We have seen that the echo of a sound can be used to determine how far away something is, and we have also seen that we can use the doppler shift of the echo to determine how fast something is going. Submarines and boats use sonar all the time. Because the echo of the sound will be very faint, it is likely that it will be hard to detect.
Radar therefore uses radio waves instead of sound.
Radio waves travel far, are invisible to humans and are easy to detect even when they are faint. Let's take a typical radar set designed to detect airplanes in flight. The burst might last a microsecond. The radar set then turns off its transmitter, turns on its receiver and listens for an echo. The radar set measures the time it takes for the echo to arrive, as well as the doppler shift of the echo.