The term Doppler is related to the phenomenon of the Doppler effect, which is a change in frequency or wavelength of a wave for an observer moving relative to its source. The Doppler effect is commonly observed with sound waves, but it can also be applied to electromagnetic radiation such as RADAR. RADAR is a detection system that uses radio waves to determine the range, angle, or velocity of objects. The Doppler effect is used in RADAR to measure the velocity of a target by measuring the frequency shift of the reflected signal.