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.
