What does the Doppler effect measure in radar technology?

In radar technology, the Doppler effect is utilized to measure the change in frequency of the returned signal due to motion. When a radar system emits a radio wave, it travels towards a target (such as a vehicle). If that target is moving, the frequency of the wave reflects back differently than it was emitted. Specifically, if the target is moving toward the radar source, the frequency of the reflected wave is increased (resulting in a higher pitch), whereas if the target is moving away, the frequency decreases (resulting in a lower pitch). This change in frequency is what allows radar systems to determine the speed and direction of the target's movement through what is known as the Doppler shift. This is fundamental for applications like speed detection in law enforcement and tracking moving objects in aviation and weather radar.

The other choices do not accurately reflect what the Doppler effect measures. The frequency of the emitted radio wave is constant and does not change due to motion, the speed of light in the atmosphere is a fixed value and not measured by the Doppler effect, and while radar can determine the distance of a target, this is achieved through time delay measurements rather than the Doppler effect.