Airplanes can measure their windspeed easily (how fast they are going through the air), but not how fast they are moving over the land (called "ground speed"). A head wind, tail wind, or cross wind changes the ground speed. If the altitude of the plane is known, then the ground speed can be found by timing how fast the plane goes past an object on the ground. (The higher you are, the slower the ground seems to move). A window on the floor of the airplane can be used to find the ground speed and the drift of the airplane.
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