When I was a kid, I played with the Doppler effect without knowing it. I was doing yeeee-oouunnn with my red little fire truck. The Doppler effect is the one that makes the sound of a car, or train or plane, running closer, with a higher pitch, and the sound of the same car, train or plane, moving away, with lower pitch.
In the optical domain, a similar effect exists, the Doppler-Fizeau effect. The received wavelength of an approaching light source is shorter than the original wavelength of the source. And the received wavelength is longer when the source moves away. aeolus uses the Doppler effect to measure wind speed. If the wavelength at reception is shorter than the wavelength at emission, the wind comes towards aeolus. If the wavelength at reception is longer than the wavelength at emission, the wind moves away from aeolus.