What does "Doppler Shift" refer to in Pulse Radar?

Doppler Shift refers to the change in frequency of radar waves reflected off a moving object. This phenomenon occurs because when an object is moving toward the radar source, the waves are compressed, leading to a higher frequency, while if the object is moving away from the radar, the waves are elongated, resulting in a lower frequency. This effect is essential in pulse radar systems for determining the speed and direction of objects, such as vehicles or aircraft. By analyzing the frequency shift of the returned radar signal, operators can accurately gauge how fast the object is moving, which is crucial for various applications, such as tracking and collision avoidance.

The other choices describe concepts that do not pertain to Doppler Shift. For instance, atmospheric distortion affects signal clarity but does not relate to frequency changes due to movement. Amplitude changes over distance are about signal strength rather than frequency, and variability in pulse width relates to the characteristics of the signal itself rather than any frequency modulation due to motion.