VELOCITY ALIASING

Figure 2-21.—Affects of return component on radial velocity. Thus, Doppler radars can only measure the component of a target’s motion that occurs along the radial axis. This component is called the radial velocity. Motion Parallel Along Radial Axis If a parcel moves directly toward or away from the antenna, we say its motion is parallel to the radial axis (beam). Truly parallel motions allow the phase shift of each wave to become fully visible to the radar. The result is that displayed velocities reflect the parcel’s true speed. In figure 2-21 (component A), the wind is blowing from the west at 20 knots, and the antenna is pointing toward 270°. The WSR-88D views the full component and measures all 20 knots. Motion Diagonally Across Radial Axis When a parcel’s motion becomes diagonal to the radial axis, only some portion of its phase shift is observed. Since less than maximum motion is realized, an underestimation of target speed is plotted and displayed. The degree of error depends entirely on the parcel’s angle and orientation. In figure 2-21 (component B), the wind is still blowing from the west at 20 knots, but the antenna is pointing toward 315°. Only a portion of the wind is blowing directly toward the antenna and the WSR-88D measures only 15 knots. Motion Perpendicular to Radial Axis Since the radar antenna rotates in a full circle (360°), the parcel’s orientation (to the beam axis) is ever changing. At some point, the parcel’s motion becomes perpendicular to the radial and exhibits zero motion toward or away from the antenna (fig. 2-21, component C). This does not necessarily mean that the target is stationary. It simply means that the target is remaining at a constant distance from the radar and because of the radar’s perpendicular viewing angle, no phase shift is observed. As far as the radar is concerned, the return pulse remains unchanged and no phase shift has occurred. From this information, the parcel is considered stationary. Zero velocity is displayed while the parcel’s actual speed goes undetected. In the following text, we will show how the WSR-88D deals with this problem. VELOCITY ALIASING Imagine two trains connected back-to-back. Since trains have the unique ability to go full speed in either direction, their orientation poses no problem. Notice the method that trains use to display speed (fig. 2-22). Figure 2-22.—Train speedometer. 2-19