The Doppler Effect
Most people have experienced the Doppler shift or Doppler effect at some point. An example is the change in pitch of the sound of a train's whistle as it passes in the distance. The Doppler effect is the observed change in the phase and frequency of sound or light waves due to the relative motion of the source and/or observer.
Doppler radar (like the WSR/88D) makes use of this phenomena. The apparent shift in the electromagnetic waves is used to determine velocities.
Electromagnetic Waves and the Doppler Shift. Each electromagnetic wave is transmitted as an arrangement of a crest and trough. If the returning waves have been displaced, they are said to have undergone a phase shift.
Radars measure phase shifts. When phase shifts are less than 180° (½ of the circle) they are clearly detectable and can be related to a correct velocity. However, shifts of 180° or more are ambiguous, as the radar cannot determine if motion is inbound toward the antenna at 60 kts, or outbound at 40 kts. Velocities become ambiguous or aliased--interpretable as two or more very different values. While aliased velocities are subject to misinterpretation, the WSR-88D has a unique way of dealing with them. The algorithms used account for the fact that direction reversal is impossible. In this case, wind speeds are exceeding radar tolerance and the algorithms are used to calculate the amount of aliasing that is occurring. All necessary corrections are then applied to the products that are received.
If the transmitted energy strikes a stationary target, the electromagnetic energy experiences no phase shift, and its waveform looks identical to the original wave. If the energy strikes a moving target, then the backscattered waves exhibit some amount of phase shift.