Doppler radar

A Doppler radar uses the Doppler effect to measure velocity. More specifically, it measures the change of wavelength caused by motion of the source. For example a police siren approaching, passing, and then receding from you. This image provided the British forces with an early warning system by generating realtime data tracking German aerial activity over, or approaching, the United Kingdom.

http://serialconsign.com/2008/07/early-radar-technology