to the FBWA controller but it depends a little bit more on airspeed measurements or estimations. I’ve had a dig through the code but all the aliases and shorthand labels easily confuse this bear of little brain and locating where smoothed_airspeed actually comes from is proving difficult for me.

The testing I have done suggests is owes more to groundspeed than anything else and certainly isn’t related to the airspeed data carried across in SITL_Xplane.cpp which appears to get lost in the fog.

Any helpful clues would be appreciated! If it arrives on a silver platter, that’d be wonderful! :smiley: