Err, excuse my ignorance but if were looking at an unreliable airspeed caused by pitot malfunction, can we not measure airspeed by GPS???
No. Airspeed is the speed at which you are moving relative to the surrounding air (which is probably also moving itself.) GPS measures the speed you are moving relative to the Earths surface (ground speed,) or more properly the speed you are moving relative to a selection of satellites in orbit.