Page "Radar" Paragraph 103

If the transmitter's output is coherent ( phase synchronized ), there is another effect that can be used to make almost instant speed measurements ( no memory is required ), known as the Doppler effect.

Most modern radar systems use this principle into doppler radar and pulse-doppler radar systems ( weather radar, military radar, etc ...).

The Doppler effect is only able to determine the relative speed of the target along the line of sight from the radar to the target.

Any component of target velocity perpendicular to the line of sight cannot be determined by using the Doppler effect alone, but it can be determined by tracking the target's azimuth over time.