English: Distance Measuring Equipment is a device for air navigation, and is installed with VOR stations. DME also can measure the speed of airplane by Doppler effect, but that is only the radial component (blue) of the real speed vector (green). The other, the tangential component (grey) is invisible for this instrument.

As the airplane flies by the radio station, the relative angle of its speed is continuously changing, so are the v_r and v_t components. When the moving direction of the airplane is just perpendicular to the direction of the station then DME indicates 0 speed.

Measuring relative direction of the station we get α, the angle between v_r and v vectors. Because v = v_r / cos α, still we do know the real v speed, too.