RADIAL VELOCITY

Figure 2-19.—(A) Stationary target causing no visible change in wave characteristics, (B) Moving target displacing energy causing observable phase shifts. As another example, look at figure 2-19, view (A). Imagine a Doppler radar repeatedly striking a stationary target such as a building. Since the speed at which energy travels is constant, each wave returns in exactly the same phase as those before it. Now look at figure 2-19, view (B). Consider a large balloon moving slowly toward the antenna. Unlike the stationary building, the balloon’s motion causes a change in the appearance of each successive wave. If the radar observes these changes (phase shifts) it will realize that motion has occurred and can then convert this information into target velocity. Keep in mind that the ability of a Doppler radar to detect phase shifts and compute velocity depends upon the system maintaining a consistent transmitter frequency and phase relationship from one pulse to the next. RADIAL VELOCITY We know that Doppler radars can tell whether a target is moving toward or away from the radar. Doppler radars can also measure the velocity of the target toward or away from the antenna. Take a look at figure 2-20. At time T1 a pulse is sent towards a target and it returns at target distance "Y." At time T2, another pulse is sent towards the same target and returns a target distance of "Y+Z." The distance to the target has changed from time Tl to T2, resulting in a phase shift between the two return signals. By measuring the phase shift, the wavelength, and the time interval from T1 to T2, the velocity the target moved toward or away from the radar can be computed. By convention, motion towards a Doppler radar is expressed in negative values and green (cool) colors on a display screen. Motion away from a Doppler radar is expressed in positive values and red (warm) colors. The WSR-88D’s sensitivity enables it to detect extremely weak reflectivities (light drizzle, ice 2-18 crystals, etc.). This sensitivity allows the radar to determine the wind speed from the size of the phase shift. Even dust or insects can act as scatterers and enable the WSR-88D to determine wind speeds in clear air. The WSR-88D will not always detect every motion in the atmosphere, nor will it display them with little or no confusion. Since only pulsed energy that returns directly to the antenna can be detected, it stands to reason that phase shifts are only observable when they occur directly along the radar beam (radial). Figure 2-20.—Radial velocity.