How to calculate wind speed without airspeed?

ArduPlane
1

Hi, I want to know how ardupilot calculate wind speed without airspeed sensor?
I find out the code that like this

bool NavEKF3_core::getWind(Vector3f &wind) const
{
wind.x = stateStruct.wind_vel.x;
wind.y = stateStruct.wind_vel.y;
wind.z = 0.0f; // currently don’t estimate this
return !inhibitWindStates;
}

stateStruct.wind_vel.x represent north wind, but I can not find where ardupilot to calculate “wind_vel.x” in condition of without airspeed sensor.